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
draft-yizhou-trill-tc-awareness-00.txt
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Abstract
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
VLANs
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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 |
| 1.link \|/ | 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
follows.
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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
following.
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. et.al., ''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. et.al., ''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
2004.
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
2011.
8. Acknowledgments
This document was prepared using 2-Word-v2.0.template.dot.
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Authors' Addresses
Yizhou Li
Huawei Technologies
101 Software Avenue,
Nanjing 210012
China
Phone: +86-25-56625375
Email: liyizhou@huawei.com
Weiguo Hao
Huawei Technologies
101 Software Avenue,
Nanjing 210012
China
Phone: +86-25-56623144
Email: haoweiguo@huawei.com
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