Configuration advice - ECMP

[AlexFer]

Hi Experts,

I've inherited a configuration - two L3 switches, across sites, spanning a VLAN. One switch has a SVI provisioned and is participating in EIGRP, the but the other doesn't. Each switch is connected to different upstream L3 switches; and downstream to Active/Standby ASA - all participating in EIGRP.

The switch with the SVI needs to be replaced, without interruption to downstream traffic.

The solution I see is to define a second SVI on the second switch, also participating in EIGRP.

Two questions:

Q1. If I define the second SVI on second switch, I think I'll have ECMP because both are in same VLAN - is there any downsides?

Q2. If I want to prefer one SVI over another, I think all I need to do is increase EIGRP delay attribute to increase composite metric on the new SVI - will this be sufficient?

R's, Alex

[Giuseppe Larosa]

Hello Alex,

you will need to enable EIGRP on the second switch on all the Vlans where the first switch speaks EIGRP and eventually also taking in account eventual passive-interfaces (that can be present in first switch configuration).

The only doubt is about licensing: verify that the second switch has the correct licenses installed and active to run full EIGRP.

It may be able to run only EIGRP stub and in that case it is not able to be a transit router.

>> Q1. If I define the second SVI on second switch, I think I'll have ECMP because both are in same VLAN - is there any downsides?

See all the above considerations

>> Q2. If I want to prefer one SVI over another, I think all I need to do is increase EIGRP delay attribute to increase composite metric on the new SVI - will this be sufficient?

You should increase delay on all "downstream" subnets/SVIs , increasing delay on the common Vlan  SVI just works for the opposite direction of traffic.

Hope to help

Giuseppe

[AlexFer]

Hi Giuseppe,

thank you very much for taking time to answer.

> You should increase delay on all "downstream" subnets/SVIs , increasing delay on the common Vlan SVI just works for the opposite direction of traffic.

my understanding is that "ip delay eigrp" adds 'delay' value to the metric of all received advertisements. So, if I apply it to the new SVI, and this SVI is between upstream and downstream routers, then the EIGRP composite metric of all advertisements it (itself) sends will be higher compared to the current SVI and hence be less preferred which is what I want (ie. old SVI to be active gateway and new SVI to be standby gateway).

Am I wrong?

R's, Alex