Posted on June 23, 2017 9:18 pm
 |  Asked by Scott Friedman
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I have a simple single area OSPF topology with two routers. The setup exchanges routes just fine. I would like to convert one of the router/switches to a MLAG configuration.

My question is what would be the correct way to approach adding the peer system into the OSPF configuration? Do I treat the two MLAG peered systems as discreet as far as OSPF is concerned? Does/can VARP play a role? I would guess not since the routing engines are separate.


Posted by Mitu Mitu
Answered on June 24, 2017 2:39 am

Hi Scott,

As I understand, you have two devices that you would like to have in MLAG and run OSPF. This can be configured since MLAG operates on the Layer 2 and the OSPF configuration is independent.

In case you wish to implement VARP on MLAG devices with OSPF, it would be good to configure the virtual address without the subnet mask (in case you are enabling OSPF for the VLAN interface with VARP IP configured).This is because while enabling OSPF for a vlan interface that has VARP IP configured with subnet mask , OSPF gets stuck in EX-START state.

In EX-START state, unicast connectivity is required. This gets broken because the ARP generated by switch is sourced from the virtual  IP/MAC address. The ARP request is consumed by peer switch since the same source IP (virtual IP) configured on it, Hence no ARP reply is generated.  

Using the “ip virtual-router address A.B.C.D/E” command with  “ip virtual-router address A.B.C.D”. In this case ARP is sourced from the physical IP for which peer switch respond and ARP gets resolved. The OSPF neighbor state is reached to “FULL”.

In case you wish to run the devices in two different MLAG domains (with other peers), that should also not cause an issue with the OSPF.




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