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             TRILL WG                                  Guillermo Ibanyez
         Internet Draft                                  Alberto Garcia
         Expires: May 2006                               Arturo Azcorra
         November 15, 2005
            Abridges as Rbridges: Transparent Routing with Simplified
            Multiple Spanning Trees.
         Status of this Memo
           By submitting this Internet-Draft, each author represents
           that any applicable patent or other IPR claims of which he or
           she is aware have been or will be disclosed, and any of which
           he or she becomes aware will be disclosed, in accordance with
           Section 6 of BCP 79.
           Internet-Drafts are working documents of the Internet
           Engineering Task Force (IETF), its areas, and its working
           groups.  Note that other groups may also distribute working
           documents as Internet-Drafts.
           Internet-Drafts are draft documents valid for a maximum of
           six months and may be updated, replaced, or obsoleted by
           other documents at any time. It is inappropriate to use
           Internet-Drafts as reference material or to cite them other
           than as "work in progress."
           The list of current Internet-Drafts can be accessed at
           The list of Internet-Draft Shadow Directories can be accessed
           This Internet-Draft will expire on May 16, 2006.
           Rbridges are link layer devices that use routing protocols as
           a control plane but do not target to scale up to large campus
           networks. This document contains an alternative proposal to
           link-state Rbridges, named Abridges. Abridges overcome
           Rbridges L2 network size restrictions allowing applicability
           to very large Ethernet campus networks while maintaining zero
           configuration and high performance, by assuming a topological
           restriction that is automatically performed. The proposal
           includes a two-layered network architecture with two
           hierarchical independent spanning tree layers. Expected
           convergence is fast, probably below two seconds.
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           Abridges use multiple simplified spanning trees rooted at
           core edge bridges to achieve results comparable to Rbridges
           with lower computational complexity. Two implementation
           variants of simplified multiple spanning trees are proposed:
           The first one is a fundamental simplification of the standard
           Multiple Spanning Tree protocol and the second one (still in
           a very preliminary stage) consists of an N-multiple
           simultaneous execution of the Rapid Spanning Tree protocol at
           each Rbridge.
           An optional mechanism of ARP/Abridge servers/registrars (with
           load splitting) is proposed to limit ARP traffic in large
           scale Ethernet networks and to enhance scalability and
           security. This mechanism can also be used for host-Designated
           Rbridge resolution as an alternative to the interchange of
           Hosts Lists between Rbridges.
           The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
           NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
           "OPTIONAL" in this document are to be interpreted as
           described in RFC-2119 [1].
         Table of Contents
         3. Network Architecture......................................4
         4. Protocols.................................................5
         4.1 RSTP Protocol............................................5
         4.2 MSTP Protocol............................................6
         4.3 Core Layer: AMSTP Protocol ..............................6
         4.4 AMSTP versus MSTP........................................8
         4.5 Designated (and Root) Abridge............................9
         4.6 Forwarding Scenario.....................................10
         4.7 Learning End Node Location..............................12
         4.8 Routing versus Learning Bridges Addresses...............12
         4.9 Header on 802 Links.....................................12
         4.10 Distributed ARP Query..................................13
         4.11 Abridge Identities and Addresses.......................13
         5. ARP/Abridge Server/Registrars............................12
         6. Issues...................................................13
         6.1 Per Ingress Spanning Tree...............................14
         6.2 Symmetrical Path Problem................................14
         6.3 Traffic Aggregation at Root bridge......................14
         6.4 VLANs ..................................................14
         6.5 Optimizing ARP/ND.......................................14
         7. Security Considerations..................................15
         8. IANA Considerations......................................15
         Author's Addresses..........................................16
         Intellectual Property Statement.............................16
         Disclaimer of Validity......................................17
         Copyright Statement.........................................17
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         1. Introduction
           Current IP-based campus networks use one prefix address per
           link to support routing. This implies administration and
           configuration of IP addresses. IP addresses are link-related,
           so the IP address of an end node varies when the point of
           attachment to the network changes.
           Bridges do not require this kind of configuration because
           they forward in the switched domain using flat layer 2
           addresses. However standard bridge protocols do not scale,
           because the spanning tree protocol only enables some selected
           links to prevent loops, and network utilization is therefore
           low. Also the routes along the spanning tree are not pair-
           wise shortest paths, and temporary loops may produce packet
           proliferation across the entire switched domain.
           Rbridges have been proposed as a hybrid of routers and
           bridges, showing the advantages of routers while preserving
           at the same time the zero configuration capability of
           However Rbridges currently do not fulfil an important
           requirement such as scaling to large Ethernet campus
           networks. The importance of this requirement is growing with
           the increasing size of campus networks and the foreseeable
           increase in connected devices (displays, IP phones, cameras,
           802.11 PDAs, sensors, etc). This lack of scalability derives
           from the use of flat MAC addresses to perform routing. Being
           non aggregatable, MAC addresses will produce long tables in
           Rbridges when used in large campus networks. Another
           potential weakness of Rbridges is that, while exhibiting
           unrestricted topological compatibility with standard bridges,
           Rbridges depend on the bridged links to communicate among
           themselves and to perform the IS-IS Designated Router
           election. This dependency increases their complexity and
           makes the whole system vulnerable to inter Rbridge
           communication problems. The overall convergence time is
           increased because the spanning tree convergence time adds up
           to the IS-IS DR election time.
           This draft proposes an architecture for Ethernet campus
           networks based on a new type of Ethernet hierarchical
           switches for campus cores. The architecture is oriented to
           provide high performance, minimal configuration, and
           scalability in very large Ethernet campus networks. The
           proposed network architecture consists of a high capacity
           core composed of an arbitrary mesh of switches named
           Abridges, and a number of access networks with standard
           bridges connected to the core.
           The document proposes an alternative implementation for
           Rbridges [10] (Routing Bridges), identified as AMSTP Bridges
           (Abridges) that combine the advantages of bridges and
           routers. Like bridges and Rbridges, Abridges require zero
           configuration and are transparent to IP nodes. Abridges also
           forward on pair-wise shortest paths like routers as Rbridges
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           We propose to use multiple L2 spanning trees between Abridges
           to forward via shortest paths in the core of the campus
           network. The AMSTP protocol is a simplification of the
           standard MSTP protocol, oriented to zero configuration. The
           core edge bridges provide backbone connectivity to lower
           layer (Access Layer) networks. The active topology of the
           Access Layer networks consists of standard spanning trees of
           switches (RSTP/STP). Each Edge Switch acts as the root bridge
           of two independent spanning trees: the spanning tree of its
           lower layer Access network, and one spanning tree instance of
           the core network. The architecture provides shortest paths in
           most traffic situations for client-server traffic (for
           servers located in a server farm) and adapts well to traffic
           aggregation. Additional mechanisms can be designed to achieve
           high network availability.
           Due to the access port mode, Abridges are compatible with
           current bridges as well as current IPv4 and IPv6 routers and
           end nodes. They are as transparent to current IP routers as
           bridges and Rbridges are. Like routers, they terminate a
           bridged spanning tree.
           Packets in the Core of Abridges must be encapsulated such
           - Forwarding is performed in the Core across per egress
           bridge tree instances, while maintaining the original L2
           header so that end destination bridges can learn about the
           location of the source by learning the source address from
           - Abridges can learn the location of end nodes. They can
           learn the location and layer 2 addresses of attached nodes
           from the source address of data packets, as bridges and
           Rbridges. However, very large campus networks with tens of
           thousands of nodes may require more scalable and safer
           solutions for locating end nodes. For this case, the use of
           ARP/Abriges Server/Registrars is proposed.
           Support of VLANs traditionally requires configuration of the
           bridges to know which ports and links belong to which VLANs.
           In order to achieve true zero configuration, we recommend
           that bridges do not separate per VLAN traffic in the campus
           core, and do not use a separate spanning tree for each
           broadcast domain. In a campus without VLANs, this means a
           single spanning tree would be used for delivery of packets
           with unknown or layer 2 group address layer 2 destinations.
           Abridges can suppress the broadcast/multicast for Neighbour
           discovery by using ARP servers/registrars or, similarly to
           Rbridges, by conventional proxy ARP (IPv4) or proxy ND
           Abridges are fully compatible with current IPv4 and IPv6
           routers and end nodes. They are as invisible to current IP
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           routers as bridges are, and they participate in two bridged
           hierarchically linked but separated spanning trees.
         2. Terminology
           AMSTP: Alternative Multiple Spanning Tree Protocol
           Abridge: An Rbridge implemented as an AMSTP Bridge
           Access network: subnetwork of standard bridges connected to
           an Abridge.
           ARP/Abridge Server/Registrar: Server that provides ARP
           resolution and the ID of a destination hosts Designated
           Abridge (see DR).
           Campus network: set of network elements (standard bridges and
           Abridges) connected to one or more routers.
           Core: set of Abridges directly interconnected through point
           to point links.
           Core port: The port of an Abridge connected to another
           Abridge through a point to point link.
           Access port: The port of an Abridge connected to a link that
           has active standard bridges connected. It executes the
           standard spanning tree protocol and provides connection to
           the Access Network.
           DR: Designated Rbridge. In the context of an Abridge, it
           means the Designated Abridge that coincides with the STP/RSTP
           Root bridge of the Access network.
           MSTP:  Multiple Spanning Tree Algorithm and Protocol.
           NRSTP: Variant implementation of AMSTP through execution of N
           independent RSTP instances.
           Rbridge: Routing Bridge as defined by Radia Perlman and TRILL
           WGs proposal.
           RSTP:  Rapid Spanning Tree Algorithm and Protocol
         3. Network Architecture
           Campus network designs are currently based on a layered
           architecture (core, distribution and access layers) to obtain
           network scalability and predictability. Segmentation of
           networks is obtained using routers or devices called
           multilayer switches that segment the network in IP segments
           or subnets.
           A similar approach is proposed here, but with the network
           segmentation performed at layer 2 instead of layer 3. The
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           proposed network architecture is shown in Figure 1. It uses a
           two-layer hierarchical L2 network to achieve scalability to
           large scale Ethernet networks. The upper layer acts as a
           Core-Distribution Layer (Core) and the lower Layer acts as an
           Access Layer. The core layer uses the AMSTP protocol for
           interconnection between core Abridges while the Access Layer
           uses the standard spanning tree protocol (RSTP or STP) to
           connect hosts of the access network with other hosts via
           their root bridge at the core (Abridge).
           The Abridges constitute the core network and are
           interconnected by dedicated links. The point to point link
           requirement derives from the need for fast convergence of
           standard layer 2 spanning tree algorithms, but it is also
           required for high performance and enhanced security (802.1X).
           Thus, point to point links are becoming a requirement for
           Ethernet networks, at least at the core and distribution
           Other bridges connect to Abridges without requiring a point
           to point connection, and form the Access Layer. The Access
           Layer is segmented in multiple access networks. Each Access
           network is formed by devices connected to a core Abridge; it
           may have arbitrary topologies but the active topology will
           use the standard spanning tree as the basic forwarding
           mechanism. More sophisticated protocols are possible for
           better infrastructure usage inside each Access network, but
           they are out of the scope of this proposal.
                                              | network|
                                            / ---------
                                    A -----A
                                   /  \   / \          Core layer
                                  /    \ /   \
                                / \      \
                      -------- /   \      \-----------
                     |network|      \     |  network |  Access Layer
                      --------       \     -----------
                                      \ ---------
                                      | network |
                            A: Abridge
                        Figure 1. Campus network topology
           Abridges must auto-configure ports to participate in the Core
           or in the Access network. The port reconfiguring mechanism is
           as follows: a port that is not connected using a point to
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           point link to another Abridge configures itself as an access
           port (an ingress and egress point for traffic to/from the
           core). Ports directly connected to another Abridge act as
           core ports. The auto-configuration of ports works as follows:
           each port detects, through the STP BPDU type (STP, RSTP or
           AMSTP) received on their link upon initialization, whether
           the device connected to the link is a standard bridge or an
           Abridge. If the BPDUs received are standard 802.1D BPDUs, the
           link will be assigned to the Access Network and the port will
           be automatically configured to access port mode. Any standard
           bridge connected to the Abridge is thus automatically
           excluded from the core function.
           Figure 1 shows an example of the proposed network topology. A
           core of Abridges constitutes the campus backbone and
           interconnects different area networks formed by standard
           802.1D bridges.
         4. Protocols
           In this section the proposed protocols are described. The
           Alternative Multiple Spanning Tree Protocol [7] is an
           evolution of the standard MSTP [6] and RSTP [3] protocols. In
           the following paragraphs RSTP and MSTP protocols are first
           summarily introduced to provide the required context to
           describe the AMSTP protocol. Differences between AMSTP and
           MSTP are summarized after a description of AMSTP.
         4.1 RSTP Protocol
           A standard protocol for bridges is the Rapid Spanning Tree
           Protocol, included in IEEE 802.1D[5]. It provides much faster
           convergence than the previous standard protocol STP [4]. To
           achieve convergence in (typically) fractions of one second,
           RSTP substitutes the timer based mechanism that STP uses, to
           ensure that the algorithm has converged with a locally
           controlled proposal-agreement mechanism between adjacent
           switches to transition the port states to forwarding in a
           controlled way. This mechanism requires point to point links
           to operate without loops. Other mechanisms are also used to
           ensure rapid convergence.
         4.2 MSTP protocol
           The Multiple Spanning Tree Protocol (IEEE 802.1Q) is based on
           RSTP (IEEE 802.1D) and creates different tree instances that
           are used by sets of VLANs according to the configuration of
           the bridge. MSTP implements a set of multiple and independent
           spanning tree instances (MSTI) in a network region. Each
           region is interconnected via a common spanning tree (CST) to
           other MST regions. Inside a region, several VLANs can be
           mapped to a single tree instance. Multiple tree instances at
           each region make it possible to improve the usage of the
           links. At each region, there is a tree instance (IST),
           identified with the number 0, that acts as the basic spanning
           tree. The CIST or total spanning tree is comprised of the CST
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           that connects all the regions, and the IST that provides
           connectivity inside each region. It allows separated
           management of the regions, appearing to the outside as a
           unique and separate "superbridge", i.e. the whole region
           connects to the CST via one Regional Root Bridge port and a
           number of designated ports like a single bridge. Therefore,
           no change in internal topology inside is influenced by
           outside tree topology changes. MSTP allows more efficient
           network infrastructure usage by assigning different spanning
           trees to different sets of VLANs.
           But MSTP is complex to configure. Tree instances must be
           planned and VLANs must be mapped to those tree instances. The
           configuration table must be checked to be exactly the same
           for all bridges of the same region. Serious malfunction
           occurs if VLAN mapping discrepancies between bridges in the
           same region exist.
         4.3 Core layer: AMSTP Protocol
           In the architecture proposed, the AMSTP Protocol works as a
           Core Layer protocol providing shortest path interconnection
           between Access Networks and providing network segmentation to
           prevent the extension of failures to the whole switched
           domain. The AMSTP Protocol has been proposed previously
           [AMSTP] for metropolitan Ethernet backbones but it can be
           extended for campus networks as well, with some
           modifications. AMSTP is a simplified multiple spanning tree
           protocol that uses one tree instance rooted at each edge
           bridge in the core to forward frames. A complete multi-tree
           is the set of all tree instances, one rooted at every edge
           bridge that interconnects all bridges in the backbone. Only
           the Abridge ports connected to other Abridges participate in
           the multiple spanning tree protocol. The rest of the ports
           participate in the standard spanning tree protocols such as
           RSTP or STP (IEEE 802.1D).
           To describe the AMSTP protocol, we consider its two main
           functionalities: building and maintaining the spanning trees
           (control plane), and processing and forwarding frames in the
           bridges (user plane).
         4.3.1   Building the Trees
            The process of tree building consists of two parts: building
           the basic (standard) RSTP tree and building the rest of the
           instances, called Alternate Multiple Spanning Tree Instances
           (AMSTI), till one tree instance per bridge is built as shown
           in figure 2. The process of building the main tree is the
           same as in RSTP.
           Every bridge emits autonomously Bridge Protocol Data Units
           (BPDU) every Hello Time (configurable from milliseconds) to
           neighbouring bridges. First the Bridge having the lowest
           Bridge ID (best configured priority plus lower MAC address
           appended) is elected as Root Bridge of the main spanning
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           tree. Every bridge receiving BPDU from this Bridge will adopt
           it as Root and propagate it in the BPDUs emitted.
           These BPDUs contain the minimum path cost from the emitting
           bridge to the elected Root Bridge. Every Bridge attaches to
           the spanning tree by selecting the port that is receiving the
           "best" BPDU as the root port. The best BPDU is the one that
           announces minimum path cost to root bridge. Each bridge
           builds its own BPDU with the result of received BPDUs from
           other bridges, selecting "superior" BPDUs according to the
           standard STP criteria (lower Bridge ID, lower path cost,
           lower port priority, lower port ID) and transmits them via
           the main tree for the continuous maintenance of the optimum
           main spanning tree.
                               A -----A
                              /  \   / \
                             /    \ /   \
            A ----A     A     A    A    A       R-----A       A---R
           /             \   /      \    \     / \     \         / \
          /               \ /        \    \   /   \     \       /   \
         R-----A----A A----R----A A---A----R A     A     A  A---A    A
         Fig.2. A five node network and its five self-rooted AMSTP
         Spanning Tree Instances (R: root bridge).
         The process of building all the other tree instances, one per
         tree, takes place as follows: Each Core Bridge appends to the
         main tree BPDU the information of all AMSTI tree instances
         which the bridges participates in. The information appended
         per tree instance is called the AM-Record and contains similar
         information for BPDU tree instance building. The key
         difference with other spanning tree protocols is that there is
         no bridge election. In AMSTP the Abridge claims itself as Tree
         Root Bridge of its own instance and accepts equally every
         other Abridge as the Root of its own instance. The bridge is
         accepted as the root by other bridges without negotiation
         (except when a malfunction is detected). This self rooted tree
         instance is identified by the bridge ID of the edge Abridge
         (root). The rest of the process is analogous to the building
         of the MSTI tree instances used by MSTP inside an MST region
         [4]: the tree is built by selecting tree paths at every bridge
         according to the same minimum path cost criteria as MSTP,
         using port priority and port ID for tie breaking. A flag
         octet, identical to the one for building the basic tree
         instance, is used by the bridges to communicate and negotiate
         transitions of port states and roles per tree instance.
         4.3.2   Frame processing in Core Switches
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         When processing a frame, a Core Switch (Abridge) may act as an
         ingress, transit or egress Abridge.
         As ingress Abridge, the switch encapsulates the frame with an
         additional Layer 2 header containing its MAC as source
         address, and as destination the MAC address of the egress
         Abridge. The ingress Abridge forwards the encapsulated frame
         through the branch belonging to the spanning tree instance
         rooted at the egress Abridge. This path is a pair-wise
         shortest path because the tree is built by minimizing path
         cost from each root to the rest of the nodes.
         Traffic forwarding in the core depends on the traffic type:
         broadcast, multicast and traffic to unknown destinations is
         forwarded via the tree instance rooted at the ingress Abridge.
         Unicast traffic (to a known Abridge) is forwarded through the
         tree instance of the egress Abridge. Forwarding takes place by
         sending the frame through the bridge root port. Broadcast and
         multicast traffic are forwarded via the tree instance rooted
         at the ingress Abridge.
         Abridges may learn from the received frames both the MAC
         addresses of other Abridges and the MACs of the connected end
         nodes by the inspection of the inner and outer Ethernet MAC
         addresses of the encapsulated frames. This learning process is
         called double MAC learning and is applicable only in networks
         with a moderate number of end nodes, like a backbone with
         routers connected to it [ ].
         The MAC learning process is based on frames broadcasted over
         the switched network. These broadcasts are commonly ARP
         packets issued by end nodes for layer 2 destination address
         resolution. In this process the bridges learn the originating
         MAC at receiving ports and the hosts add the IP-MAC pair to
         their ARP table. In networks with a high number of end nodes,
         processing a high number of ARP requests by every endnode may
         result in significant load for endnodes. A different mechanism
         is needed to prevent ARP packets from
         broadcasting/multicasting in large Ethernet campus networks.
         The ports of switches that are not connected to AMSTP capable
         Core Switches do not run AMSTP, so they are kept out of the
         core forwarding mechanism. For Core Switches running AMSTP to
         interoperate with legacy switches running STP or RSTP, a
         mechanism is needed, like the standard port migration protocol
         used by MSTP, RSTP and STP. Basically the mechanism is that if
         a port of an MSTP switch receives BPDUs of protocol version 0
         (STP protocol) it will emit STP BPDUs only. Recovery is not
         automatic; the port will not emit MSTP BPDUs until a
         configuration command restarts the protocol migration process,
         forcing renegotiation between neighbouring switches.
         4.3.3   AMSTP BPDU layout
         AMSTP BPDUs have a structure that resembles MSTP BPDUs [4]
         since both are comprised essentially of a basic BPDU and
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         several AM-Records appended. The AMSTP BPDU structure is shown
         in figure 3. The basic BPDU is used for basic tree (0)
         negotiation between switches. Each of the appended AM-Records
         is used to negotiate a specific tree instance (AMSTI).
         As in the MSTP case the BPDUs carrying the rapid spanning tree
         information distributed via instance 0 also carry the
         information of all the spanning tree instances appended to the
         RSTP BPDU as AM records. This reduces broadcasting and
         simplifies BPDU processing at the switches.
         !   Basic RSTP BPDU      !
         !   Tree instance 0      !
         --------------------------   -------------------------
         !   [AMSTP header]       !  /!    AMSTI flags        !
         !                        ! / -------------------------
         --------------------------/  !  Root bridge ID (edge)!
         !   Tree Instance 1      !   -------------------------
         !   Root 1               !   !    Root path cost     !
         !                        !   -------------------------
         --------------------------   !   Dest. Port Address  !
         !   Tree Instance 2      !\  !   of Root bridge      !
         !   Root 2               !|  -------------------------
         !                        ! \ !    Port priority      !
         -------------------------- | -------------------------
         ...........                 \!    Remaining hops     !
         --------------------------   -------------------------
         !   Tree Instance 1      !
         !   Root N               !
         Fig. 3.  AMSTP BPDU layout
         Every AM-record includes an octet flag identical to the one
         described for the RSTP tree. These flags are used to negotiate
         all transitions of each tree instance between connected ports
         of neighbouring switches.
         Minimum configuration is an important requirement for Core
         Switches. While multiple spanning tree algorithms enable much
         better usage of the existing infrastructure, they are usually
         complex to configure because a way to assign frames to tree
         instances is needed. In the case of MSTP, this means that the
         mapping of VLANs to tree instances (MSTIs) has to be
         configured manually at each bridge, resulting in a complex and
         error-prone process.
          AMSTP uses Self rooted Spanning Tree instances instead of
         VLAN mapped trees and all tree instances are automatically
         created, so no tree configuration is needed. The parameters to
         configure are those common to RSTP, such as selection of the
         Root Bridge and configuration of the Backup Bridges for the
         region and their priorities.
         Multicast (L2 addresses) traffic. Multicast traffic in the
         campus core is forwarded via same tree instances as unicast
         traffic, via pair-wise shortest paths to destination Abridges.
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         The difference with unicast traffic is that the spanning tree
         used is rooted at the ingress Abridge, instead of the tree
         rooted at the destination Abridge. The multicast trees are
         therefore always optimized for minimum hops without the
         construction of additional tree instances. As for Rbridges,
         Abridges may treat multicast traffic as broadcast or may use
         current techniques like IGMP snooping to limit broadcast.
         4.4 AMSTP versus MSTP
          Table I below shows a comparison of the main protocol
         differences between MSTP and AMSTP. The first difference is
         the criteria used for assignment of frames to a tree instance
         for processing, in other words, how the bridge knows which
         spanning tree instance to use to forward the frame. The second
         one is the criteria used to create a tree instance.
                                  TABLE I
         Protocol feature                MSTP                AMSTP
         Criteria for
         frame assignment                              Destination MAC
                                                         of frame(root)
         to a tree instance    VLAN tag on frame (802.1Q)
         Tree instance            Configured :           Automatic: One
         formation               Sets of VLANs are      per core bridge
         criteria                mapped to every tree
         Number of tree instances Configured :1 to 64  One per core
                                                         bridge  (*)
         Root bridge      As RSTP (lower bridge        No election.
         election.       ID including bridge priority) Every bridge is
                                                       the root of its
                                                       tree instance
         Bridge ID       4 MSB byte priority, 12 bit VLAN ID 6 byte MAC
         Single or                          Multiple             Single
         Multiple MST regions
         Main application
         Environment          Interconnected VLAN based regions
         Cores, backbones
         (*) An Abridge with no access ports (transit Abridge instead
         of edge Abridge) does not create a self rooted instance.
         4.5 Designated (and Root) Abridge
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         Similarly to Rbridges, an Abridge of each link has special
         duties. This Abridge acts as the Designated Rbridge of that
         link. The DR function combines very well with being the root
         bridge of the spanning tree of that link. To achieve automatic
         election of Abridges as roots of the respective access
         networks of the campus it would suffice that the default
         bridge ID of Abridges have a lower value than that of standard
         bridges (midrange). An Abridge may in this way become the root
         bridge of any link. DR election and root bridge election are
         one and the same operation, performed according to the
         standard procedure [5]. In this way DR election does not
         depend on any external mechanism and convergence time at links
         does not add up to the convergence time of DR election at IS-
         IS as in the Rbridge case. The complete DR election process is
         4.6 Forwarding scenario
         Now the basic forwarding scenario is described. Figure 4 shows
         two hosts H1 and H2 connected at different access networks.
         First the ARP and destination Abridge resolution are
         described, and then the forwarding process.
         4.6.1 ARP and Abridge Resolution
         Using ARP servers is the optional mechanism proposed to limit
         broadcast/multicast traffic. However, the standard ARP
         mechanism must be kept to ensure that hosts that silently move
         from one part of the campus to another can be located.
         Besides ARP for host resolution, the servers may also be used
         for resolution of the destination Abridge. Each server stores
         a table with tuples containing the IP, L2 address of the end
         node and L2 address of the Designated Bridge (Root Abridge).
         The set of stored tuples corresponds to IP addresses that
         produce identical (few bits) hash results of IP destination
         end node.
         The sequence for communication between H1 and H2 at figure 4
         is as follows:
         Host H1 first sends a broadcast ARP packet to get the
         resolution of host H2s L2 address. The packet is distributed
         through the spanning tree of the access network and arrives at
         the root Abridge. The root Abridge detects the ARP, calculates
         hash(IP destination address) and with the result obtains the
         server responsible for that IP address. The server performs a
         look up using H2s IP destination address and obtains the H2 L2
         address and the (egress) Abridge ID of that access network,
         then sends the reply in a packet to the ingress Abridge. The
         Abridge extracts the information and forwards a standard ARP
         response packet to host H1. Host H1 can then proceed to send
         packets with the L2 address of host H2. The ingress Abridge
         also registers the originating host by sending a registration
         packet containing the ARP packet to the corresponding
         ARP/Abridge server, obtained by computing hash(IP origin).
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                  b: standard bridge                 /  Access Layer
                  A: Abridge                        b---b
            Path: H1-b-b-b-A-A-A-b-b-b-H2          / .............
                                            A     A
                                            \\    \\    Core layer
                                             \\    \\
                                       /       \     \  ..........
                                      /         \     \  Access Layer
                       H1---- b---b--b           b     b---b---b----H2
                            /       /           / \     \
                         b-/   b---b        b- b   b-b   b---b---b
         Figure 4. End to End forwarding scenario
         Note: If the destination host is connected to the same access
         network, the host will reply directly by emitting an ARP
         response packet.
         Note: The Abridge registers a host at the corresponding ARP
         Server/Registrar whenever it detects a frame from an unknown
         host connected at its access network.
         4.6.2 Forwarding
         The frame forwarding process is as follows: the standard frame
         sent by host (IP(H2), L2(H2)) arrives to the Access network
         root bridge (Abridge). Its DA Ethernet Address contains the
         end node destination address. The root Abridge (Designated)
         looks at its cache for the ID of the destination end nodes
         designated Abridge (that was filled just before with the
         ARP/Abridge server response). The Abridge still has in its
         cache the pair (L2 address, L2 egress Abridge) obtained before
         and encapsulates the frame with a header like this: (DA egress
         Abridge, SA ingress Abridge, Ethertype: AMSTP). It then
         determines the applicable tree instance by looking at the
         destination Abridge and forwards it through the port that was
         elected root for the Abridge destination instance. The packet
         arrives at the Designated Port of the next Abridge, which then
         inspects it and forwards it to the outer destination MAC
         address using the corresponding tree instance to obtain the
         root port of that instance. The packet is forwarded again via
         the root port till the egress Abridge is reached. The egress
         Abridge detects that it is the destination of the frame,
         removes the encapsulation header of the frame and forwards the
         original frame via the access port where the L2 host has been
         learnt or via all access ports if H2 is unknown. The packet
         goes from the egress bridge (root) to H2 following a branch of
         the tree rooted at the egress bridge. Frame forwarding in the
         access networks is performed in the standard way with the
         spanning tree set up by STP or RSTP. A packet exiting the
         Abridge by an access port must look to ordinary bridges like
         an ordinary layer 2 packet and must not be encapsulated.
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         The Abridge may learn the destination Abridge by host list
         interchange. The forwarding behaviour of Rbridges is as
         follows: "When a DR R1 receives a native packet with layer 2
         address S and layer 2 destination address D, R1 looks up the
         location of D. If D is claimed by egress Rbridge R2, then R1
         encapsulates the packet, directing it towards R2". Abridges
         may use the same behaviour, but in this case network size
         might not scale to one hundred thousand end nodes--the Campus
         Transit Tables (CTT) would be too big.
         In contrast to an Rbridge, when an Abridge receives an
         encapsulated packet, it forwards it based on the DA Abridge
         and does not change the DA for the "next-hop" address. The
         next hop is selected by forwarding the frame via the root port
         of tree instance rooted at the destination Abridge. A packet
         in the core must look like an Ethernet frame, but must be
         differentiable from a native layer 2 packet by Abridges. To
         accomplish this, a new layer 2 protocol type ("Ethertype") is
         4.7 Learning End node Location
         Abridges learn end node location in access ports as standard
         bridges do. Abridges learn root bridge IDs of the multiple
         instances of core from AMSTP BPDUs received.
         Similarly to Rbridges, the Core (Edge) Abridge, acting as root
         and Designated Rbridge, might work in two modes:
            - As a standard Designated Rbridge, that learns the L2
              addresses of attached end nodes, initiates a distributed
              ARP when an ARP query is received for an unknown
              destination, and answers ARP queries when the target node
              is known. This mechanism is an alternative to the use of
              ARP Servers/Registrars
            - From data packets. They learn (layer 3, layer 2) pairs
              (for the purpose of supporting proxy ARP/ND) from
              listening to ARP or ND replies.
         4.8 Routing in Abridges vs Learning Bridges Addresses
         Some recent proposals like Shortest Path Bridging (SPB), as
         proposed at the IEEE [12][13], use also multiple tree
         instances rooted at edge bridges. However it presents the
         problem of asymmetrical spanning trees. This happens when the
         tree rooted at bridge A differs in chosen path A-B from the
         path chosen by the tree rooted at B to A. The problem occurs
         when there are ties in the path costs of tree instances. In
         the instance with node A as root the tie may be solved by
         choosing one path. In the instance with node B as root the tie
         may be solved choosing a different path. But the spanning
         trees must be symmetrical for the address learning to work
         correctly: the address learnt at one port of B sent by A (via
         spanning tree A to B), if forwarded via same port through the
         opposite direction spanning tree (B to A) might find the path
         blocked due to a different root port election at A for the
         tree instance rooted at B.
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         Abridges work differently because they do not learn addresses.
         Abridges only build spanning trees and assign traffic to them
         according to the destination Abridge. AMSTP uses always the
         root port to send frames to the destination bridge (instance
         rooted at destination), so the routing function for Abridge is
         as follows:
            - The bridge ID of the destination corresponding to the
              destination end node is obtained from the Abridge Server.
            - The bridge ID of the destination is translated to the
              port MAC destination address of the destination Abridge
              at the internal Abridge table.
            - The frame is encapsulated with an external L2 header with
              Destination Bridge ID.
            - Abridges only forward a frame received at a designated
              port, upstream, via the root port. The L2 external
              destination address can be the Destination Bridge ID
              itself. When the encapsulated frame arrives at the
              destination bridge, it must identify its Bridge ID in the
              DA and remove the L2 encapsulation of the frame and
              forward it downstream to the access network via access
         4.9 Header on 802 Links
           Abridges, as Rbridges, must coexist with ordinary bridges.
           The encapsulated L2 format must be compatible with the
           Ethernet format. No additional fields like TTL are required
           if the fast convergence mechanism procedure of RSTP is used.
           An encapsulated packet would look as follows:
                             | outer header |original packet |CRC|
                                Figure 5 Encapsulated packet
            The outer header contains:
            o L2 destination = destination (egress) Abridge
            o L2 source = origin (ingress) Abridge
            o protocol type = "to be assigned...Abridge encapsulated
         packet" (AMSTP)
         4.10 Distributed ARP Query
           Abridges may perform distributed ARP Query as Rbridges do,
           but for large campus networks, it is recommended the use of
           ARP/Abridge servers/ registrars to reduce multicast traffic
           and processing load at end nodes.
         4.11 Abridge identities and addresses.
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           Each ABridge needs a unique ID within the campus.  The
           simplest such address is a unique 6-byte ID, since such an ID
           is easily obtainable as any of the EUI-48's owned by that
           A new Ethertype must be assigned to indicate an Abridge-
           encapsulated packet.
           A layer 2 multicast address is used as the "all Abridges"
           destination address in distributed ARP queries and any other
           intercommunication message.
           An optional layer 2 multicast address is needed to address to
           "all ARB/Abridge" servers" (if used), to communicate among
           them the available servers and the hash value(s) supported.
           The AMSTP protocol distributes BPDUs addressed to the local
           multicast protocol addresses used by the spanning tree
           protocol (Bridge Group Address 01-80-C2-00-00-00). These
           addresses are neither forwarded by bridges nor by Rbridges or
         5. ARP/Abridge Servers/Registrars
           Abridges, as Rbridges, may suppress the broadcast/multicast
           for neighbour discovery by doing proxy ARP (IPv4) or proxy ND
           (IPv6). However the mechanism proposed for large campus
           networks to suppress broadcast/multicast for neighbour
           discovery consists of ARP servers/registrars, where end nodes
           are registered upon frame detection by the Designated
           Although all ARP/Abridge servers might work in parallel, it
           seems more efficient to perform statistical uniform load
           distribution between servers, distributing the IP addresses
           to resolve among the available servers by a hashing based
           mechanism. The process is as follows: When a host issues an
           ARP packet, the packet is forwarded up across the spanning
           tree of the access network up to the root bridge (Abridge).
           The Abridge, acting as Designated Abridge, performs hashing
           of the destination IP. With this hash result the Abridge
           obtains the ARP/Abridge server ID in charge of that IP
           address. This server ID was previously obtained from
           announcement packets from ARP servers containing its IP
           address, L2 address, server ID and hash values that it
           The Abridge encapsulates the ARP packet originated by endnode
           with an additional L2 header with the destination address of
           the corresponding server for ARP resolution.
           The Abridge also prepares a registering packet with the IP
           origin in order to register (or refresh) the host originating
           the ARP into the corresponding ARP/Abridge server.
           To avoid redundant load on ARP/Abridge servers, they must
           share the load by assigning server IDs according to the
           result of hash (IP destination). The total number of servers
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           may be dimensioned according to the length of the hash
           results used or by additional grouping. An additional
           protocol between ARP/Abridge servers can be designed to
           handle dynamic load splitting among the available
           servers/registrars as they come into and out of service. A
           server coming into service takes charge of a hash value
           handed out by a running server. The new server performs the
           new registrations, and forwards unsolved requests to the
           previous server.  After the expiration time of the first
           registration performed at new server is reached, the handover
           process is complete as no valid registries remain in previous
         6. Issues
           In this section the identified issues, either for Rbridges,
           Abridges or both, are described or commented.
         6.1 Per Ingress Spanning Tree.
           Per Ingress multicast spanning Tree is implemented by default
           with Abridges. Multicast paths always traverse minimum hops.
           There is no issue here.
         6.2 Symmetrical Paths Problem.
           Shortest Path Bridging [SPB], the current proposal at IEEE
           for pair-wise shortest path, depends on symmetrical tree
           instances between bridges pairs for the L2 addresses learning
           to work properly. In case of a path cost tie during tree
           instances calculation, different paths might be elected in
           opposite directions. The proposal at [13] describes a change
           in MSTP Protocol to prevent this, but convergence times
           Abridges are not subject to this problem because they forward
           unicast traffic through one branch of the destination Abridge
           tree instance. Packets are forwarded in Abridges via its port
           elected as the root of the destination Abridge tree instance.
           Unicast forwarding in the core campus always follows the path
           from Designated Port to root port at each Abridge traversed
           till reaching the destination. No address learning is used
           for filtering as the packet is always forwarded via one port
           (root port of Abridge).
          6.3 Traffic Aggregation at Root.
           A usual argument against spanning trees is that the traffic
           accumulates near the root bridge, provoking congestion. The
           real situation in campus networks is that traffic,
           predominantly client-server, distributes in a tree form.
           However, bridge design and Ethernet technologies with their
           various speeds (100 Mbps, 1 Gbps, 10 Gbps) currently make
           efficient switch designs possible (like N*100 Mbps with two 1
           Gbps uplinks) that aggregate traffic efficiently.
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          6.4 VLANs
           VLAN usage in campus core requires detailed configuration of
           which Abridge port belongs to which VLAN.
           Abridges may learn, as VLAN aware bridges, which port belongs
           to which VLAN by inspecting the incoming VLAN tagged frames.
           This may help simplify VLAN configuration in Abridges but
           does not eliminate the need to configure VLANs in campus
           networks: Tagged VLAN frames must be generated either by
           manually configured bridges or by hosts originating the
           frames. In the hosts case, a system to assign a VLAN to each
           host must be set up via a dynamic VLAN server that requires
           VLANs are used to separate broadcast domains. Frames are
           broadcast in Abridges when the destination is unknown. The
           tree instance used by the ingress Abridge to broadcast is its
           own tree instance rooted at that Abridge. To limit broadcast
           to the ports belonging to the VLAN, it is necessary to filter
           by VLAN, which means that separate tree instances must be
           built for VLAN forwarding, increasing the complexity or at
           least requiring additional filtering on the tree instance
           used for broadcast,  performed using the VLAN tag inside the
           encapsulated frame.
           The recommendation, as default behaviour, is that VLAN tagged
           frames are encapsulated in the same way as non VLAN tagged
           frames and no VLAN specific forwarding is performed in the
         6.5 Optimizing ARP/ND
            Mechanisms proposed for Rbridges for ARP/ND optimization
           [10] are feasible in Abridges as well. However, if proposed
           ARP/Abridge servers are used for ARP and destination Abridge
           resolution they become redundant.
         7. Security Considerations [To be added]
            As for Rbridges, the objective of Abridges is to keep at
         least the same security level of bridged networks, not
         introducing additional risks.
            However the position of Abridges and their role as Root
            Bridges combined with the use of ARP Servers/Registrars
            allow efficient means to enhance the network security due to
            easier localization of attackers, fast detection of spoofed
            MACs by successive and duplicated, inconsistent registries,
            If IEEE 802.1X is used in link ports connecting Abridges,
            security is greatly enhanced in the network core, although
            it can not prevent malicious behaviour of trusted
            authenticated Abridges.
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            However, authentication requires some additional
            configuration, which contradicts in part the zero
            configuration objective of Rbridges and Abridges.
         8. IANA Considerations.
           A new Ethertype must be assigned to indicate an Abridge-
           encapsulated packet.
           A layer 2 multicast address is used as the "all Abridges"
           destination address in distributed ARP queries and any other
           intercommunication message.
           An optional layer 2 multicast address is needed to address to
           "all ARB/Abridge servers" (if used), to communicate among
           them the available servers and the hash value(s) supported.
           A new Ethertype is required for AMSTP protocol.
           If ARP/Abridge servers-registrars are used, a L2 group
           multicast address is required.
         9. NRSTP Protocol.
           This concept is in its early stages, and requires detailed
           analysis and is described summarily here due to its
           An alternative to implementing multiple simplified spanning
           trees like AMSTP might consist of a simultaneous and
           independent construction of N spanning trees (one per
           Abridge) by full independent execution of N RSTP protocols
           (single code, multiple data) at each Abridge. Each Abridge
           executes  RSTP protocol N times simultaneously to participate
           in N tree instances. In one of the N protocol executions, the
           Abridge claims itself as the nonnegotiable root bridge. At
           the same time, with the other N-1 RSTP protocol executions,
           the Abridge joins the N-1 RST tree instances proposed by the
           other N-1 Abridges of the core. As for AMSTP, the destination
           Abridge tree instance is used to forward unicast frames,
           while for broadcast and multicast, the originating Abridge
           tree instance is used. The number of BPDUs is multiplied, but
           processing and implementation may be simplified.
         10. Conclusions
           An alternative implementation for Rbridges has been
           described. It provides pair-wise shortest paths using
           multiple L2 spanning trees across Abridges instead of link
           state L2 routing. The proposal has lower computational
           complexity than Rbridges and is scalable to large scale
           Ethernet campus networks. A topological restriction,
           automatically controlled, is introduced: core forwarding only
           operates on dedicated links that interconnect Abridges.
           Obtainable convergence is likely similar to that obtained by
           the standard IEEE Rapid Spanning Tree protocol, less than 2
           seconds, typically in the hundreds of milliseconds range. The
           design is compatible with current IP nodes and routers and
           with standard bridges, but any connected standard bridge
           connected to an Abridge always works outside the network
           core, in the access layer.
         11. Acknowledgments
           This draft used the current Rbridges draft as a basis for the
           structure,         and for some of the text, to aid
           comprehension and to aid comparison between the two.
           Thanks to Matt Hutton who performed the English language
           For feedback and contributions, join the Rbridge mailing list
           at http://www.postel.org/rbridge
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         12. References
           [1] Bradner, S."Key words for use in RFCs to Indicate
           Requirement Levels"  BCP 14, RFC 2119, March 1997.
           [2] The Rbridge archives. http://www.postel.org/pipermail/
           [3] Rapid Reconfiguration of Spanning Tree. http://www.
           [4] IEEE 802.1D.IEEE-1998 IEEE standard for local and
           metropolitan area networks--Common specifications--Media
           access control (MAC) Bridges.
           [5] IEEE 802.1D-2004 IEEE standard for local and metropolitan
           area Networks-- Common specifications--Media access control
           (MAC) Bridges.
           [6] IEEE 802.1Q-2003 IEEE standard for Local and Metropolitan
           Area Networks- Virtual Bridged Local Area Networks.
           [7] G. Ibanyez, A. Garcia, A. Azcorra. Alternative Multiple
           Spanning Tree  Protocol (AMSTP) for Optical Ethernet
           Backbones. IEEE HSLN (LCN 2004). Tampa, Nov. 2004
           [8] Plummer, D., "Ethernet Address Resolution Protocol: Or
           converting network protocol addresses to 48.bit Ethernet
           address for transmission on Ethernet hardware", STD 37, RFC
           826, November 1982.
           [9] Narten, T., Nordmark, E. and W. Simpson, "Neighbour
           Discovery for IP Version 6 (IPv6)", RFC 2461 (Standards
           Track), December 1998.
           [10] Perlman, R., "RBridges: Transparent Routing", Proc.
           Infocom 2004.
           [11] R. Perlman, J. Touch, A. Yegin. Rbridges: Transparent
           Routing draft-perlman-rbridge-03.txt May 2005.
           [12] M. Seaman. Shortest Path Bridging. http://www.ieee802.
           org/1/files/public/docs2005/ new-seaman-shortest-path-par-
           [13] N. Finn. "An Update on Networking Technologies".
           [14] A. Iwata, et al., "Global Open Ethernet Architecture for
           a Cost-Effective Scalable VPN Solution,"IEICE Trans. On
           Communications, E87-B, 1, pp.142-151, Jan. 2004.
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         Author's Addresses
            Guillermo Ibanyez
            Universidad Carlos III Madrid
            Email: gibanez@it.uc3m.es
            Alberto Garcia
            Universidad Carlos III Madrid
            Email: alberto@it.uc3m.es
            Arturo Azcorra
            Universidad Carlos III Madrid
            Email: azcorra@it.uc3m.es
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            INTERNET DR         Rbridge     November 16, 2005
         This document is subject to the rights, licenses and
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