Topology Change Propagation - Cisco ASR 9000 Series Configuration Manuallines

Implementing Multiple Spanning Tree Protocol
Figure 59: Access Networks
Note Although the above figure shows access rings, in general there are no restrictions on the access network
topology or the number or location of links to the gateway devices.
Access gateway ensures loop-free connectivity in the event of these failure cases:
• Failure of a link in the access network.
• Failure of a link between the access network and the gateway device.
• Failure of an access device.
• Failure of a gateway device.

Topology Change Propagation

There is one case where the two gateway devices need to exchange BPDUs between each other, and this is
to handle topology changes in the access network. If a failure in the access network results in a topology
change that causes a previously blocked port to move to forwarding, the access device sends a topology change
notification out on that port, so as to notify the rest of the network about the change and trigger the necessary
MAC learning flushes. Typically, the topology change notification is sent towards the root bridge, in the case
of access gateway, that means it is sent to one of the gateway devices.
As described above, this causes the gateway device itself to take any necessary action; however, if the failure
caused the access network to become partitioned, it may also be necessary to propagate the topology change
notification to the rest of the access network, that is, the portion connected to the other gateway device. This
can be achieved by ensuring there is connectivity between the gateway devices, so that each gateway device
can propagate any topology change notifications it receives from the access network to the other device. When
a gateway device receives a BPDU from the other gateway device that indicates a topology change, it signals
this in the static BPDUs (that it is sending towards the access network).
Topology Change Propagation is only necessary when these two conditions are met:
• The access network contains three or more access devices. If there are fewer than three devices, then any
• The access devices send traffic to each other, and not just to or from the core or aggregation network. If
possible failure must be detected by all the devices.
all the traffic is to or from the core or aggregation network, then all the access devices must either already
L2VPN and Ethernet Services Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 6.3.x
Topology Change Propagation
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