TRILL Working Group Yizhou Li
Internet Draft Weiguo Hao
Intended status: Standards Track Huawei Technologies
Jon Hudson
Brocade
Naveen Nimmu
Broadcom
Anoop Ghanwani
DELL
Expires: April 2013 October 21, 2012
Aware Spanning Tree Topology Change on RBridges
draft-yizhou-trill-tc-awareness-01.txt
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Abstract
When a local LAN running spanning tree protocol connecting to TRILL
campus via more than one RBridge, there are several ways to perform
loop avoidance. One of them illustrated by RFC6325 [RFC6325] A.3 was
to make relevant ports on edge RBridges involving in spanning tree
calculation. When edge RBridges are emulated as a single highest
priority root, the local bridged LAN will be naturally partitioned
after running spanning tree protocol. This approach achieves better
link utilization and intra-VLAN load balancing in some scenarios.
This document describes how the edge RBridges react to topology
change occurring in bridged LAN in order to make the abovementioned
spanning tree approach function correct.
Table of Contents
1. Introduction ................................................ 3
1.1. Motivations ............................................ 5
2. Conventions used in this document ........................... 6
3. BPDU RBridge Channel......................................... 6
4. Operations .................................................. 7
4.1. Sending BPDU using RBridge channel ..................... 8
4.2. Receiving BPDU in RBridge channel ...................... 9
4.3. Informing the remote site ............................. 10
5. Security Considerations..................................... 11
6. IANA Considerations ........................................ 12
7. References ................................................. 12
7.1. Normative References................................... 12
7.2. Informative References................................. 13
8. Acknowledgments ............................................ 13
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1. Introduction
The TRILL protocol [RFC6325] provides the appointed forwarder
mechanism [RFC6439] for loop avoidance where, for part of the loop,
the frame would be in TRILL encapsulated format, for example in the
scenario shown by Figure 1. Only one of the RBridges is responsible
for encapsulating/decapsulating a given VLAN's data frames on a link.
Bridges in the local bridged LAN runs normal spanning tree protocol
for local loop avoidance. RBridges 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 a
root bridge change. Root bridge changes trigger 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 conformable to [RFC6325] never encapsulates or forwards any
BPDU frame it receives.
------------------
/ \
| Trill Network |
\ /
------------------
| |
DRB| |
+------+ +------+
AF --->| RB1 | | RB2 |
+------+ +------+
| |
+---------------------------------------------+
| | | |
| STP | | |
| +----+ root+----+ +----+ |
| | B4 |-------| B1 |-------| B2 | |
| +----+ +----+ +----+ |
| | | |
| | | |
| | |<---blocked |
|Bridged | +----+ | |
|LAN +-----| B3 |----+ |
| +----+ |
+---------------------------------------------+
Figure 1 TRILL and bridged LAN topology
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[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
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.
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
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.
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------------------
/ \
| Trill Network |
\ /
------------------
| |
| |
-----+-----------+----
/ +------+ +------+ \ <---emulated highest
| | RB1 | | RB2 | | priority root Bx
-------------| +------+ +------+ |---------
| \-----+-----------+-----/ |
| | | |
| | | |
| | | |
| +----+ +----+ \|/ +----+ |
| | B4 |-------| B1 |--- ---| B2 | |
| +----+ p1 +----+ /|\ +----+ |
| | | | |
| | blocked \|/ |
| | - ----blocked |
|Bridged | /|\ |
|LAN | +----+ | |
| +-----| B3 |----+ |
| p1 +----+ p2 |
-----------------------------------------------
Figure 2 RBs function as STP tree root topology
1.1. Motivations
Bridged LANs may have topology changes at any time. When RB1 & RB2
serve as one single STP tree root as shown in Figure 2, it is
required that RB1 and RB2 have to tunnel some BPDUs to help the
bridged LAN convergence in certain circumstances. Figure 2 will be
used to illustrate such motivation for rest of this subsection.
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 by the spanning tree protocol. RB1 and RB2 will not
receive TRILL-Hello from each other. Bridged LAN is logically
partitioned into two parts. RB1 is DRB and AF for all VLANs in left
partition and RB2 is DRB and AF in right partition.
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If B1-B3 link fails for some reason, alternate port p2 on B3 will
send topology change (TC) BPDU to B2 as RSTP specifies [802.1D]. 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 an emulated single
tree root in a spanning tree instance in local bridged LAN. The group
has at least two RBridges.
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 .
. .
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
Figure 3 RBridge Channel Format for BPDU
BPDU field is used to put the original BPDU frame.
The fields of TRILL header and inner Ethernet header SHOULD be set as
per [TRILLChannel] unless specified in this draft.
4. Operations
Figure 4 shows TC BPDU tunneled from RB2 to RB1 using RBridge Channel.
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-------------------------
/ \
/ \
| Trill Network |
| |
| +---------------+ |
| | 4.tunnel BPDU | |
| | in channel | |
\ | +-----------+ | /
\ | | | | /
\ ---|-|-----------|-|-- /
/ +-+ +--+ +--+ +-+ \ <--- emulated highest
| | RB1 | | RB2 | | priority 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 4 Tunneled TC BPDU
4.1. Sending BPDU using RBridge channel
In figure 4, when B1-B3 link fails, alternate 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.
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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. Static configuration of
root bridge group is also allowed.
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 sequential unicast BPDU
tunneled. In figure 4, 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. M bit in TRILL
header SHOULD be 0.
If TRILL Campus was partitioned temporarily in some unusual cases,
RBridges in the same root bridge group may not reach each other. For
instance, if RB2 was not able to reach RB1 through TRILL campus at
some transition period due to network fault, RB1 would not receive
the tunneled TC BPDU from RB2. Then the approach illustrated in this
document will take effect again only after RB1 and RB2 connectivity
via TRILL recovers from the network fault.
It is possible to statically configure a root bridge group, especial
when network is relatively small and stable. Therefore when an
RBridge tunnels the TC BPDU to other members in the same root bridge
group, it has to make sure the destination is reachable.
If edges RBridges configured in the same root bridge group connect to
separate TRILL campus intentionally, it is not recommended to use
spanning tree partition mechanism and such root bridge group
provisioning is normally considered as mis-configuration.
4.2. Receiving BPDU in RBridge channel
When an RBridge receives a TC BPDU from RBridge channel, it
determines the frame was sent from an RB in the same root bridge
group. Then RBridge decapsulates the frame and sends the original TC
BPDU to its local bridged LAN. TC BPDU will be flooded throughout in
the left partition to merge MAC table of bridges.
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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 update 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
naturally though data frame learning.
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 is aged out at the remote
RBridge.
A lightweight method is to use RBridge channel to carry MAC purge
information. In Figure 4, When RB2 receives TC BPDU from B2, 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 ... |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
Figure 5 RBridge Channel Format for Purge
Number of nicknames: number of the following nicknames, which will be
used by the receivers to purge their relevant MAC-to-nickname
correspondence table entries.
Num of VLAN blocks: number of the following VLAN block. A VLAN block
is specified by a start and an end VLAN IDs. When start and end VLAN
IDs are the same, it implies only one VLAN ID is in the block. When
number of VLAN block is 0, it implies no VLAN ID is specified.
For any nickname x specified and any VLAN y specified in this TLV,
the receivers should purge MAC-to-nickname correspondence table
entries with (any-MAC, VLAN-y, nickname-x). When number of VLAN block
is 0, the receivers should purge entries with (any-MAC, any-VLAN,
nickname-x).
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].
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Forged TC BPDU may trigger RBridges continuously sending tunneled
BPDU and MAC purges. 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
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.
[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.
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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
John Hudson
Brocade
120 Holger Way
San Jose, CA 95134
USA.
Email: jon.hudson@gmail.com
Naveen Nimmu
Broadcom
9th Floor, Building no 9, Raheja Mind space
Hi-Tec City, Madhapur,
Hyderabad - 500 081, INDIA
Phone: +1-408-218-8893
Email: naveen@broadcom.com
Anoop Ghanwani
DELL
350 Holger Way
San Jose, CA 95134
USA.
Phone: +1-408-571-3500
Email: Anoop@duke.alumni.duke.edu
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