BVI interface and mpls mtu

[matthew]

hi,

Whilst I was converting from IOS to IOS-XR, specifically from a 7604, I followed all the guides, switching from SVI to a BVI core facing interface.

However all was going well until I couldn't set mpls mtu on a BVI which you can on an SVI.

you can set ipv4 mtu ok and ipv6 mtu ok like traditional SVIs.

On standard interfaces under IOS XR you can set the mpls mtu command so why not on a BVI?

I prefer to do it that way so I can set a higher interface MTU.

I raised a Cisco TAC case and was told to set the MTU on the bridge domain but then this would affect all traffic and not just mpls traffic.

Has it just been missed off all these years? Can it be put back in a future release?

thanks

matt

[Aleksandar Vidakovic]

hi Matt,

running MPLS over BVI is an unusual deployment scenario for an XR router. LDP typically runs on physical or bundle interfaces, while BVI is typically facing the access. Hence no one has asked so far to change MPLS MTU on BVI.

SVI on c7600 was a "native" interface type in the c7600 EARL architecture and was used widely. The BVI on asr9k (I suppose you are migrating to an asr9k) is not a "native" interface type and hence comes with some performance implications (please see BRKSPG-2904 from Cisco Live Berlin 2017). Does your design solution really require a BVI interface as MPLS core facing interface? Maybe you can adjust the design and have the benefit of running a kind of solution that's well tested and deployed widely.

/Aleksandar

[matthew]

hi,

ok thanks for the reply.

Ideally yes I did need it due to varying MTUs on different connectivity from suppliers. 

However what I've done for now is to change the routers connected to the ASR9001 as Layer 3 PtP's. This isn't ideal but it works and I can set the mpls mtu.

thanks

matt