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Versions: (draft-touch-trill-rbridge-arch) 00 01 02 03 04 05

     Network Working Group                             Eric Gray, Editor
     Internet Draft                                             Ericsson
     Expires: August, 2008
     Intended Status: Informational
                                                       February 25, 2008
     
     
     
                The Architecture of an RBridge Solution to TRILL
                      draft-ietf-trill-rbridge-arch-05.txt
     
     
     Status of this Memo
     
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     Abstract
     
        RBridges are link layer (L2) devices that use a routing protocol
        as a control plane. This combines several of the benefits of the
        link layer with those of the network layer. For example RBridges
        use existing link state routing, without necessarily requiring
        configuration, to improve aggregate throughput, for RBridge to
     
     
     
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        RBridge traffic. RBridges also may support IP multicast and IP
        address resolution optimizations. They are intended to be
        applicable to L2 network sizes similar to those of conventional
        bridges and are intended to be backward compatible with those
        bridges as both ingress/egress and transit. They also support
        VLANs (although this generally requires configuration) while
        otherwise attempting to retain as much 'plug and play' as is
        already available in existing bridges. This document proposes an
        architecture for RBridge systems as a solution to the TRILL
        problem, defines terminology, and describes basic components and
        desired behavior. One (or more) separate documents will specify
        protocols and mechanisms that satisfy the architecture presented
        herein.
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
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     Table of Contents
     
     
        1. Introduction................................................4
        2. Background..................................................7
           2.1. Existing Terminology...................................7
           2.2. RBridge Terminology...................................10
        3. Components.................................................12
           3.1. RBridge Device........................................13
           3.2. RBridge Data Model....................................14
              3.2.1. Unicast TRILL Forwarding Database................14
              3.2.2. Multi-destination TRILL Forwarding Database......14
              3.2.3. Ingress TRILL Forwarding Database................16
        4. Functional Description.....................................17
           4.1. TRILL Campus Auto-configuration.......................17
           4.2. RBridge Peer Discovery................................18
           4.3. Topology Discovery....................................18
           4.4. Determination of End Station Points of Attachment.....18
           4.5. Learning..............................................19
           4.6. Tunneling.............................................19
        5. RBridge Operation..........................................20
           5.1. RBridge General Operation.............................20
           5.2. Ingress/Egress Operations.............................21
           5.3. Transit Forwarding Operations.........................24
              5.3.1. Unicast..........................................25
              5.3.2. Broadcast, Multicast and Flooding................25
                 5.3.2.1. Broadcast...................................25
                 5.3.2.2. Multicast...................................27
                 5.3.2.3. Flooding....................................28
           5.4. Routing Protocol Operation............................30
           5.5. Other Bridging and Ethernet Protocol Operations.......30
              5.5.1. Wiring Closet Problem............................31
        6. How RBridges Address the TRILL Problem Space...............32
        7. Conclusions................................................32
        8. Security Considerations....................................33
        9. IANA Considerations........................................34
        10. Acknowledgments...........................................34
        11. References................................................34
           11.1. Normative References.................................34
           11.2. Informative References...............................34
        12. Author's Addresses........................................35
        Intellectual Property Statement...............................35
        Disclaimer of Validity........................................36
        Copyright Statement...........................................36
        Acknowledgment................................................36
     
     
     
     
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     1. Introduction
     
        This document describes an architecture that addresses the TRILL
        problem and applicability statement [2]. This architecture
        describes a solution that is composed of a set of devices called
        RBridges.  RBridges cooperate together in an Ethernet network to
        provide a layer two delivery service that makes efficient use of
        available links using a link state routing protocol. The service
        provided is analogous to creation of a single, virtual device
        composed of an overlay of tunnels, constructed between RBridge
        devices, using paths determined by link state routing. RBridges
        thus support increased aggregate RBridge to RBridge bandwidth,
        and fault tolerance, when compared to conventional Ethernet
        bridges (which forward frames via a spanning tree, in a non-VLAN
        or single VLAN context, or multiple spanning trees), while still
        being compatible with bridges and hubs.
     
        The principal objectives of this architecture is to provide an
        overview of the use of these RBridges in meeting the following
        goals:
     
          1) Provide a form of optimized layer two delivery service.
     
          2) Use existing technology as much as possible.
     
          3) Allow for configuration free (or minimal configuration)
             deployment.
     
        In providing a (optimized) layer two (L2) service, key factors
        we want to maintain are: transparency to higher layer (layer 3
        and above) delivery services and mechanisms, and use of location
        independent addressing. Optimization of the L2 delivery service
        consists of: use of an optimized subset of all available paths
        and support for optimization of ARP/ND and pruning of multicast
        traffic delivery paths.
     
        Not all optimizations are necessarily expected to be supported
        in initial specification and some subset of these optimizations
        may be specified at a later time.  This architecture should
        allow some level of optimization support to be provided in
        compliant implementations, in as many case as possible.
     
        To accomplish the goal of using existing technologies as much as
        possible, we intend to specify minimal extensions to an existing
        link-state routing protocol, as well as defining specific sub-
        sets of existing bridging technologies that this architecture is
        intended to makes use of.
     
     
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        The extent to which routing protocol extensions may be required
        depends on the closeness of the "fit" of the chosen routing
        protocol (in this case, IS-IS) to RBridge protocol requirements.
        The specific of routing protocol use - along with appropriate
        extensions and enhancements - will be defined in corresponding
        RBridge protocol specifications (see [3] for example).
     
        Specific protocol specifications will also describe the details
        of interactions between the RBridge protocol and specific L2
        technologies - i.e. - Virtual Local Area Networking (VLAN), L2
        Multicast, etc.  This document describes the general nature of
        the RBridge solution without restricting related specifications.
     
        As an overview, however, the intention is to use a link-state
        routing protocol to accomplish the following:
     
          1) Discover RBridge peers.
     
          2) Determine RBridge link topology.
     
          3) Potentially advertise L2 reachability information; note
             that - at this time - the default method for acquiring L2
             reachability information specified in [3] depends on use of
             data-plane learning (see Bridge Learning in the terminology
             section below).
     
          4) Establish L2 delivery using shortest path (verses STP, RSTP
             or MSTP).
     
        There are additional RBridge protocol requirements - above and
        beyond those addressed by any existing routing protocol - that
        are identified in this document and need to be addressed in
        corresponding RBridge protocol specifications.
     
        To allow for configuration free deployment, specific protocol
        specifications should explicitly define the conditions under
        which RBridges may - and may not - be deployed as-is (plug and
        play), and the mechanisms that are required to allow this. For
        example, the first requirement any RBridge protocol must meet is
        to derive information required by link-state routing protocol(s)
        for protocol start-up and communications between peers - such as
        higher-layer addressing and/or identifiers, encapsulation header
        information, etc.
     
        At the abstract level, RBridges need to maintain the following
        information:
     
     
     
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          1) Peer information,
     
          2) Topology information,
     
          3) Forwarding information -
     
               a. unicast,
     
               b. flooded, and
     
               c. multicast.
     
        In addition, RBridge specifications may suggest (or require) the
        maintenance of other information as needed to support ARP/ND and
        multicast optimizations.
     
        Peer information may be acquired via the routing protocol, or
        may be discovered as a result of RBridge-specific peer discovery
        mechanisms.  Details of specific peer information requirements -
        as well as how this information will be acquired is specified in
        protocol specifications (e.g. - [3]).
     
        Topology information is expected to be acquired via the link-
        state routing protocol.
     
        In addition to the requirement that the routing protocol should
        be an existing link-state routing protocol, which may provide a
        mechanism for (or re-use of an existing) neighbor/peer
        discovery, the routing protocol should be able to work with
        minimal (or no) configuration - using algorithmically derived
        addressing, for example, assuming the use of addresses is
        required.
     
        Given the potential choices, IS-IS routing has been chosen at
        this time, and is assumed in this architecture.  The fact that
        this is assumed in this architecture is - in no way - intended
        to preclude use of another link-state routing protocol, or any
        other routing protocol, in any solution not described in this
        architecture.
     
        Forwarding information is derived from the combination of
        attached MAC address learning, snooping of multicast-related
        protocols (e.g. - IGMP), and routing advertisements and path
        computations using the link-state routing protocol.
     
        Other information - such as the mapping of MAC and IP addresses,
        or multicast pruning information - may be learned using snooping
     
     
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        of ARP/ND or IGMP (for example) and it is possible that RBridges
        may need to participate actively in these protocols.
     
        The remainder of this document outlines the TRILL architecture
        of an RBridge-based solution and describes RBridge components,
        interactions and functions. Note that this document is not
        intended to represent the only solution to the TRILL problem
        statement, nor does it specify the protocols that instantiate
        this architecture - or that only one such set of protocols is
        prescribed. The former may be contained in other architecture
        documents and the latter would be contained in separate
        specification documents (see - e.g. - [3]).
     
     2. Background
     
        This architecture is based on the RBridge system described in an
        Infocom paper [1]. That paper describes the RBridge system as a
        specific instance; this document abstracts architectural
        features only. The remainder of this section describes the
        terminology of this document, which may differ from that of the
        original paper.
     
     2.1. Existing Terminology
     
        The following terminology is defined in other documents. A brief
        definition is included in this section for convenience and - in
        some cases - to remove any ambiguity in how the term may be used
        in this document, as well as in derivative documents intended to
        specify components, protocol, behavior and encapsulation
        relative to the architecture described in this document.
     
        o  IEEE 802.1D and IEEE 802.1Q: IEEE documents which include
           specification for bridged Ethernet, including Media Access
           Control (MAC) bridges and the BPDUs used in spanning tree
           protocol (STP) [5], [8].
     
        o  ARP: Address Resolution Protocol - a protocol used to find an
           address of form X, given a corresponding address of form Y.
           In this document, ARP refers to the well-known protocol used
           to find L2 (MAC) addresses, using a given L3 (IP) address.
           See [7] for further information on IP ARP.
     
        o  Bridge: an Ethernet (L2, 802.1D) device with multiple ports
           that receives incoming frames on a port and transmits them on
           zero or more of the other ports; bridges support both bridge
           learning and STP. Transparent bridges do not modify the L2
           PDU being forwarded.
     
     
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        o  Bridge Learning: process by which a bridge determines on
           which (if any) single outgoing port to transmit (forward or
           copy) an incoming unicast frame. This process depends on
           consistent forwarding as "learning" uses the source MAC
           address of frames received on each interface. Layer 2 (L2)
           forwarding devices "learn" the location of L2 destinations by
           peeking at layer 2 source addresses during frame forwarding,
           and store the association of source address and receiving
           interface.  L2 forwarding devices use this information to
           create "filtering database" entries and - gradually -
           eliminate the need for flooding.
     
        o  Bridge Protocol Data Unit (BPDU): the frame type associated
           with bridge control functions (for example: STP/RSTP).
     
        o  Bridged LAN: see IEEE 802.1Q-2005, Section 3.3 [8].
     
        o  Broadcast Domain: the set of (layer 2) devices that must be
           reached (or reachable) by (layer 2) broadcast traffic
           injected into the domain.
     
        o  Broadcast Traffic: traffic intended for receipt by all
           devices in a broadcast domain.
     
        o  Ethernet: a common layer 2 networking technology that
           includes, and is often equated with, 802.3.
     
        o  Filtering Database: database containing association
           information of (source layer 2 address, arrival interface).
           The interface that is associated with a specific layer 2
           source address, is the same interface which is used to
           forward frames having that address as a destination.  When a
           layer 2 forwarding device has no entry for the destination
           layer 2 address of any frame it receives, the frame is
           "flooded".
     
        o  Flooded Traffic: traffic that is subject to flooding - i.e. -
           being forwarded on all interfaces, except the one on which it
           was received, within a LAN or VLAN.
     
        o  Flooding: the process of forwarding traffic to ensure that
           frames reach all possible destinations when the destination
           location is not known.  In "flooding", an 802.1D forwarding
           device forwards a frame for any destination not "known" (i.e.
           - not in the filtering or forwarding database) on every
           active interface except that one on which it was received.
           See also VLAN flooding and flooded traffic.
     
     
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        o  Frame: in this document, frame refers to an Ethernet (L2)
           unit of transmission (PDU), including header, data, and
           trailer (or payload and envelope).
     
        o  Hub: Ethernet device with multiple ports that transparently
           transmits frames arriving on any port to all other ports.
           This is a functional definition, as there are devices that
           combine this function with certain bridge-like functions that
           may - under certain conditions - be referred to as "hubs".
     
        o  IS-IS: Intermediate System to Intermediate System routing
           protocol. [6] for further information on IS-IS.
     
        o  LAN: Local Area Network, is a computer network covering a
           small geographic area, like a home, office, or group of
           buildings, e.g., as based on IEEE 802.3 technology, see also
           IEEE 802.1Q-2005, Section 3.11 [8].
     
        o  MAC: Media Access Control - mechanisms and addressing for L2
           frame forwarding.
     
        o  Multicast Forwarding: forwarding methods that apply to frames
           with broadcast or multicast destination MAC addresses.
     
        o  Node: a device with an L2 (MAC) address that sources and/or
           sinks L2 frames.
     
        o  Packet: in this document, packet refers to L3 (or above) data
           transmission units (PDU - e.g. - an IP Packet (RFC791 [4]),
           including header and data.
     
        o  PDU: Protocol Data Unit - unit of data to be transmitted by a
           protocol. To distinguish L2 and L3 PDUs, we refer to L2 PDUs
           as "frames" and L3 PDUs as "packets" in this (and related)
           document(s).
     
        o  Router: a device that performs forwarding of IP (L3) packets,
           based on L3 addressing and forwarding information. Routers
           forward packets from one L2 broadcast domain to another (one,
           or more in the IP multicast case) - distinct - L2 broadcast
           domain(s). A router terminates an L2 broadcast domain.
     
        o  Spanning Tree Protocol (STP): an Ethernet (802.1D) protocol
           for establishing and maintaining a single spanning tree among
           all the bridges on a local Ethernet segment. Also, Rapid
           Spanning Tree Protocol (RSTP). In this document, STP and RSTP
           are considered to be the same.
     
     
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        o  SPF: Shortest Path First - an algorithm name associated with
           routing, used to determine a shortest path graph traversal.
     
        o  TRILL: Transparent Interconnect over Lots of Links - the
           working group and working name for the problem domain to be
           addressed in this document.
     
        o  Unicast Forwarding: forwarding methods that apply to frames
           with unicast destination MAC addresses.
     
        o  Unknown Destination - a destination for which a receiving
           device has no filtering database entry.  Destination (layer
           2) addresses are typically "learned" by (layer 2) forwarding
           devices via a process commonly referred to as "bridge
           learning" (see definition above).
     
        o  VLAN: Virtual Local Area Network, see IEEE 802.1Q-2005 [8].
     
        o  VLAN Flooding: flooding as described previously, except that
           frames are only forwarded on those interfaces configured for
           participation in the applicable VLAN.
     
     2.2. RBridge Terminology
     
        The following terms are defined in this document and intended
        for use in derivative documents intended to specify components,
        protocol, behavior and encapsulation relative to the
        architecture specified in this document.
     
        o  Adjacent RBridges: RBridges that communicate directly with
           each other without relay through other RBridges.
     
        o  Cooperating RBridges: a set of communicating RBridges that
           will share a consistent set of forwarding information.
     
        o  Designated RBridge (DRB): one approach to resolving several
           issues associated with having multiple RBridges as candidate
           ingress and egress points for a single LAN (or VLAN) is to
           designate an RBridge to act as the ingress and egress in that
           case.  The RBridge designated to handle ingress and egress
           traffic to a specific Ethernet link (or VLAN associated with
           that link) having shared access and multiple RBridges is such
           a link's "Designated RBridge", or DRB. The Designated RBridge
           may (for example) be determined by an election process among
           those RBridges having shared access via a single LAN, or may
           be selected by some other means.
     
     
     
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        o  Edge RBridge (edge of a TRILL Campus): describes RBridges
           that may serve to ingress frames into the TRILL Campus and
           egress frames from the TRILL Campus. L2 frames transiting an
           TRILL Campus enter, and leave, it via an edge RBridge.
     
        o  Egress RBridge: for any specific frame, the RBridge through
           which that frame leaves the TRILL Campus. For frames
           transiting a TRILL Campus, the egress RBridge is an edge
           RBridge where RBridge encapsulation is removed from the
           transit frames prior to exiting the TRILL Campus.
     
        o  Encapsulation database: in the TRILL context, the database
           that the Ingress RBridge uses to map the layer 2 destination
           address in the received frame to the egress Rbridge.
     
        o  Forwarding Tunnels: in this document, Campus Forwarding
           Tunnels (or Forwarding Tunnels) is used to refer to the paths
           for forwarding transit frames, encapsulated at an RBridge
           ingress and decapsulated at an RBridge egress.
     
        o  Ingress RBridge: for any specific frame, the RBridge through
           which that frame enters the TRILL Campus. For frames
           transiting a TRILL Campus, the ingress RBridge is the edge
           RBridge where RBridge encapsulation is added to the transit
           traffic entering the TRILL Campus.
     
        o  Multi-Destination Frames: Broadcast or Multicast frames, or
           Unicast frames destined to a MAC DA that is unknown i.e. -
           flooded frames (see flooded traffic).  Frames that need to be
           delivered to multiple egress RBridges, via the RBridge
           Distribution Tree.
     
        o  Peer RBridge: The term "Peer RBridge", or (where usage is not
           ambiguous) the term "Peer", are used in the RBridge context
           to refer to any of the RBridges that make up a TRILL campus.
     
        o  RBridge: a logical device as described in this document,
           which incorporate both routing and bridging features, thus
           allowing for the achievement of TRILL Architecture goals. A
           single RBridge device which can cooperate with other RBridge
           devices to create a TRILL Campus.
     
        o  RBridge Distribution Tree: This term or (where usage is not
           ambiguous) the term "distribution tree", refers to a tree
           used by RBridges to deliver multi-destination frames. An RDT,
           or distribution tree, is computed using a specific RBridge as
           the root. May also be referred to as an R-tree.
     
     
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        o  TRILL Campus: this term, or the term "Campus" (where usage is
           not ambiguous) is used in the RBridge context to refer to the
           set of cooperating RBridges and TRILL Links that connect them
           to each other.
     
        o  TRILL Forwarding Database: this term, or the term "forwarding
           database" (where not ambiguous) is used in an RBridge context
           to refer to the database that maps the egress TRILL address
           to the next hop TRILL link.
     
        o  TRILL Header: a 'shim' header that encapsulates the ingress
           L2 frame and persists throughout the transit of a TRILL
           Campus, which may be further encapsulated within a hop-by-hop
           L2 header (and trailer). The hop-by-hop L2 encapsulation in
           this case includes the source MAC address of the immediate
           upstream RBridge transmitting the frame and destination MAC
           address of the receiving RBridge - at least in the unicast
           forwarding case.
     
        o  TRILL Link: this term, or the term "Link" (where its usage is
           not ambiguous) is used in the RBridge context to refer to the
           Layer 2 connection that exists either between RBridges, or
           between an RBridge and Ethernet end stations.
     
     3. Components
     
        A TRILL Campus is composed of RBridge devices and the forwarding
        tunnels that connect them; all other Ethernet devices, such as
        bridges, hubs, and nodes, operate conventionally in the presence
        of an RBridge.
     
         +----------------------------------------------------------+
         |                 Higher Layer Entities                    |
         +--+--------------+----------------------+--------------+--+
         |  \ TRILL Layer  | RBridge Relay Entity | TRILL Layer  /  |
         +---+-------------+----------------------+-------------+---+
         | Data Link Layer |                      | Data Link Layer |
         +-----------------+                      +-----------------+
         | Physical Layer  |                      |  Physical Layer |
         +--------+--------+                      +--------+--------+
                  |                                        |
                 P 1                                      P 2
     
              Figure 1:  Simplified Architecture of an RBridge
     
        Figure 1 shows an RBridge that contains:
     
     
     
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        o  An RBridge Relay Entity connecting two RBridge ports
     
        o  At least one physical port (two in this example)
     
        o  Higher layer Entities, including at least the IS-IS protocol
     
        o  At the TRILL Layer, an RBridge encapsulates incoming Ethernet
           frames with a TRILL header to forward them to other RBridges.
     
     3.1. RBridge Device
     
        An RBridge is a device - having some of the characteristics of
        both bridges and routers - that forwards frames on an Ethernet
        link segment. It has one or more Ethernet ports which may be
        wired or wireless; the particular physical layer is not
        relevant. An RBridge is defined more by its behavior than its
        structure, although it logically contains three tables, which
        may be used to describe the externally visible behavior of an
        RBridge relative to its peers and may also distinguish RBridges
        from conventional bridges.
     
        Conventional bridges contain a learned filtering (or forwarding)
        database, and spanning tree port state information. The bridge
        learns which nodes are accessible from a particular port by
        assuming bi-directional consistency: the source addresses of
        incoming frames indicate that the incoming port is to be used as
        output for frames destined to that address. Incoming frames are
        checked against the learned filtering (forwarding) database and
        forwarded to the particular port if a match occurs, otherwise
        they are flooded out all active ports (except the incoming
        port).
     
        Spanning tree port state information indicates the ports that
        are active in the spanning tree. Details of STP operation are
        out of scope for this document, however the result of STP is to
        disable ports which would otherwise result in more than one path
        traversal of the spanning tree.
     
        RBridges, by comparison, have a TRILL forwarding database, used
        for forwarding of RBridge encapsulated frames across the TRILL
        Campus and by the ingress RBridge to determine the encapsulation
        to use for frames received as un-encapsulated from non-RBridge
        devices. The TRILL forwarding database is described in the
        following sections.
     
     
     
     
     
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     3.2. RBridge Data Model
     
        The following tables represent the logical model of the data
        required by RBridges in forwarding unicast and multicast data
        across a TRILL Campus.
     
     3.2.1. Unicast TRILL Forwarding Database
     
        The Unicast TRILL Forwarding Database is a forwarding table for
        unicast traffic within the TRILL Campus, allowing tunneled
        traffic to transit the TRILL Campus from ingress to egress. The
        size of a fully populated Unicast TRILL Forwarding Database at
        each RBridge is maximally bounded by the product of the number
        of Adjacent RBridge peers and VLANs.
     
        RBridges may have separate Unicast TRILL Forwarding Databases
        for each VLAN, if this is supported by configuration. Note that
        scaling concerns may dictate otherwise, either in specific of
        RBridge protocol specification, or in deployment.  The Unicast
        TRILL Forwarding Database is continually maintained by RBridge
        routing protocols and/or MAC learning. (see Section 5.4).
     
        The Unicast TRILL Forwarding Database contains data specific to
        RBridge forwarding for unicast traffic. The specific fields
        contained in this table are to be defined in RBridge protocol
        specifications. In the abstract, however, the table should
        contain forwarding direction and encapsulation associated with
        an RBridge encapsulated frame received - determined by the TRILL
        "shim" header destination and VLAN (if applicable).
     
     3.2.2. Multi-destination TRILL Forwarding Database
     
        The Multi-destination TRILL Forwarding Database consists of a
        set of forwarding entries used for support of RBridge
        Distribution Trees (RDT). Multi-destination TRILL Forwarding
        Database entries are distinct from typical Unicast TRILL
        Forwarding Database entries because there may be zero or more of
        them that match for any incoming frame.
     
        The Multi-destination TRILL Forwarding Database may overlap the
        Unicast TRILL Forwarding Database, or be instantiated as a
        separate table, in specific compliant implementations.
     
        In discussing entries to be included in the Multi-destination
        TRILL Forwarding Database, the following entities are
        temporarily defined, or further qualified:
     
     
     
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        o  Root RBridge - the RBridge that is the head end of an RDT.
           All RBridges within a TRILL Campus are potential Root
           RBridges.
     
        o  Egress RBridge - an RBridge that is the tail end of a path
           corresponding to a specific Multi-destination TRILL
           Forwarding Database entry. All RBridges within a TRILL Campus
           are potential egress RBridges. Not all RBridges within a
           TRILL Campus will be on the shortest path between any ingress
           RBridge and any other egress RBridge.
     
        o  Local RBridge - the RBridge that forms and maintains the
           Multi-destination TRILL Forwarding Database entry (or
           entries) under discussion. The local RBridge may be a root
           RBridge, or an egress RBridge with respect to any set of
           entries in the Multi-destination TRILL Forwarding Database.
     
        o  RBridge TRILL Campus Egress Interface - an interface on any
           RBridge where a transit RBridge encapsulated frame would be
           decapsulated prior to forwarding. With respect to such an
           interface, the local RBridge is the egress RBridge.
     
        Each local RBridge will maintain - as a logical representation -
        a set of entries for at least the following, corresponding to a
        subset of all possible forwarding paths:
     
        o  Zero or more entries grouped for each root RBridge - keyed by
           some root RBridge identifier - used to determine forwarding
           of broadcast, multicast, and flooded frames originally
           RBridge encapsulated by that ingress within the TRILL Campus.
     
        o  Corresponding to each of these entry groups, one entry for
           each of zero or more egress RBridge - where the local RBridge
           is on the shortest path toward that egress RBridge.
     
        o  Corresponding to each of these entry groups, one entry for
           each of zero or more TRILL Campus egress interfaces.
     
        Each entry would contain an indication of which single interface
        a broadcast, multicast or flooded frame would be forwarded for
        each (root RBridge, egress RBridge) pair.  Entries would also
        contain any required encapsulation information, etc. required
        for forwarding on a given interface, and toward a corresponding
        specific egress RBridge.
     
     
     
     
     
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        Note that the above information is one logical representation of
        the information required to perform a reverse path forwarding
        check (or RPFC) as is discussed in [3].
     
        A local RBridge could maintain a full set of entries from every
        RBridge to every other RBridge, however - depending on topology
        - only a subset of these entries would ever be used.  In
        addition, a topology change that changed selection of shortest
        paths would also very likely change other elements of the
        entries, negating possible benefits from having pre-computed
        Multi-destination TRILL Forwarding Database entries.
     
        Multi-destination TRILL Forwarding Database entries should also
        include VLAN identification information relative to each set of
        Root RBridges, to allow scoping of broadcast, multicast and
        flooding forwarding by configured VLANs.
     
        Multi-destination TRILL Forwarding Database entries may also
        include Multicast-Group Address specific information relative to
        each egress RBridge that is a member of a given well-known
        multicast group, to allow scoping of multicast forwarding by
        multicast group.
     
        Implicit in this data model is the assumption that the TRILL
        "shim" header encapsulation will contain information that
        explicitly identifies the TRILL Campus ingress RBridge for any
        broadcast, multicast or flooded frame.
     
        Maintenance of this Multi-destination TRILL Forwarding Database
        will be defined in appropriate protocol specifications used to
        instantiate this architecture. Note that doing this does not
        strictly require those specification to adopt this data model.
        The protocol specification needs to include mechanisms and
        procedures required to establish and maintain the Multi-
        destination TRILL Forwarding Database in consideration of
        potential SPF recomputations resulting from network topology
        changes.
     
     3.2.3. Ingress TRILL Forwarding Database
     
        The Ingress TRILL Forwarding Database determines how arriving
        traffic will be encapsulated, for forwarding toward the egress
        RBridge, via the TRILL Campus. It becomes configured in much the
        same way that bridge learning occurs: by snooping incoming
        traffic, and assuming bi-directional consistency.
     
     
     
     
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        This learned information at an egress RBridge may be propagated
        to all other RBridges in the TRILL Campus via the RBridge
        routing protocol, as an alternative to direct MAC learning from
        data frames. However, the information propagated in this fashion
        may be quite large and filtering to prevent overwhelming edge
        RBridges would require extensive per-VLAN state information in
        core RBridges.  Hence the current model is that the default mode
        for learning L2 reachability information is via learning from
        the data plane directly in a manner very analogous to bridge
        learning.
     
        Using this approach, the ingress TRILL Forwarding Database may
        be as large as the number of nodes on the Ethernet LAN, for all
        VLANs in which a specific ingress RBridge is a participant.
     
        The Ingress TRILL Forwarding Database essentially determines the
        tunnel encapsulation used to transport each specific frame
        across the TRILL Campus, for frames entering at this ingress.
     
     4. Functional Description
     
        The RBridge Architecture is largely defined by RBridge behavior;
        the logical components are minimal, as outlined in Section 3.
     
     4.1. TRILL Campus Auto-configuration
     
        Cooperating RBridges self-organize to compose a single TRILL
        Campus system. The details for how this occurs are given in
        protocol specification(s).
     
        At an architectural level, it is sufficient to note that every
        end station attached to a TRILL Campus should have a primary
        point of attachment to the TRILL Campus, as might be defined
        (for example) by a Designated RBridge.  Furthermore, if it is
        possible that there are 802.1Q (or 802.1D) bridges in a local
        LAN, the association of specific RBridges and the end stations
        for which they act as primary point of attachment must be
        determined in a way that is consistent with forwarding in an
        802.1Q (or 802.1D) network.  If a DRB election process were
        used, each TRILL Link (or VLAN set within a TRILL Link) attached
        to a TRILL Campus would have a single Designated RBridge (either
        for the link, or for a subset of the VLANs on the link); using
        DRBs as an example, the DRB would be where all traffic intended
        to transit a TRILL Campus enters and exits.
     
        Other approaches might be used as well.  For example, the
        primary point of attachment for each end station (or set of end
     
     
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        stations), might be configured, or determined in some other way,
        on a per end station (or set of end stations) basis.  Any such
        approach must either allow for the possibility of LAN bridges or
        not be used when such a possibility exists.
     
        This rule applies strictly on a per-VLAN basis.
     
        High-level steps that may be included in auto-configuration are:
        RBridge peer discovery, topology discovery, determination of end
        station points of attachment (DRB election, for example),
        learning and forwarding (tunneling) TRILL encapsulated frames.
     
     4.2. RBridge Peer Discovery
     
        Proper operation of the TRILL solution using RBridges depends on
        the existence of a mechanism for discovering peer RBridges.
        Failure to discover all peer RBridges leads inevitably to an
        incomplete discovery of the RBridge topology.
     
        RBridge peer discovery can be accomplished in a relatively easy
        re-use of well-known techniques based on broadcast - such as the
        use of IS-IS "hello" messages.
     
     4.3. Topology Discovery
     
        Proper operation of RBridges also depends on the existence of a
        mechanism for determining the RBridge topology. An accurate
        determination of RBridge topology is required in order to
        determine how traffic frames will flow in the topology and thus
        avoid the establishment of persistent loops in frame forwarding,
        or construction of a partitioned local LAN.
     
        Fortunately, accurate topology determination is a fundamental
        requirement of a functioning link-state routing protocol. The
        complexity that applies in this architecture directly relates to
        the existence of multiple VLANs on a TRILL Link.
     
        For this reason, RBridges (in terms of protocol definition,
        implementation and deployment) should avoid unnecessary use of
        multiple VLANs - in particular on links that will be, or may be,
        used for transit of TRILL encapsulated frames.
     
     4.4. Determination of End Station Points of Attachment
     
        The mechanisms and details of how end stations are associated
        with a specific RBridge - when multiple RBridges are available
        (connected to a local LAN or VLAN) - for the purpose of
     
     
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        providing ingress to and egress from the Rbridge Campus, will be
        provided by protocol specification(s).  A potential approach to
        be considered is the use of a "Designated RBridge (DRB)
        Election", on either a per link, or per VLAN (set) basis.
     
        Architecturally, it is important to note that this determination
        must be based on an accurate view of the topology, including
        availability of certain links in a given topology for traffic
        associated with any given VLAN.  Otherwise, it is possible to
        partition a TRILL Link-local LAN (assuming that RBridges may be
        deployed and configured to replace existing 802.1Q bridges) as a
        result of a failure - under circumstances in which such a
        partition would not have occurred with previously deployed
        802.1Q bridges.
     
        Protocol specification(s) need to define how accurate VLAN
        topology is to be determined and applied in determination of end
        station primary points of attachment.  Protocol specification(s)
        also need to state the limitations that any chosen mechanisms
        may impose on the solution (in terms of scalability and ease of
        deployment, for example).  Finally, protocol specification(s)
        need to describe how determination is made with respect to which
        RBridge(s) are responsible for learning new end station
        information, and for flooded and broadcast frames for reaching
        known and unknown end stations on any link or VLAN.
     
     4.5. Learning
     
        The protocol specifications need to define how learning of MAC-
        layer reachability information is expected to occur - at least
        in the default case.
     
        As described previously, a major consideration is the complexity
        associated with receiving reachability information for a lot of
        end-stations for which an ingress RBridge has no interest.  This
        is the case, for example, where a large number of VLANs are in
        use (see [8]).  This issue does not arise if learning is based
        on the data plane (similar to bridge learning) - as is currently
        described as a default learning mode in [3].
     
     4.6.Tunneling
     
        RBridges pass encapsulated frame traffic to each other
        effectively using tunnels. These tunnels use an Ethernet link
        layer header, together with a TRILL header.
     
     
     
     
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        Specifics of encapsulation are to be defined in appropriate
        protocol/encapsulation specifications.
     
        It is the combination of the local MAC desitnation (which is for
        a locally attached RBridge) and the TRILL encapsulation that
        distinguishes RBridge to RBridge traffic from other traffic.
        The link-layer header includes source and destination addresses,
        which typically identify the local RBridges (the sending and
        receiving RBridges relative to the local TRILL Link).
     
        The TRILL header is required to support loop mitigation for (at
        least) unicast traffic within the TRILL Campus; traffic loops in
        forwarding between RBridges and non-RBridge devices, as well as
        across non-RBridge devices between RBridges, is beyond the scope
        of this document.
     
        The TRILL header and encapsulation:
     
        o  must clearly identify the traffic as RBridge traffic - the
           outer Ethernet header may, for instance, use an Ethertype
           number unique to RBridges;
     
        o  should also identify a specific (egress) RBridge - the TRILL
           header may, for example, include an identifier unique to the
           egress RBridge, in the unicast case;
     
        o  should include the RBridge transit route, a hopcount, or a
           timestamp to prevent indefinite looping of a frame.
     
     5. RBridge Operation
     
        This section is intended primarily to serve as a tutorial for
        RBridge operations. As such in any case where this section says
        anything in diagrement with specific protocol specifications,
        the protocol specification over-rides.
     
     5.1. RBridge General Operation
     
        As described in sections above, operations that apply to all
        RBridges include peer and topology discovery (including hello
        messaging, negotiation of RBridge identifiers and link-state
        routing), determination of RBridge primary points of attachment
        (for local end stations - possibly via DRB election), shortest
        path (SPF) computation and either learning or advertising reach-
        ability for specific L2 (MAC Ethernet destination) addresses
        within a broadcast domain.
     
     
     
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        In addition, all RBridges will compute RBridge Distribution
        Trees for delivery of (potentially VLAN scoped) broadcast,
        multicast and flooded frames to each peer RBridge. Setting up
        these trees early is important as there is otherwise no means
        for frame delivery across the TRILL Campus during the learning
        phase. Because it is very likely to be impossible (at an early
        stage) for RBridges to determine which RBridges are edge
        RBridges, it is preferable that each RBridge compute these trees
        for all RBridges as early as possible - even if some entries
        will not be used.
     
        The specifics of each of these operational steps will be defined
        in protocol specifications (such as [3]).
     
     5.2.Ingress/Egress Operations
     
        Operation specific to edge RBridges involves RBridge learning,
        advertisement, encapsulation and decapsulation (at ingress and
        egress RBridges, respectively).
     
        As described previously, RBridge learning is similar to typical
        bridge learning - i.e. - all RBridges listen promiscuously to L2
        Frames on each local LAN and acquire end station location
        information associated with source MAC addresses in L2 frames
        they observe.
     
        If multiple RBridges are available (i.e. - connected to a local
        LAN or VLAN), a determination must be made as to which RBridge
        is the primary point of attachment for each end station (or end
        stations by groups - using, for example, the VLAN associations
        of VLAN aware end stations) requiring ingress and egress
        services from an RBridge Campus.  This is a minimal requirement,
        and there is no reason why this same determination might not be
        made in all cases, even the degenerate case in which only one
        RBridge may be used.  This could be the case, for instance, if a
        DRB election is done in all cases, including when the DRB can
        only be the one and only RBridge to which local end stations
        might be attached.
     
        In the degenerate case - where only one RBridge is connected to
        a specific Ethernet LAN - obviously that RBridge will be (or
        become) the primary point of attachment for local end stations
        requiring ingress/egress services from the RBridge Campus.
     
        Minimally, only the RBridge determined as the primary point of
        attachment for a given (set of) end station(s) is required to
        perform RBridge learning for that (set of) end station(s) while
     
     
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        they remain associated with interface(s) connected to the local
        LAN or VLAN.
     
        Note that - in some cases - the determination of primary points
        of attachment and learning may be tightly bound operations.
        This might be the case if - for example - the determination is
        based on literal configuration of end station and RBridge
        attachments.  In other cases, learning would occur in a more
        conventional - and flexible - way, if (for example) an automated
        process of selecting a DRB (such as DRB election) is used on a
        per-link or per-VLAN (set) basis.
     
        Assuming learning is required by a specific solution described
        in any protocol specification(s), as RBridges learn segment-
        local MAC source addresses, it creates an entry in its learned
        filtering/forwarding database that associates that MAC source
        address with the interface on which it was learned.
     
        Similarly - to support ARP/ND optimization - IP-to-MAC mappings
        may also be learned by snooping corresponding protocol messages.
        Protocol specifications may include either optional or required
        behaviors to support ARP/ND, or multicast, learning and
        distribution methods.
     
        Periodically, as determined by RBridge protocol specification,
        each RBridge may advertise this learned information to its
        RBridge peers.  These advertisements would propagate to all edge
        RBridges (as potentially scoped by associated VLAN information
        for each advertisement). Each edge RBridge would incorporate
        this information in the form of a Unicast TRILL Forwarding
        Database entry.
     
        Note that currently, [3] specifies that this is not the default
        mode, and that learning primarily occurs via the data plane at
        egress, as well as at ingress.
     
        The trade-off is between the complexity associated with flooding
        data verses the complexity associated with flooding reachability
        information.
     
        For applications in which it is likely that most edge RBridges
        will not want to receive most of the reachability information,
        flooding avoidance requires either that the method is not used,
        or that intermediate (core, in at least some cases) RBridges
        need to keep VLAN specific state information to limit the scope
        of advertisement flooding.
     
     
     
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        If it is desired - in any specific solution - to support
        discovery of new end station attachments, RBridges may also
        discover that a new end station has become locally attached (for
        which they may be, or become, an edge RBridge) as a result of
        receiving un-encapsulated frames that require forwarding.  Any
        such solution must specify how a primary point of attachment is
        determined when this occurs.  Several possible approaches exist.
     
        If an automated DRB selection process (such as DRB election) is
        the approach in use for a specific solution (on a per-link, or
        per-VLAN, basis), this determination is automatic for any link
        (or VLAN on a local link).  If an RBridge is the DRB for a local
        link or VLAN,, and has not previously learned that the MAC
        destination for a frame is local (this could be the case - for
        instance - for the very first frame it observes), then the
        RBridge could be required to forward (or flood) the frame via
        the TRILL Campus to all other RBridges (potentially within a
        VLAN scope).
     
        When a frame is received, which must be forwarded across the
        RBridge Campus, the responsible RBridge would flood the frame
        unless it has already created a Unicast TRILL Forwarding
        Database entry for the frame's MAC destination address.  If it
        has a corresponding Unicast TRILL Forwarding Database entry,
        then it would use that.  The RBridge in this instance would be
        an ingress RBridge for the frame being forwarded across the
        RBridge Campus.
     
        The encapsulation used by this ingress RBridge would be
        determined by the Unicast TRILL Forwarding Database entry - if
        one exists - or the Unicast TRILL Forwarding Database-equivalent
        entry for the RBridge Distribution Tree.
     
        When the encapsulated frame arrives at egress RBridge(s), it is
        decapsulated and forwarded via the egress interface(s) onto the
        local link (or VLAN on the local link).
     
        In using the approach of learning from the data plane, the
        egress RBridge stores information related to content of the
        frame's TRILL encapsulation for use in subsequent reverse
        traffic in a manner directly analogous to bridge learning.
     
        Note that an egress RBridge will - in most case - be the RBridge
        determined to be the primary point of attachment for a
        destination end station on the local link or VLAN accessed via
        its egress interface(s). Exceptions to this might exist under
        circumstances in which use of distinct RBridges for ingress and
     
     
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        egress for a common (set of) end station(s) does not produce
        local forwarding ambiguity.  Any protocol specification that
        allows this must also unambiguously describe the precise
        circumstances under which it is allowed - as well as the
        limitations and issues this introduces in the solution
        specified.
     
        Also note that specific solution(s) in protocol specification(s)
        will need to describe how determination of an end station's
        primary point of attachment (RBridge) occurs for the case where
        a specific end station has not yet been discovered at any
        ingress or egress interface - except under circumstances where
        discovery of new end stations is not supported, or explicitly
        disallowed.  In the example in which a DRB election is used,
        this determination is both trivial and automatic.  In an
        approach where end station and RBridge attachment/association is
        configured, this should be relatively obvious - if inflexible.
     
        In the DRB example, if the destination MAC address of a received
        frame does not correspond to a learned MAC destination address
        at an egress interface, it will forward the frame on all
        interfaces for which it is either the designated RBridge. If a
        received frame does correspond to a learned MAC destination
        address at an egress interface, the RBridge will forward the
        frame via that interface only.
     
        Any specific solution's protocol specification(s) should also
        allow for the fact that flooded frames may arrive at a single
        local LAN (or VLAN) - where local end stations may be attached -
        via multiple RBridges. Protocol specification(s) need to account
        for how determination of which RBridge is exclusively
        responsible for forwarding such frames is to be made.  This is
        essential to avoid having the same frame arrive multiple times
        and potentially from widely disparate directions (i.e. - on
        different interfaces of individual bridges).
     
     5.3. Transit Forwarding Operations
     
        There are two models for transit forwarding within a TRILL
        Campus: unicast frame forwarding for known destinations, and
        everything else.  The difference between the two is in how the
        encapsulation is determined. Exactly one of these models will be
        selected - in any instantiation of this architecture- for each
        of the following forwarding modes:
     
        o  Unicast frame forwarding
        o  Forwarding of non-unicast frames
     
     
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           o  Broadcast frame forwarding
           o  Multicast frame forwarding
           o  Frame flooding
     
     5.3.1. Unicast
     
        In unicast forwarding, the TRILL header is specific to the
        egress RBridge and MAC destination in the outer Ethernet
        encapsulation is specific to the next hop RBridge.
     
        As the frame is prepared for transmission at each RBridge, the
        next hop MAC destination information is determined at that local
        RBridge using a corresponding Unicast TRILL Forwarding Database
        entry based on the TRILL "shim" header.
     
     5.3.2. Broadcast, Multicast and Flooding
     
        RBridge Distribution Trees are used for forwarding of broadcast,
        multicast and unknown destination frames across the TRILL
        Campus. In a simple implementation, it is possible to use the
        Multi-destination TRILL Forwarding Database entries for all
        frames of these types.
     
        However, this approach results in possibly severe inefficiencies
        in at least the multicast case.
     
        As a consequence, instantiations of this architecture should
        allow for local optimizations on a hop by hop basis.
     
        Examples of such optimizations are included in the sections
        below.
     
     5.3.2-1. Broadcast
     
        The path followed in transit forwarding of broadcast frames will
        have been established through actions initiated by each RBridge
        (as any RBridge is eligible to subsequently become an ingress
        RBridge) in the process of computing Multi-destination TRILL
        Forwarding Database entries.
     
        The protocol specification will most likely require each RBridge
        to assume that it may be a transit as well as an ingress and
        egress RBridge and establish forwarding information relative to
        itself and each of its peer RBridges, and stored in the Multi-
        destination TRILL Forwarding Database.  At least one exception
        case exists and that is when RBridges are configured to treat a
        given link as a point to point link between two RBridges.
     
     
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        Forwarding information should logically exist in two forms:
        transit encapsulation information for interfaces over which the
        RBridge will forward a multipoint frame to one or more adjacent
        RBridges and a decapsulation indication for each interface over
        which the RBridge may egress frames from the TRILL Campus. In
        each case, the Multi-destination TRILL Forwarding Database
        includes some identification of the interface on which a frame
        is forwarded toward any specific egress RBridge for frames
        received from any specific ingress RBridge.
     
        Note that an interface over which an RBridge may egress frames
        is any interface for which the RBridge is the primary point of
        end station attachment for one or more end stations, or the
        RBridge determined to be responsible for broadcast delivery.
     
        RBridges must not wait to determine that one (or more) Ethernet
        end stations are present on an interface before deciding to
        forward decapsulated broadcast frames on that interface.  Again,
        an exception case would exist if RBridges have been configured
        to treat a local link as a point to point connection between two
        RBridges, or otherwise configured to ignore possible presence of
        end stations in this case.
     
        Forwarding information is selected for each broadcast frame
        received by any RBridge (based - for example - on identifying
        the ingress RBridge, or distribution tree root, for the frame)
        for all corresponding Multi-destination TRILL Forwarding
        Database entries. Each RBridge may thus be required to replicate
        one RBridge encapsulated broadcast frame for each interface that
        is determined from Multi-destination TRILL Forwarding Database
        entries corresponding to the frame's ingress RBridge (or
        distribution tree root). This includes decapsulated broadcast
        frames for each interface for which the RBridge is responsible
        for providing egress for broadcast frames (as might have been
        determined previously by DRB election, for example).
     
        Note that frame replication and forwarding should be scoped by
        VLAN, if VLAN support is provided. Also note that an egress
        RBridge may be required to transmit a decapsulated frame on the
        same interface on which it previously received the corresponding
        RBridge encapsulated frame.
     
        This approach for broadcast forwarding might be considered to
        add complexity because replication occurs at all RBridges along
        the ingress RBridge tree, potentially for both RBridge
        encapsulated and decapsulated broadcast frames. However, the
        replication process is similar to replication of broadcast
     
     
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        traffic in 802.1D bridges with the exception that additional
        replication may be required at each interface for egress from
        the TRILL Campus.
     
        Note that the additional replication associated with TRILL
        Campus egress may be made to exactly conform to 802.1D bridge
        broadcast replication in implementations that model a TRILL
        Campus egress as a separate logical interface.
     
        Using this approach results in one and only one copy of the
        broadcast frame being delivered to each egress RBridge.
     
     5.3.2-2. Multicast
     
        Multicast forwarding is reducible to broadcast forwarding in the
        simplest (default) case. However, protocol specifications may
        require, or recommend and implementations may choose - using
        mechanisms that are out of scope for this document - to optimize
        multicast forwarding.  In order for this to work effectively,
        however, support for awareness of multicast "interest" is
        required for all RBridges.
     
        Without optimization, multicast frames are injected by the
        ingress RBridge onto an RDT by - for instance - encapsulating
        the frame with a MAC destination multicast address, and
        forwarding it according to its local Multi-destination TRILL
        Forwarding Database. Again, without optimization, each RBridge
        along the path toward all egress RBridges will similarly forward
        the frame according to their local Multi-destination TRILL
        Forwarding Database.
     
        Using this approach results in one and only one copy of the
        multicast frame being delivered to appropriate egress RBridges.
        However, using this approach, multicast delivery is identical to
        broadcast delivery - hence very inefficient.
     
        In any optimization approach, RBridge encapsulated multicast
        frames will use either a broadcast or a group MAC destination
        address. In either case, the recognizably distinct destination
        addressing allows a frame forwarding decision to be made at each
        RBridge hop. RBridges may thus be able to take advantage of
        local knowledge of multicast distribution requirements to
        eliminate the forwarding requirement on interfaces for which
        there is no recipient interested in receiving frames associated
        with any specific group address.
     
     
     
     
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        As stated earlier, in order for RBridges to be able to implement
        multicast optimization, distribution of learned multicast group
        "interest" information must be provided - and propagated - by
        all RBridges.  Mechanisms for learning and propagating multicast
        group participation by RBridges is out of scope in this document
        but may be defined in RBridge protocol specification(s).
     
        Note that, because the multicast optimization would - in
        principle - further scope and reduce broadcast traffic, two
        things may be said:
     
        o  It is not necessary that all implementations in a deployment
           implement the optimization (though all must support the data
           required to implement it in RBridge peers) in order for any
           local multicast optimization (consistent with the above
           description) to work;
        o  Introduction of a multicast optimization will not result in
           potential forwarding loops where broadcast forwarding would
           not do so.
     
        In the simplest case, the ingress RBridge for a given multicast
        frame will re-use the MAC destination group address of a
        received multicast frame.  However this may not be required as
        it is possible that the mechanisms specified to support
        multicast will require examination of the decapsulated MAC
        destination group address at each RBridge that implements the
        optimization.
     
        Specifics of multicast forwarding are to be defined in protocol
        specifications.
     
     5.3.2-3. Flooding
     
        Flooding is similarly reducible to broadcast forwarding in the
        simplest (default) case - with the exception that a frame being
        flooded across the TRILL Campus is typically a unicast frame for
        which no Unicast TRILL Forwarding Database entry exists at the
        ingress RBridge. This is not a minor distinction, however,
        because it impacts the way that addressing may be used to
        accomplish flooding within the TRILL Campus.
     
        An ingress RBridge that does not have a Unicast TRILL Forwarding
        Database entry for a received frame MAC destination address,
        will inject the frame onto the ingress RBridge Tree by - for
        instance - encapsulating the frame with a MAC destination
        broadcast address, and forwarding it according to its local
        Multi-destination TRILL Forwarding Database. Without
     
     
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        optimization, each RBridge along the path toward all egress
        RBridges will similarly forward the frame according to their
        local Multi-destination TRILL Forwarding Database.
     
        Using this approach results in one and only one copy of the
        flooded frame being delivered to all egress RBridges.
     
        However implementations may choose to optimize flooding. A
        Flooding optimization will only work at any specific RBridge if
        that RBridge re-evaluates the original (decapsulated) unicast
        frame.
     
        Any flooding optimization would operate similarly to the
        multicast optimization described above, except that - instead of
        requiring local information about multicast distribution - each
        RBridge implementing the optimization will need only to lookup
        the MAC destination address of the original (decapsulated) frame
        in its local Unicast TRILL Forwarding Database. If an entry is
        found, the frame could then be forwarded only if the specific
        RBridge is on the shortest path between the originating ingress
        RBridge and the appropriate egress RBridge.  This could be
        implemented - for example - as a specialized Multi-destination
        TRILL Forwarding Database entry.
     
        Note that, because a flooding optimization would - in principle
        - further scope and reduce flooded traffic, two things may be
        said:
     
        o  It is not necessary that all implementations in a deployment
           support the optimization in order for any local flooding
           optimization (consistent with the above description) to work
           (hence such an optimization is optional);
        o  Introduction of the flooding optimization will not result in
           potential forwarding loops where flooded forwarding would not
           do so.
     
        Because a forwarding decision can be made at each hop, it is
        possible to terminate flooding early if a Unicast TRILL
        Forwarding Database for the original MAC destination was in the
        process of being propagated when flooding for the frame was
        started.  It is therefore possible to reduce the amount of
        flooding to some degree in this case.
     
        Specifics of a flooding optimization - beyond the above proof of
        the concept that such a thing could be done safely - is out of
        scope for this document and should be out of scope generally in
        all protocol specifications for which the above analysis holds.
     
     
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     5.4. Routing Protocol Operation
     
        The details of routing protocol operation are determined by the
        choice to use IS-IS routing.  These details would be defined in
        appropriate protocol specification(s). Protocol specifications
        in this case may include both RBridge protocols (such as [3]),
        and specifications offering a generalized enhancement to IS-IS.
     
        Protocol specifications should identify the means by which IS-IS
        meets the peer and topology discovery, and path computation
        needs of the specific protocol - including which IS-IS optional
        features and enhancements (if any) are required for support of
        specified RBridge operations.
     
     5.5. Other Bridging and Ethernet Protocol Operations
     
        In defining this architecture, several interaction models have
        been considered for protocol interaction between RBridges and
        other L2 forwarding devices - in particular, 802.1D bridges.
        Whatever model we adopt for these interactions must allow for
        the possibility of other types of L2 forwarding devices. Hence,
        a minimal participation model is most likely to be successful
        over the long term, assuming that RBridges are used in a L2
        topology that would be functional if RBridges were replaced by
        other types of L2 forwarding devices.
     
        Toward this end, RBridges - and the TRILL Campus as a whole -
        could (in theory) participate in Ethernet link protocols,
        notably the spanning tree protocol (STP) on the ingress/egress
        links using exactly one of the following interaction models:
     
        o  Transparent Participation (Transparent-STP)
        o  Active Participation (Participate-STP)
        o  Blocking Participation (Block-STP)
     
        Only one of these variants would be supported by an instance of
        this architecture. All RBridges within a single TRILL Campus
        must use the same model for interacting with non-RBridge
        protocols. Furthermore, it is the explicit intent that only one
        of these models is ultimately supported - at least as a default
        mode of compliant implementations.
     
        This architecture assumes RBridges block STP.
     
     
     
     
     
     
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     5.5.1. Wiring Closet Problem
     
        There is at least one remaining issue with this assumption and
        that has been referred to as the "wiring closet problem."  The
        essential problem is described in this subsection.
     
        Given this configuration of bridges in a wiring closet, and an
        RBridge core:
     
           -----> B-1 <----------------> RB-a <-----.
                   |                                 \
                   /                                  > RBridge CORE
                   |                                 /
           -----> B-2 <----------------> RB-b <-----'
     
        The link between (802.1D) bridges B-1 and B-2 is meant to be
        disabled by STP.  In the RBridge case, however, there is no
        indication (from STP) that this link is redundant.  Moreover, in
        order to avoid breaking bridge learning, either RB-a or RB-b
        will be the DR and - as a result, only one of the links (B-
        1<=>RB-a, B-2<=>RB-b) will get used.
     
        One solution to this problem is to include - as a configuration
        option, for instance - the ability to enable negotiation of (or
        use of a pre-defined, or configurable) pseudo-bridge identifier
        to be used in any of the variations of STP.
     
        One - (near) zero-configuration - option we've considered would
        be to use a well-known bridge identifier that each RBridge would
        use as a common pseudo-bridge identifier.  Such an ID, used in
        combination with other STP configuration parameters, would most
        likely have to be guaranteed to win the root bridge election
        process in order to be a reasonable and useful default.
     
        However, because this architecture assumes RBridges block STP,
        participation in any form of STP is assumed to take place in an
        in-line, co-located bridge function. Such a bridge function is
        in addition to RBridge architectural functionality described in
        this document.  Implementations may include such functionality
        and will very likely require some minimal configuration to turn
        it on, in vendor specific RBridge implementations.  An example
        of a minimal configuration would be to assign a pseudo-bridge
        identifier to (the local in-line co-located bridge associated
        with) a specific RBridge port.
     
        For reasons of interoperability, specific protocol proposals to
        address the needs of this architecture may specify exactly how a
     
     
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        co-located bridge will operate in this case (if such co-located
        bridge functionality is included in an implementation), as well
        as whether or not inclusion of such co-location is required.
     
        As a further note, one of the problems that should be addressed
        - assuming that this problem is to be resolved - is how to make
        certain the solution is robust against configuration error.  In
        any solution that requires configuration of a pseudo-bridge ID
        that is common across a TRILL Campus, for example, it is
        possible to guard against configuration errors by using an
        election process (based on the root bridge election process) to
        determine which configured ID will be used by all RBridges in
        common - assuming that multiple pseudo-bridge IDs are
        inadvertently configured.
     
        Finally, note that there is a chicken-and-egg problem associated
        with RBridge participation in STP where RBridges may themselves
        be connected by spanning trees.
     
     6. How RBridges Address the TRILL Problem Space
     
        The RBridge architecture addresses the following aspects of the
        requirements identified in reference [2] through the use of a
        link-state routing protocol and defined forwarding behaviors:
     
        o  Inefficient Paths
     
        o  Robustness to Link Interruption
     
        In addition, using a logical model of "separation of functions"
        this architecture allows specifications and implementations to
        address existing and developing Ethernet extensions and
        enhancements, and provides a background against which protocol
        specifications may address: concerns about convergence under
        dynamic network changes, and optimizations for VLAN, ARP/ND,
        Multicast, etc.
     
     7. Conclusions
     
        This document discusses options considered and factors affecting
        any protocol specific choices that may be made in instantiating
        the TRILL architecture using RBridges.
     
        Specific architectural and protocol instantiations should take
        these into consideration. In particular, protocol, encapsulation
        and procedure specifications should allow for potential
     
     
     
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        optimizations described in the architectural document to the
        maximum extent possible.
     
        Also, this document addresses considerations relative to
        interaction with existing technology and "future-proofing"
        solutions.  For both simplicity in description, and robust long
        term implementation of the technology, this document recommends
        the use of clear distinction - at all possible points - of
        definitions, protocols, procedures, etc. from related (but not
        identical) specifications and interactions.
     
        In particular, this document recommends the use of a
        "collocation model" in addressing issues with combining RBridge,
        Router and 802.1D bridge behavior.
     
     8. Security Considerations
     
        As one stated requirement of this architecture is the need to be
        able to provide an L2 delivery mechanism that is potentially
        configuration free, the default operation mode for instances of
        this architecture should assume a trust model that does not
        require configuration of security information. This is - in fact
        - an identical trust model to that used by Ethernet devices in
        general.
     
        In consequence, the default mode does not require - but also
        does not preclude - the use of established security mechanisms
        associated with the existing protocols that may be extended or
        enhanced to satisfy this document's architectural definitions.
     
        In general, this architecture suggest the use of a link-state
        routing protocol - modified as required to support L2 reach-
        ability and link state between RBridges. Any mechanisms defined
        to support secure protocol exchanges between link-state routing
        peers may be extended to support this architecture as well.
     
        This architecture also suggests use of additional encapsulation
        mechanisms and - to the extent that any proposed mechanism may
        include (or be extended to include) secure transmission - it may
        be desirable to provide such (optional) extensions.
     
        To the extent possible, any extensions of protocol or
        encapsulation should allow for at least one mode of operation
        that doesn't require configuration - if necessary, for limited
        use in a physically secure deployment.
     
     
     
     
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     9. IANA Considerations
     
        This document has no direct IANA considerations. It does
        suggest, that protocols that instantiate the architecture use a
        TRILL header as a wrapper on the payload for RBridge to RBridge
        traffic, and this TRILL header may be identified by a new
        Ethertype in the tunneled Ethernet link header. This Ethertype,
        identified in an Ethernet header, could be allocated by the
        IEEE.
     
     10. Acknowledgments
     
        The initial work for this document was largely done by Joe
        Touch, based on work he and Radia Perlman completed earlier.
        Subsequent changes are not to be blamed on either of them.
     
        In addition, the current text has been helped substantially by
        comments and suggestions from the TRILL working group, working
        group chairs, and from Ron Bonica, Stewart Bryant, Joel Halpern,
        Guillermo Ibanez and Russ White in particular.  Also, a great
        deal of work was recently done - by Joe Touch, Radia Perlman,
        Dinesh Dutt and Silvano Gai - in an effort to align terminology
        and concepts used in this document with those also used in the
        other TRILL documents.
     
     11. References
     
     11.1. Normative References
     
        None.
     
     11.2. Informative References
     
        [1]   Perlman, R., "RBridges: Transparent Routing", Proc.
              Infocom 2005, March 2004.
     
        [2]   Touch, J., R. Perlman, (ed.) "Transparent Interconnection
              of Lots of Links (TRILL): Problem and Applicability
              Statement", work in progress, draft-touch-trill-prob-
              00.txt, November, 2005.
     
        [3]   Perlman, R., S. Gai, D. Dutt, D. Eastlake III, "RBridges:
              Base Protocol Specification", work in progress, draft-
              ietf-trill-rbridge-protocol-05.txt, July, 2007.
     
        [4]   Postel, J., "INTERNET PROTOCOL", RFC 791, September, 1981.
     
     
     
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        [5]   802.1D-2004 IEEE Standard for Local and Metropolitan Area
              Networks: Media Access Control (MAC) Bridges
     
        [6]   Callon, R., "Use of OSI IS-IS for Routing in TCP/IP and
              Dual Environments", RFC 1195, December, 1990.
     
        [7]   Plummer, D., "An Ethernet Address Resolution Protocol --
              or -- Converting Network Protocol Addresses to 48.bit
              Ethernet Address for Transmission on Ethernet Hardware",
              RFC 826, November, 1982.
     
        [8]   802.1Q-2005 IEEE Standard for Local and Metropolitan Area
              Networks: Virtual Bridged Local Area Networks
              (Incorporates IEEE Std 802.1Q-1998, IEEE Std 802.1uT-2001,
              IEEE Std 802.1vT-2001, and IEEE 802.1sT-2002)
     
     12. Author's Addresses
     
        Editor:
     
        Eric Gray
        Ericsson
        900 Chelmsford Street
        Lowell, MA, 01851
        Phone: +1 (978) 275-7470
        EMail: Eric.Gray@Ericsson.com
     
        Contributors:
     
        Joe Touch
        USC/ISI
        4676 Admiralty Way
        Marina del Rey, CA 90292-6695, U.S.A.
        Phone: +1 (310) 448-9151
        EMail: touch@isi.edu
        URL:   http://www.isi.edu/touch
     
        Radia Perlman
        Sun Microsystems
     
        EMail: Radia.Perlman@sun.com
     
     
     Intellectual Property Statement
     
        The IETF takes no position regarding the validity or scope of
        any Intellectual Property Rights or other rights that might be
     
     
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