Step 19 root-bridge id
Example:
RP/0/RSP0/CPU0:router(config-mstag-if-inst)# root-id 001c.0000.0011
Sets the root bridge ID for the BPDUs sent from the current port.
Step 20 root-priority priority
Example:
RP/0/RSP0/CPU0:router(config-mstag-if-inst)# root-priority 4096
Sets the root bridge priority for the BPDUs sent from this port.
Repeat steps 4 to 19 to configure each interface, and repeat steps 13 to 19 to configure each MSTI for each
interface.
Note
Step 21 Use the commit or end command.
commit - Saves the configuration changes and remains within the configuration session.
end - Prompts user to take one of these actions:
• Yes - Saves configuration changes and exits the configuration session.
• No - Exits the configuration session without committing the configuration changes.
• Cancel - Remains in the configuration mode, without committing the configuration changes.
Configuring MSTAG Topology Change Propagation
MSTAG Topology Change Propagation is configured simply by configuring connectivity between the
MSTAG-enabled interfaces on the two gateway devices:
1. Configure MSTAG as described above. Take note of the untagged subinterface that is used.
2. Configure connectivity between the gateway devices. This may be via an MPLS Pseudowire, or may be
a VLAN subinterface if there is a direct physical link.
3. Configure a point-to-point (P2P) cross-connect on each gateway device that contains the untagged
subinterface and the link (PW or subinterface) to the other gateway device.
Once the untagged subinterface that is configured for MSTAG is added to the P2P cross-connect, MSTAG
Topology Change Propagation is automatically enabled. MSTAG forwards BDPUs via the cross-connect to
the other gateway device, so as to signal when a topology change has been detected.
For more information on configuring MPLS pseudowire or P2P cross-connects, refer to the Implementing
Point to Point Layer 2 Services module.
Verifying MSTAG
These show commands allow you to verify the operation of MSTAG:
• show spanning-tree mstag mst-name
L2VPN and Ethernet Services Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 6.3.x
438
Implementing Multiple Spanning Tree Protocol
Configuring MSTAG Topology Change Propagation
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