Users Guide

1038 Link Aggregation

configured in a unique MST instance not shared with the MLAG domain. If

the VLAN assigned to the redundant link is also configured on the peer link,

traffic on that VLAN is blocked by MLAG.

To configure the redundant link to be the forwarding for the redundant MST

instance, the link cost needs to be reduced in order to be the root port.

However, with MLAG, even changing the cost of the redundant port will not

make the desired port as root port, as the MLAG peer link cost is internally

set to 0 for all instances. Also it is not permitted by spanning-tree to set a link

cost to 0. The lowest possible value allowed is 1. The spanning tree cost of the

peer link is set to 0 with a specific intent — to ensure that the peer link is

always elected as the designated /root forwarding port in the case of any

redundant links between the primary and secondary switches. It is not

possible to configure a non-peer-link/redundant port cost lower than the peer-

link cost. The alternative is to configure the priority of the bridge (which is

currently a non-root bridge) for the desired MST instance and increase the

internal cost of the peer link interface for that same instance on the bridge

(which is originally the root bridge) so that the port becomes designated

forwarding instead of root forwarding. A example configuration is given later

in this chapter.

Layer-2 Configuration Steps

This section describes how to configure two MLAG peers in a basic layer-2

switching configuration with the default spanning-tree configuration.

Enable MLAG globally and create the MLAG VLANs:

console#config

console(config)#feature vpc

console(config)#vlan 10-17

console(config-vlan10-17)#exit

Configure the keep-alive protocol:

Configure the device priority using the role command, if desired. This

should be configured differently for each of the MLAG peers.

Configure the VPC domain ID and also configure the local priority, if

desired. The VPC domain ID is not important except that it MUST be

the same as the MLAG peer and SHOULD be different than any

other MLAG partner. The MLAG system MAC address may optionally

be configured in the step.

console(config)#vpc domain 1