Dell Networking N4000 Series Configuration Manual page 951

Stackable layer 2 and 3 switches

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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 L3 routing, not by the MLAG.

Figure 28-16. Degenerate Routing Topology

IP address 192.168.10.1/24

IP route 192.168.30.1 192.168.20.1 200

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 L3 routing.

IP address 192.168.30.1/24

Router or Switch

Configuring Link Aggregation

IP address 192.168.10.2/24

IP route 192.168.30.1 192.168.10.1 200

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