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

Gert Doering gert at greenie.muc.de
Thu Jun 4 15:05:06 EDT 2015


On Mon, May 18, 2015 at 08:21:10AM +0200, Marian ??urkovi?? wrote:
> the fact that both entries are present in RIB and CEF tables is expected
> behaviour. It's part of the new design, where both active and backup paths are
> preinstalled into the CEF table, which allows much faster convergence when the
> active path disappears. Note that the backup path has different CEF flags, which
> should instruct the forwarding engine *not* to use it when another regular path
> is present in CEF table.
> It should be investigated why your router uses the backup path for traffic -
> this is clearly wrong. One possible reason might be that the affected linecards
> are running different firmware version than required by the IOS.

The answer I received in the TAC case (SR 635067397) is that "this is 
totally expected behaviour, because the CEF team thinks it needs to
work that way".

If I do not want load-sharing here, I am supposed to use labelled next-hops,
so router sending prefixes *to* me can steer whether they want to use 
the "best-external" path or not.

To me this sounds like total bullshit...

(Yes, during reconvergence, microloops will happen if ebgp best-external
is used.  Otherwise, traffic blackholing happens if a prefix goes away
and the peer router does not have an alternative path.  So, right.  Leave
that decision to *me*, not force my network design because someone doesn't
understand trade-offs)

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