The following MSTAG parameters are configurable for each interface, for each spanning tree instance:
• VLAN IDs
• Root Bridge Priority and ID
• Bridge Priority
• Port Priority
• Internal Port Path Cost
To ensure consistent operation across the access network, these guidelines should be used when configuring:
• Both gateway devices should be configured with a Root Bridge Priority and ID (for each spanning tree
instance) that is better (lower) than the Bridge Priority and Bridge ID of any device in the access network.
It is recommended to set the Root Bridge Priority and ID to 0 on the gateway devices.
To avoid an STP dispute being detected by the access devices, the same root
priority and ID should be configured on both gateway devices.
Note
• Both gateway devices should be configured with a Port Path Cost of 0.
• For each spanning tree instance, one gateway device should be configured with the bridge priority and
ID that is higher than the root bridge priority and ID, but lower than the bridge priority and ID of any
other device in the network (including the other gateway device). It is recommended to set the bridge
priority to 0.
• For each spanning tree instance, the second gateway device should be configured with a bridge priority
and ID that is higher than the root bridge priority and ID and the first gateway device bridge priority and
ID, but lower than the bridge priority and ID of any device in the access network. It is recommended to
set the bridge priority to 4096 (this is the lowest allowable value greater than 0).
• All of the access devices should be configured with a higher bridge priority than the gateway devices. It
is recommended to use values of 8192 or higher.
• For each spanning tree instance, the port path cost and other parameters may be configured on the access
devices so as to ensure the desired port is put into the blocked state when all links are up.
There are no checks on MSTAG configuration—misconfiguration may result in incorrect operation of the
MSTP protocol in the access devices (for example, an STP dispute being detected).
Caution
The guidelines above are illustrated in the following figure.
These guidelines do not apply to REPAG, as in that case the access devices ignore the information received
from the gateway devices apart from when a topology change is signalled.
Note
L2VPN and Ethernet Services Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 6.3.x
433
Implementing Multiple Spanning Tree Protocol
Configuring MSTAG parameters
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