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Response: Next-Hop and VTEP Reachability in EVPN Networks

Jeff Tantsura published a great response to my Can We Trust BGP Next Hops blog post on LinkedIn, and I asked him for permission to save it in a more permanent form. Here it is (slightly edited)…

I’d like to bring back EVPN context. The discussion is more nuanced, the common non-arguable logic here - reachability != functionality.

The fact that an end-point is reachable doesn’t mean it is functional, and other way around, well, if it is functional but unreachable - it doesn’t really matter. There are 2 ways to validate the functionality: locally to the device or through a path-trace that emulates encapsulation and semantics of the real traffic that is destined to the end-point (VTEP is the end-point for VxLAN encapsulated traffic).

Local validation

For obvious reasons it doesn’t make sense to withdraw the underlay route (VTEP) [ even if the VTEP is not functional — IP ].

EVPN provides means to signal per EVI membership, namely throughout IMET (type 3) route. If local-to-the-device validation could detect dysfunctional/malfunctioning VTEP, withdrawing type 3 route could signal such condition. If the hosts behind the malfunctioning VTEP are single-homed, it doesn’t matter that much since traffic is not going to reach them anyway.

When the hosts are multi-homed and EVPN ESI is in use, this is much more interesting case, since now the goal is to stop load-sharing/exclude a device with malfunctioning VTEP from being load-shared to.

As we know - ESI functionality is provided by type 1(per ES; per EVI) and type 4 (DF election) routes. Withdrawing type 1 per ES route (function called for a reason - mass withdraw) would achieve exactly that.

Remote validation

We could also use path trace that is VxLAN aware. In NX-OS realm NG-OAM (draft-tissa-nvo3-oam-fm) could validate VTEP functionality on the device and potentially notify sending device of the failure.

Ivan’s summary

As far as I’m aware no vendor implemented either option to check VTEP functionality in real time and adjust the forwarding behavior (NG-OAM ping and traceroute can be run as diagnostic commands).

I hope I’m wrong, in which case you probably know how to reach me.

Also, running EVPN sessions between loopback (VTEP) addresses and expecting that presence of BGP sessions validates correct data-plane VTEP functionality is no more than wishful thinking.

In the end we’re back to “networks work reasonably well, stay calm, stop worrying (too much) and get a life.

Next: Can We Trust BGP Next Hops (Part 2)? Continue

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