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Aware Spanning Tree Topology Change on RBridges

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This is an older version of an Internet-Draft whose latest revision state is "Expired".
Authors Yizhou Li , Hao Weiguo
Last updated 2012-07-08
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TRILL Working Group                                         Yizhou Li
Internet Draft                                             Weiguo Hao
Intended status: Standards Track                   Huawei Technologies

Expires: November 2012                                   July 09, 2012

              Aware Spanning Tree Topology Change on RBridges

Status of this Memo

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   This Internet-Draft will expire on January 9, 2009.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1. Introduction ................................................ 2
      1.1. Motivations ............................................ 4
   2. Conventions used in this document............................ 5
   3. BPDU RBridge Channel......................................... 5
   4. Operations .................................................. 6
      4.1. Sending BPDU using RBridge channel ..................... 7
      4.2. Receiving BPDU in RBridge channel ...................... 7
      4.3. Informing the remote site............................... 8
   5. Security Considerations...................................... 9
   6. IANA Considerations ......................................... 9
   7. References ................................................. 10
      7.1. Normative References................................... 10
      7.2. Informative References................................. 10
   8. Acknowledgments ............................................ 10

1. Introduction

   TRILL protocol [RFC6325] [RFC6439] described appointed forwarder
   mechanism for loop avoidance in the scenario shown by Figure 1. Only
   one of the RBridges is responsible for encapsulating/decapsulating a
   given VLAN data frame on a link. Local bridged LAN runs normal
   spanning tree protocol for loop avoidance. RBridge keeps track of the
   root bridge by listening to BPDUs received on the local port. This
   information is reported per VLAN by the RBridge in its LSP and is
   used to detect the root change. Root change willtrigger the reset of
   the inhibition timer of the appointed forwarder. When an RBridge
   ceases to be appointed forwarder for a VLAN on a port, it sends
   topology change BPDUs to purge the MAC table on local bridged LAN
   switches. An RBridge never encapsulates or forwards any BPDU frame it
   receives [RFC6325].

   [RFC6325] A.2 & A.3 presented the problems using the conventional
   approach shown in Figure 1. Native frames enter and leave a link via
   the link's appointed forwarder for the VLAN of the frame can cause
   congestion or suboptimal routing. Four methods was illustrated in
   [RFC6325] to solve the problem,

   1. Use RBridge instead of conventional bridge

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   2. Re-arrange network topology

   3. Carefully select the different appointed forwarders for VLANs if
      end stations on local bridged LAN can be separated into multiple

   4. Configure the RBridges to be like one STP tree root in local
      bridged LAN. The RBridge ports that are connected to the bridged
      LAN send spanning tree configuration BPDUs. Then the bridged LAN
      is forced into partitions. Figure 2 shows its network topology.

                        /                  \
                       |   Trill Network    |
                        \                  /
                            |           |
                         DRB|           |
                        +------+     +------+
                AF  --->| RB1  |     | RB2  |
                        +------+     +------+
                            |           |
         |                  |           |              |
         |            STP   |           |              |
         |  +----+   root+----+       +----+           |
         |  | B4 |-------| B1 |-------| B2 |           |
         |  +----+       +----+       +----+           |
         |                |               |            |
         |                |               |            |
         |                |               |<---blocked |
         |Bridged         |     +----+    |            |
         |LAN             +-----| B3 |----+            |
         |                      +----+                 |

                  Figure 1 TRILL and bridged LAN topology

   Method 1 and 2 highly depends on the network topology and equipment
   types and therefore have very limited applicability. Method 3 and 4
   have broader applicability. Method 4 is more applicable than method 3
   if all end stations in bridged LAN are on the same VLAN or intra VLAN
   load balancing is required to avoid per VLAN congestion and
   suboptimal routing. The traffic discontinuity was caused by
   inhibition timer setting in case of root change in method 3. Proper
   timeout value has to be carefully chosen for tradeoff between
   unnecessary traffic continuity and potential loop. Method 4

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   eliminates the requirement of setting inhibition timer in case of
   root change.  Therefore method 4 is considered as a very common
   practice in real deployment.

1.1. Motivations

   Bridged LAN may have topology change at any time. When RB1 & RB2
   serve as one single STP tree root, it is required that RB1 and RB2
   have to tunnel some BPDUs to help the bridged LAN convergence in
   certain circumstances. Figure 2 is used to show such motivation in
   the given topology.

                       /                   \
                      |                     |
                      |   Trill Network     |
                      |                     |
                       \                   /
                           |           |
                           |           |
                     / +------+     +------+ \  <---highest pri
                    |  | RB1  |     | RB2  | |      root Bx
        ------------|  +------+     +------+ /---------
        |            \-----+-----------+-----         |
        |                  |           |              |
        |                  |           |              |
        |                  |           |              |
        |  +----+       +----+  \|/  +----+           |
        |  | B4 |-------| B1 |--- ---| B2 |           |
        |  +----+ p1    +----+  /|\  +----+           |
        |                |     |       |              |
        |                |     blocked  \|/           |
        |                |              - ----blocked |
        |Bridged         |              /|\           |
        |LAN             |     +----+    |            |
        |                +-----| B3 |----+            |
        |                   p1 +----+ p2              |
              Figure 2 RBs function as STP tree root topology

   RB1 & RB2 use the same bridge ID to emit spanning tree BPDUs as the
   highest priority root Bx. All bridges in LAN see RB1 and RB2 as a
   single tree root. Therefore B1-B2 and B2-B3 links are blocked for
   loop avoidance after running spanning tree protocol. RB1 and RB2 will
   not receive TRILL-Hello from each other. Bridged LAN is logically

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   partitioned into two parts. RB1 is DRB and AF for all VLANs in left
   partition and RB2 is DRB and AF in right partition.

   If B1-B3 link fails for some reason, alternate port p2 on B3 will
   send topology change (TC) BPDU to B2. B2-B3 link will start
   forwarding frames. TC BPDU is then sent from B2 to RB2. As RB2 never
   forwards BPDU frame to TRILL campus, left partition has no way to
   know the topology change. Therefore B4 will not able to correctly
   purge the MACs learnt from port p1 for end stations connected to B3.
   MAC table entry aging is the last resort in this case. In addition, a
   remote end station may keep sending traffic to an end station
   connected to B3 via RB1-B1 which causes frame loss. Therefore some
   mechanism must be used to purge the MACs learned both in the left
   partition of the bridged LAN and the remote Rbridges when topology
   changes. This draft proposes to use RBridge channel [TRILLChannel] to
   tunnel the TC BPDU to solve the issue.

2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC-2119 [RFC2119].

   This document uses the terminologies defined in [RFC6325] along with
   the following:

   Root Bridge Group - A group of RBridges acting as a single tree root
   in a spanning tree instance in local bridged LAN

3. BPDU RBridge Channel

   A new channel protocol is defined to carry BPDU.

   Channel protocol code: TBD (BPDU)

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             | 0| 1| 2| 3| 4| 5| 6| 7| 8| 9|10|11|12|13|14|15|
             |                RBridge Channel                |
             |                     Header                    |
             |                    Reserved                   |
             |                                               |
             .                      BPDU                     .
             .                                               .

4. Operations

   Figure 3 shows TC BPDU tunneled from RB2 to RB1 using RBridge Channel.

4.1. Sending BPDU using RBridge channel

   In figure 3, when B1-B3 link fails, port p2 on B3 will start to send
   TC BPDU and go to forwarding state. RB2 receives TC BPDU from B2
   sequentially. RB2 encapsulates the TC BPDU in RBridge channel and
   sends it to RB1.

   Interested VLANs and Spanning Tree Roots Sub-TLV [RFC6326] carries
   spanning tree root bridge IDs seen for all ports for which the
   RBridge is the appointed forwarder for a VLAN. As RB1 and RB2 use the
   same bridge ID and that bridge ID is the spanning tree root, RB1 and
   RB2 are considered as in a root bridge group.

   When RBridge receives TC BPDU from an access port, it tunnels the
   frame to all the other RBridges in the same root bridge group using
   RBridge channel protocol specified in section 3. Normally the number
   of RBridges in a root bridge group is limited, say 2 or 3; such
   tunneling is performed using TRILL unicast encapsulation. N members
   in a root bridge group results in N-1 unicast tunneled BPDU sent. In
   figure 3, RB2 knows RB1 is in the same root bridge group from LSP
   exchange; hence RB2 uses RB1's nickname as egress nickname and
   encapsulates the TC BPDU in RBridge channel.

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                 /                          \
                |                            |
                |       Trill Network        |
                |                            |
                |      +---------------+     |
                |      | 4.tunnel BPDU |     |
                |      |   in channel  |     |
                \      | +-----------+ |     /
                 \     | |           | |    /
                  \ ---|-|-----------|-|-- /
                   / +-+ +--+     +--+ +-+ \  <---highest pri
                  |  | RB1  |     | RB2  | |      root Bx
      ------------|  +------+     +------+ /------------------
      |            \-----+-----------+-----                  |
      |                  |           |                       |
      |   5. TC BPDU  |  |           |  /|\  3. TC BPDU      |
      |              \|/ |           |   |                   |
      |                  |           |                       |
      |  +----+       +----+  \|/  +----+                    |
      |  | B4 |-------| B1 |--- ---| B2 |                    |
      |  +----+       +----+  /|\  +----+                    |
      |                |       |       |                     |
      |                |     blocked   |                     |
      |                |               |<---blocking to      |
      |      \|/              |    forwarding       |
      |     failure -->                |                     |
      |               /|\              |                     |
      |                |               |                     |
      |                |  +----+ p2    | /|\                 |
      |                +--| B3 |-------+  |                  |
      |Bridged            +----+ ---------+                  |
      |LAN                       2. TC BPDU                  |
      |                                                      |
                         Figure 3 Tunneled TC BPDU

4.2. Receiving BPDU in RBridge channel

   When an RBridge receives a TC BPDU from RBridge channel, it
   determines the frame was sent from a RB in the same root bridge group.
   Then RBridge decapsulates the frame and sends the original TC BPDU to

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   its local bridged LAN. TC BPDU will be flooded throughout in left
   partition to clear MAC table in bridges.

4.3. Informing the remote site

   When local topology changes, the correspondence of end station and
   its attaching RBridge cached by remote RB may become invalid. The
   RBridges who is the appointed forwarder for the specified VLAN in
   remote sites should be informed to clear the stale correspondence
   table entry.

   When traffic is bi-directional, the remote RBridge will receive the
   data frames from the newly attached RBridge of the local end station.
   The remote RBridge will update its MAC-Nickname correspondence table.

   When traffic is uni-directional from the remote to local site or
   traffic from local to remote has to be triggered by traffic from
   remote to local, remote RBridge will not receive the data frame from
   local RBridge to refresh its table. Then traffic discontinuity may
   last for some time until the table entry aged out at remote RBridge.

   A lightweight method is to use RBridge channel to carry MAC purge
   information. In Figure 3, When RB2 receives TC BPDU, it derives the
   corresponding VLAN list. For example, if MSTP is used, RB2 will get
   the VLAN IDs in the same MSTP instance as TC BPDU. RB2 sends out MAC
   purge information using RBridge channel with VLAN information and
   RBidges nicknames in the same root bridge group. All remote RBridges
   received MAC purge should clear its MAC-to-nickname correspondence
   table for entries with the specified nicknames and VLAN IDs. If no
   VLAN list is specified, the remote RBridges should clear the
   correspondence in all VLANs relevant to the given nicknames. The MAC
   purge is recommended to send on the management VLAN in which all
   RBridges joins.

   A new channel protocol code for MAC purge should be defined as

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             | 0| 1| 2| 3| 4| 5| 6| 7| 8| 9|10|11|12|13|14|15|
             |                RBridge Channel                |
             |                     Header                    |
             |     Number of nicknames  |     nickname 1     |
             |          nickname 1      |     nickname 2     |
             |          nickname 2      |       ...          |
             |           ...            |     nickname n     |
             |          nickname n      |  Num of VLAN blocks|
             |          Start.VLAN                  |        |
             |           End.VLAN                   |        |
             |       Other Start/End VLAN list ...           |

5. Security Considerations

   This document does not change the general RBridge security
   considerations of the TRILL base protocol and TRILL RBridge Channel.
   See Section 6 of [RFC6325] and section 7 of [TRILLChannel].

   Forged TC BPDU may trigger RBridge continuously sending tunneled BPDU
   and MAC purge. It may cause denial-of-service in TRILL campus.
   Similar as the traditional bridged LAN running spanning tree, it is
   suggested to monitor the receiving rate of TC BPDU on bridged LAN
   facing port of RBridges. If the receiving rate is beyond the
   threshold, RBridge should only process and tunnel the TC BPDU in the
   configured rate.

6. IANA Considerations

   IANA is requested to allocate the new channel protocol codes as

   Channel protocol code X1: BPDU

   Channel protocol code X2: MAC purge

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7. References

7.1. Normative References

   [RFC6325] Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
             Ghanwani, "Routing Bridges (RBridges): Base Protocol
             Specification", RFC 6325, July 2011.

   [RFC6326] Eastlake, D., Banerjee, A., Dutt, D., Perlman, R., and A.
             Ghanwani, "TRILL Use of IS-IS", RFC 6326, July 2011.

   [6326bis] Eastlake, D., ''Transparent Interconnection of Lots
             of Links (TRILL) Use of IS-IS'', draft-eastlake-isis-
             rfc6326bis-07.txt, Work in Progress, December 2011.

   [RFC6439] Eastlake, D., ''RBridge: Appointed Forwarder'', RFC
             6439, November 2011.

   [TRILLChannel] - Eastlake, D., V. Manral, Y. Li, S. Aldrin, D. Ward,
             "RBridges: RBridge Channel Support in TRILL", draft-ietf-
             trill-rbridge-channel, work in progress.

   [RFC6327] Eastlake 3rd, D., Perlman, R., Ghanwani, A., Dutt, D.,
             and V. Manral, "Routing Bridges (RBridges): Adjacency", RFC
             6327, July 2011

   [802.1D] "IEEE Standard for Local and metropolitan area networks
             /Media Access Control (MAC) Bridges", 802.1D-2004, 9 June

7.2. Informative References

   [RFC6165] Banerjee, A. and D. Ward, "Extensions to IS-IS for Layer-2
             Systems", RFC 6165, April 2011.

   [802.1Q-2011] "IEEE Standard for Local and metropolitan area networks
             /Virtual Bridged Local Area Networks", 802.1Q-2011, 31 Aug

8. Acknowledgments

   This document was prepared using

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Authors' Addresses

   Yizhou Li
   Huawei Technologies
   101 Software Avenue,
   Nanjing 210012

   Phone: +86-25-56625375

   Weiguo Hao
   Huawei Technologies
   101 Software Avenue,
   Nanjing 210012

   Phone: +86-25-56623144

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