[c-nsp] Port-channel between Cisco 4948 and ASR 9k going err-disable

[Tom Hill]

On 12/10/16 18:06, David Wilkinson wrote:
> Should split horizon stop the loops when connecting downstream switches
> in a resilient configuration?

It can't when you've the ability to loop a broadcast frame around via
devices that aren't party to the split horizon forwarding. I'm not
certain this is really how VPLS was supposed to be used, in all honesty.

Your 4948s at each site /should not/ be able to broadcast between each
other; they ought to both go to a single PE. Anything between them then
relies on the PEs (and split horizon forwarding) for loop avoidance.

Assuming that you can't do that for some reason, then perhaps just
removing the LAG/STP misconfiguration protection (and sticking with
PVST) will solve your current woes.

I do, however, wonder if MST-AG might be safer for you in the long run:

https://supportforums.cisco.com/document/61401/asr9000xr-using-mst-ag-mst-access-gateway-mst-and-vpls

Mainly because the PEs would then know what's going on. It might provide
faster convergence across the VFI, too.

To add some further resilience, you could look at multi-homed VPLS (or
EVPN) which would involve MC-LAG from both local PEs towards each 4948.
You'd still use the same number of 10G links as you are now. Less, if
the 4948s aren't interconnected.

In general though, this is a lot of work that could be unpicked very
easily if just one of your customers creates a loop within their own
network, with effects very similar to those that you've experienced
running this topology without PVST. :)

-- 
Tom