Administrator Guide

Table Of Contents

Link Aggregation 1075

required to handle the case where a link from the router to one of the MLAG

peers fails. Static routes must be added to the primary and secondary MLAG

peers to route traffic addressed to the connected router across the backup

routed link in the case of a failure of an MLAG link to the router.

This is not a recommended topology, as the same scenario can be achieved

without the use of MLAG by simply configuring the middle switches as

routers and using ECMP to load-balance across the links to the redundant

router pair. In this type of solution, MLAG adds no value, as the redundancy

is provided by layer-3 routing, not by the MLAG.

Figure 28-16. Degenerate Routing Topology

In the one-armed scenario in Figure 28-16, the MLAG cannot associate the

failure of the VLAN 30 link with VLAN 20. Traffic from the routed or

switched network towards the upstream router is routed over the backup

router link when the MLAG link fails solely based on the routing

configuration. Traffic from the upstream router on VLAN 30 to the

switched/routed network is handled by the MLAG failover scenario and is

switched across the peer-link on VLAN 30, but it could just as easily be

handled by layer-3 routing.

Peer-Link

Router

MLAG

VLAN 30

VLAN 20

Router or Switch

VLAN 30

IP address 192.168.30.1/24

Failed Link

VLAN 10

IP address 192.168.10.1/24

IP route 192.168.30.1 192.168.20.1 200

VLAN 10

IP address 192.168.10.2/24

IP route 192.168.30.1 192.168.10.1 200

VLAN 20