[c-nsp] IOS XR / advertise best-external

[Gert Doering]

Hi,

On Sun, May 17, 2015 at 01:19:15PM -0700, Arie Vayner wrote:
> The best-external feature is needed to get a backup path through the 2nd PE
> for a dual attached CE site... This is used by PIC edge...
> 
> If you have an active/standby design, where a PE has the backup link, it
> would be configured to prefer the remote PE's iBGP route (the remote PE
> hosts the primary link).
> 
> Still, we want the primary PE to learn about the alternate path from the
> backup PE.
> 
> This feature allows the backup PE to advertise the eBGP route even though
> it is not selected as the best path.

I do understand what best-external is there for, but I don't see why it
has to put the prefix in the RIB and FIB...  as that, unless I'm overlooking
something, makes it totally unsuitable for routers that are not PEs - 
like, in this example, an IXP peering router.

(Even in your the active/standby case, I might not *want* the backup PE to
send traffic to the standby CE - quite likely, there is a good reason to
make this active/standby and not active/active, like "stateful gear on
the customer side")

Yes, not having the best-external prefix in the FIB is likely to result in 
microloops during reconvergence - but then, having a load-balanced prefix 
"iBGP best path, eBGP best-external" path will also cause microloops 
during convergence, just "for half the packets" (and given cef hashing, 
it's not like "after one loop, the packet is free to go to the other 
path")...

gert
-- 
USENET is *not* the non-clickable part of WWW!
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Gert Doering - Munich, Germany                             gert at greenie.muc.de
fax: +49-89-35655025                        gert at net.informatik.tu-muenchen.de
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