[c-nsp] Route map matching, tags and community question

David Hubbard dhubbard at dino.hostasaurus.com
Mon Apr 22 16:25:11 EDT 2013

Hi all, we've recently set up real time blackholing via a
trigger router and a route map that applies to our
'redistribute static' clause in the BGP config.  That route
map just looks for a specific tag, changes local pref, sets
the discard route and sets some communities that correspond
to the required communities that our upstreams provide for
this.  Our neighbors are set to send-community of course.

The problem I'm running into is on some of our neighbors,
I send specific communities just for them, such as path
prepending, via an outbound route map.  I've noticed that
on the neighbors where I do that, the communities set via
the outbound route map are not added to the blackhole
community (if present) coming from the redistribution,
they replace it.

So I need a way to send certain communities to only certain
neighbors, but also send the blackhole community if needed.

Trying to accomplish this, would the following be a valid

1) Remove the upstream communities from my redistribute
route map and just keep it doing the other necessary
things like discard route, etc.

2) Create or modify the outbound route maps specific to
each neighbor so that they set any required communities
(if any) for that neighbor, followed by a sequence that
tests for my blachole tag, and if present, sets the
relevant upstream provider community; i.e.:

route-map upstream-one permit 10
 set community 1:123
route-map upstream-one permit 11
 set community 1:234
route-map upstream-one permit 12
 match tag 67
 set community 1:67

Will that successfully announce my routes to that neighbor
with 1:123 and 1:234 for all routes, and only
1:67 on the ones that are tagged 67? 



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