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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: January, 2008
     Intended Status: Informational
                                                            July 9, 2007
     
     
     
                The Architecture of an RBridge Solution to TRILL
                      draft-ietf-trill-rbridge-arch-03.txt
     
     
     Status of this Memo
     
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        This Internet-Draft will expire on January 9, 2008.
     
     
     
     Abstract
     
        RBridges are link layer (L2) devices that use routing protocols
        as a control plane. They combine several of the benefits of the
        link layer with network layer routing benefits. RBridges use
        existing link state routing (without requiring configuration) to
        improve RBridge to RBridge aggregate throughput. RBridges also
     
     
     
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        provide support for IP multicast and IP address resolution
        optimizations. They are intended to be applicable to similar L2
        network sizes as 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) and otherwise attempt to retain as much
        'plug and play' as is already available in existing bridges.
        This document proposes an RBridge system as a solution to the
        TRILL problem. It also defines the RBridge architecture, defines
        its terminology, and describes basic components and desired
        behavior. One or more separate documents specify the protocols
        and mechanisms that satisfy the architecture presented herein.
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
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     Table of Contents
     
     
        1. Introduction................................................4
        2. Background..................................................6
           2.1. Existing Terminology...................................6
           2.2. RBridge Terminology...................................10
        3. Components.................................................12
           3.1. RBridge Device........................................12
           3.2. RBridge Data Model....................................13
              3.2.1. Unicast TRILL Forwarding Database................13
              3.2.2. Multi-destination TRILL Forwarding Database......13
              3.2.3. Ingress TRILL Forwarding Database................15
        4. Functional Description.....................................16
           4.1. TRILL Campus Auto-configuration.......................16
           4.2. RBridge Peer Discovery................................18
           4.3. Tunneling.............................................19
           4.4. RBridge General Operation.............................20
           4.5. Ingress/Egress Operations.............................21
           4.6. Transit Forwarding Operations.........................23
              4.6.1. Unicast..........................................23
              4.6.2. Broadcast, Multicast and Flooding................23
                 4.6.2-1. Broadcast...................................24
                 4.6.2-2. Multicast...................................25
                 4.6.2-3. Flooding....................................26
           4.7. Routing Protocol Operation............................28
           4.8. Other Bridging and Ethernet Protocol Operations.......28
              4.8.1. Wiring Closet Problem............................29
        5. How RBridges Address the TRILL Problem Space...............30
        6. Conclusions................................................30
        7. Security Considerations....................................31
        8. IANA Considerations........................................32
        9. Acknowledgments............................................32
        10. References................................................32
           10.1. Normative References.................................32
           10.2. Informative References...............................32
        Author's Addresses............................................33
        Intellectual Property Statement...............................34
        Disclaimer of Validity........................................34
        Copyright Statement...........................................34
        Acknowledgment................................................35
     
     
     
     
     
     
     
     
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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 within 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 link state routing.
        RBridges thus support increased RBridge to RBridge bandwidth and
        fault tolerance, when compared to conventional Ethernet bridges
        (which forward frames via a spanning tree), 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 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.
     
        To accomplish the goal of using existing technologies as much as
        possible, we intend to specify minimal extensions (if required)
        to one or more existing link-state routing protocols, as well as
        defining the specific sub-set of existing bridging technologies
        this architecture is intended to makes use of.
     
        The extent to which routing protocol extensions may be required
        depends on the closeness of the "fit" of any chosen routing
        protocol to RBridge protocol requirements. See [6] for further
        information on these requirements. The use of a specific routing
        protocol - along with appropriate extensions and enhancements -
        will be defined in corresponding RBridge protocol specifications
        (see [3] for example).
     
     
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        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) Advertise L2 reachability information.
     
          4) Establish L2 delivery using shortest path (verses STP).
     
        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:
     
          1) Peer information,
     
          2) Topology information,
     
          3) Forwarding information -
     
               a. unicast,
     
               b. flooded, and
     
               c. multicast.
     
     
     
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        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.  Topology information is expected to be acquired via
        the link-state routing protocol.
     
        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
        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.
     
     
     
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        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], [11].
     
        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 [10] 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.
     
        o  Bridge Learning: process by which a bridge determines on
           which 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 [11].
     
        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.
     
     
     
     
     
     
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        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.
     
        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: an Ethernet device with multiple ports which
           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. See [8] 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 [11].
     
        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.
     
     
     
     
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        o  Node: a device with an L2 (MAC) address that sources and/or
           sinks L2 frames.
     
        o  OSPF: Open Shortest Path First routing protocol. See [7] and
           [9] for further information on OSPF.
     
        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.
     
        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".
     
        o  VLAN: Virtual Local Area Network, see IEEE 802.1Q-2005 [11].
     
     
     
     
     
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        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 (DR): the RBridge that is elected to
           handle ingress and egress traffic to a particular Ethernet
           link having shared access among multiple RBridges; that
           RBridge is such a link's "Designated RBridge". The Designated
           RBridge is determined by an election process among those
           RBridges having shared access via a single LAN.
     
        o  Edge RBridge (edge of a TRILL Campus): describes RBridges
           that 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 Designated RBridge (ingress) 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.
     
     
     
     
     
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        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: a tree used by RBridges to deliver
           multi-destination frames. An RDT is computed using a specific
           RBridge as the root. May also be referred to as an R-tree..
     
        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.
     
     
     
     
     
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        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.
     
     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 contains three tables which distinguish
        it from conventional bridges.
     
        Conventional bridges contain a learned filtering (or forwarding)
        database, and a 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 directly connected RBridge peers (where "directly connected"
        in this context refers to RBridges connected to each other
        without transiting one or more additional RBridges) and VLANs.
        RBridges may have separate Unicast TRILL Forwarding Databases
        for each VLAN, if this is supported by configuration. The
        Unicast TRILL Forwarding Database is continually maintained by
        RBridge routing protocols and/or MAC learning. (see Section
        4.7).
     
        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 instantiated as a separate
        table, in 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 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
           the 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.
     
        A local RBridge could maintain a full set of entries from every
        RBridge to every other RBridge, however - depending on topology
     
     
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        - 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 should 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.
     
        How the Multi-destination TRILL Forwarding Database is
        maintained will be defined in appropriate protocol
        specifications used to instantiate this architecture. 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 to the egress
        RBridge, via the TRILL Campus. The Ingress TRILL Forwarding
        Database can be considered a version of the learned filtering
        (forwarding) database that treats the TRILL Campus, as a whole,
        as another port. It becomes configured in much the same way: by
        snooping incoming traffic, and assuming bi-directional
        consistency.  The information may be learned at the egress
        RBridge and propagated to all other RBridges in the TRILL Campus
        via the RBridge routing protocol, as an alternative to direct
        MAC learning from data frames. The Ingress TRILL Forwarding
        Database may be as large as the number of nodes on the Ethernet
        LAN, across all VLANs. RBridges may have separate Ingress TRILL
     
     
     
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        Forwarding Databases for each VLAN, if separate VLANs are
        supported by configuration.
     
        The Ingress TRILL Forwarding Database essentially determines the
        tunnel encapsulation used to transport each specific frame
        across the TRILL Campus.
     
     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. Consider first a set of bridges on a single
        Ethernet LAN (Figure 1). Here bridges are shown as 'b', hubs as
        'h', and nodes as 'N'; bridges and hubs are numbered. Note that
        the figure does not distinguish between types of nodes, i.e.,
        hosts and routers; both are end nodes at the link layer, and are
        otherwise indistinguishable to L2 forwarding devices. Bridges in
        this topology organize into a single spanning tree, as shown by
        double lines ('=', '||', and '//') in the figure.
     
                                  N       N---b3---N
                                  |           ||
                                  |           ||
                             N---h1--b4===b5==h2==b6
                                     |   //   |   ||
                                     |  //    N   ||
                                     | //         ||
                                 N---b7====b8-----b9-----N
                                           |      |\
                                           |      | \
                                           N      N  N
     
                  Figure 1 Conventionally bridged Ethernet LAN
     
        It is useful to note that hubs are relatively transparent to
        bridges, both for traffic from nodes to bridges (h1) and for
        traffic between bridges (h2). Also note that the same hub can
        support traffic between bridges and from a host to a bridge
        (h2), but that the spanning tree is exclusively between bridges.
        Bridges are thus compatible with hubs, both as transits and
        ingress/egress.
     
     
     
     
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        A TRILL Campus operates similarly, and can be viewed as a
        variant of the way bridges self-organize. Figure 2 shows the
        same topology where some of the bridges are replaced by RBridges
        (shown as 'r' in the figure). In this figure, stars ('*')
        represent the paths the RBridge is capable of utilizing, due to
        the use of link state routing. RBridges can tunnel directly to
        each other (r4-r5), or through hubs (h2) or bridges (b8).
     
        Note that the former b8-b9 path, which is b8-r9 in Figure 2 and
        had been disable by the hypothetical spanning tree in Figure 1,
        is now usable.
     
                                  N       N---b3---N
                                  |           ||
                                  |           ||
                             N---h1--r4***r5**h2**r6
                                     *   *    |   *
                                     *  *     N   *
                                     * *          *
                                 N---r7****b8*****r9-----N
                                           |      |\
                                           |      | \
                                           N      N  N
     
                         Figure 2 RBridged Ethernet LAN
     
        Every node in a TRILL Campus is considered to have a primary
        point of attachment to the TRILL Campus, as defined by the
        Designated RBridge. Each Ethernet link segment attached to a
        TRILL Campus has a single Designated RBridge; that RBridge is
        where all traffic that transits the TRILL Campus enters and
        exits. In Figure 2, it is easy to see that the nodes off of h1
        must attach at r4; the nodes off of b3, however, attach at
        either r5 or r6, depending on which is the Designated RBridge.
     
        Without loss of generality, an RBridge topology can be
        reorganized (ignoring link length) such that all nodes, hubs,
        and bridges are arranged around the periphery, and all RBridges
        are considered directly connected by their tunnels (Figure 3).
        Note that this view ignores the ways in which hubs and bridges
        may serve both on the ingress/egress and for transit, hence this
        view is not useful for traffic analysis. Using this view, it is
        easy to distinguish between RBridge to RBridge traffic and other
        traffic on shared devices, such as h2 and b8, because RBridge to
        RBridge traffic content is hidden from non RBridge devices by
        the RBridge encapsulation.
     
     
     
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                                  N       N---b3---N
                                  |           ||
                                  |           ||
                                  |           h2
                                  |          /| \
                                  |         / N  \
                                  |        /      \
                             N---h1--r4***r5******r6
                                     *   *        *
                                     *  *         *
                                     * *          *
                                 N---r7***********r9-----N
                                      \          /|\
                                       \        / | \
                                        \      /  N  N
                                         \    /
                                          \  /
                                           b8
                                           |
                                           N
     
                   Figure 3 Reorganized RBridge Ethernet LAN
     
     4.2. RBridge Peer Discovery
     
        Proper operation of the TRILL solution using RBridges depends on
        the existence of a mechanism for discovering peer RBridges and
        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.
     
        The discovery mechanisms must use protocol messages which will
        be propagated throughout a LAN (or broadcast domain) until they
        are consumed by another RBridge.  This must happen in order to
        ensure that RBridges in the same broadcast domain are discovered
        by their peers as required to allow for accurate determination
        of RBridge topology.
     
        These protocol messages should be distinguished in a manner that
        is consistent with the chosen RBridge routing protocol, or any
        other discovery mechanism used. It is very likely that peer
        discovery will actually be done as part of the RBridge routing
        protocol's peer discovery; however this is to be determined by
        specific RBridge protocol specification(s).
     
     
     
     
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        An RBridge intercepts protocol messages that it recognizes as
        being of this type (peer discovery), performs any processing
        required and forwards these messages as required by the
        discovery protocol. For example, a receiving RBridge may first
        determine if it has seen this message before and insert itself
        in a list of RBridges traversed by this message prior to
        forwarding the message on at least all interfaces other than the
        one on which it was received.
     
        Note that forwarding the modified message on all interfaces in
        the example above is safe, if somewhat wasteful.
     
        RBridges must forward all other protocol messages in a manner
        consistent with L2 addressing and forwarding - as would be done
        by a typical 802.1D bridge.
     
        Handling of 802.1D BPDUs is as determined in section 4.8.
     
     4.3. 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.
     
        Specifics of encapsulation are to be defined in appropriate
        protocol/encapsulation specifications.
     
        It is the combination of the encapsulation that distinguishes
        RBridge to RBridge traffic from other traffic. The link header
        includes source and destination addresses, which typically
        identify the ingress and egress RBridges. For incoming multicast
        and broadcast traffic, one of these addresses may represent the
        multicast group or broadcast address. Additionally, these
        addresses may be VLAN-specific, i.e., such that each ingress and
        egress address have per-VLAN addresses.
     
        The additional TRILL header is required to support loop
        mediation for traffic within the TRILL Campus; traffic loops in
        forwarding between RBridges and non-RBridge nodes, as well as
        across non-RBridge devices between RBridges, is limited by loop
        mediation and/or prevention mechanisms that are beyond the scope
        of this document (but may include a TTL-like mechanism,
        mechanisms to establish a loop free topology - such as
        STP/RSTP/MSTP - or both) on the applicable LAN links.
     
        The TRILL header and encapsulation:
     
     
     
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        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;
     
        o  should include the RBridge transit route, a hopcount, or a
           timestamp to prevent indefinite looping of a frame.
     
     4.4. RBridge General Operation
     
        Operations that apply to all RBridges include peer and topology
        discovery (which may include negotiation of RBridge
        identifiers), Designated RBridge election, link-state routing,
        SPF computation and advertising reach-ability for specific L2
        (MAC Ethernet destination) addresses within a broadcast domain.
     
        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 initial phase is the peer and topology discovery phase. This
        should continue for a sufficient amount of time to reduce the
        amount of re-negotiation (Designated RBridge and - possibly -
        identifiers) and re-computation that will be triggered by
        discovery of new peers. The timer values selected for delaying
        the next phase should take into account the time required for
        local STP and availability of segment connectivity between
        RBridge peers.
     
        The next phase is election of Designated RBridges for all shared
        access segments. This phase cannot complete before completion of
        peer and topology discovery. In parallel, RBridge routing
        protocol should begin the process of building the link-state
        information - assuming this was not done during the peer and
        topology discovery phase.
     
     
     
     
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        At about this time, RBridges should establish RBridge
        Distribution Trees.
     
        Once RBridges have established RBridge Distribution Trees, the
        learning and forwarding phase may begin. In this phase, RBridges
        initially forward frames by flooding via RBridge Distribution
        Tree(s). Also during this phase, RBridges begin "learning" MAC
        address locations from local segments and propagating L2 reach-
        ability information via the RBridge routing protocol to all
        other RBridges.  Gradually, the Unicast TRILL Forwarding
        Database will be built up for all RBridges, and fewer frames
        will require flooding via the RBridge Distribution Tree(s).
     
        ARP/ND optimization may occur during this phase as information
        learned from ARP/ND queries may be propagated across the TRILL
        Campus - potentially significantly reducing the impact of at
        least one source of broadcast traffic.
     
        The learning phase typically does not complete as new MAC
        attachment information continues to be learned and old
        information may be timed out and discarded. Consequently, the
        learning phase is also the operational phase. During the
        combined learning and operational phase, all RBridges maintain
        both RBridge Distribution Trees and a Unicast TRILL Forwarding
        Database. RBridges not elected as the Designated RBridge may be
        required to become one in the event that the DR goes off-line.
     
     4.5. Ingress/Egress Operations
     
        Operation specific to edge RBridges involves RBridge learning,
        advertisement, encapsulation (at ingress RBridges) and
        decapsulation (at egress RBridges).
     
        As described elsewhere, RBridge learning is similar to typical
        bridge learning - i.e. - all RBridges listen promiscuously to L2
        Frames on a local LAN segment and acquire location information
        associated with source MAC addresses in L2 frames they observe.
     
        By convention, a Designated RBridge election always occurs. In
        the degenerate case - where only one RBridge is connected to a
        specific Ethernet segment - obviously that RBridge will "win"
        the election and become the designated RBridge.
     
        With this convention, only the Designated RBridge performs
        RBridge learning for interface(s) connected to that segment.
     
     
     
     
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        As each RBridge learns 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 - for ARP/ND optimization - IP-to-MAC association
        information 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 advertises this learned information to its RBridge
        peers.
     
        These advertisements propagate to all edge RBridges (as
        potentially scoped by associated VLAN information for each
        advertisement). Each edge RBridge incorporates this information
        in the form of a Unicast TRILL Forwarding Database entry.
     
        RBridges also discover that they are an edge RBridge as a result
        of receiving un-encapsulated frames that require forwarding. If
        an RBridge is the Designated RBridge for a segment, and it has
        not previously learned that the MAC destination for a frame is
        local (this will be the case - for instance - for the very first
        frame it observes), then the RBridge would be required to
        forward (or flood) the frame via the TRILL Campus to all other
        RBridges (potentially within a VLAN scope).
     
        The RBridge in this case 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, then it would use that.  This
        RBridge would be an ingress RBridge with respect to the frame
        being forwarded.
     
        The encapsulation used by this ingress RBridge would be
        determined by the Unicast TRILL Forwarding Database - if one
        exists -  or the Unicast TRILL Forwarding Database-equivalent
        entry for the RBridge Distribution Tree. The encapsulation - as
        discussed elsewhere - should include (in the TRILL header)
        information to identify the egress RBridge (for example, the
        RBridge identifier negotiated previously during the peer and
        topology discovery phase).
     
     
     
     
     
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        When the encapsulated frame arrives at egress RBridge(s), it is
        decapsulated and forwarded via the egress interface(s) onto the
        local segment.
     
        Note that an egress RBridge will be the Designated RBridge on
        the local segment accessed via its egress interface(s). If the
        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 - or only -
        RBridge. If the received frame does correspond to a learned MAC
        destination address at an egress interface, the RBridge will
        forward the frame via that interface only.
     
     4.6. Transit Forwarding Operations
     
        There 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
           o  Broadcast frame forwarding
           o  Multicast frame forwarding
           o  Frame flooding
     
     4.6.1. 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.
     
     4.6.2. 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.
     
     
     
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        However, this approach results in potentially extreme
        inefficiencies in the multicast and unknown destination flooding
        cases.
     
        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.
     
     4.6.2-1. 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. Each RBridge assumes that it may be
        a transit as well as an ingress and egress RBridge and will
        establish forwarding information relative to itself and each of
        its peer RBridges, and stored in the Multi-destination TRILL
        Forwarding Database. Multi-destination TRILL Forwarding Database
        entries are computed at each RBridge for paths going toward all
        other RBridges - at least in cases where the RBridge performing
        Multi-destination TRILL Forwarding Database computations is on
        the shortest path.
     
        Forwarding information is in two forms: transit encapsulation
        information for interfaces over which the RBridge will forward a
        broadcast frame to one or more peer 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 a Designated RBridge.
        RBridges must not wait to determine that one (or more) non-
        RBridge Ethernet nodes is present in an interface before
        deciding to forward decapsulated broadcast frames on that
        interface.
     
        Forwarding information is selected for each broadcast frame
        received by any RBridge (based on identifying the ingress
        RBridge for the frame) for all corresponding Multi-destination
        TRILL Forwarding Database entries. Each RBridge is thus required
     
     
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        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. This includes decapsulated broadcast frames for each
        interface for which it is the designated RBridge.
     
        Note that frame replication and forwarding should be scoped by
        VLAN if VLAN support is provided. Also note that a Designated
        RBridge (DR) may be required to transmit a decapsulated frame on
        the interface on which it received the 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
        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.
     
     4.6.2-2. Multicast
     
        Multicast forwarding is reducible to broadcast forwarding in the
        simplest (default) case. However 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.
     
     
     
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        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.
     
        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.
     
     4.6.2-3. Flooding
     
        Flooding is similarly reducible to broadcast forwarding in the
        simplest (default) case - with the exception that a frame being
     
     
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        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
        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 the 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.
     
     
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        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.
     
     4.7. Routing Protocol Operation
     
        The details of routing protocol operation can be determined once
        a specific routing protocol has been selected.  These details
        would be defined in appropriate protocol specification(s).
     
        Protocol specifications should identify means for determining
        the content of the Unicast TRILL Forwarding Database and Multi-
        destination TRILL Forwarding Database.
     
     4.8. 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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     4.8.1. 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.
     
     5. 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.
     
     6. 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.
     
     7. 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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     8. 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, would be allocated by the
        IEEE.
     
     9. 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 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.
     
     10. References
     
     10.1.Normative References
     
        None.
     
     10.2.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., J. Touch, "RBridges: Base Protocol
              Specification", work in progress, draft-ietf-trill-
              rbridge-protocol-04.txt, January, 2006.
     
        [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]   Gray, E., (ed.) "TRILL Routing Requirements in Support of
              RBridges", work in progress, draft-ietf-trill-routing-
              reqs-02.txt, September, 2006
     
        [7]   Moy, J., "OSPF Version 2", RFC 2328, April, 1998.
     
        [8]   Callon, R., "Use of OSI IS-IS for Routing in TCP/IP and
              Dual Environments", RFC 1195, December, 1990.
     
        [9]   Coltun, R., D. Ferguson & J. Moy, "OSPF for IPv6", RFC
              2740, December, 1999.
     
        [10]  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.
     
        [11]  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)
     
     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
     
     
     
     
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        Radia Perlman
        Sun Microsystems
     
        EMail: Radia.Perlman@sun.com
     
     
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