[c-nsp] IOS XR BGP default route - prepending AS

Brian Knight brian.t.knight at gmail.com
Wed Apr 20 02:50:44 EDT 2016

At $DAYJOB we use MPLS VPNs from other carriers to provide Internet access
to customers connected to these VPNs.  There is always a primary path for
outbound access, plus one or more backup paths.  I am trying to configure a
backup path, but can't seem to set AS path prepending on the default route
we announce to the MPLS VPN.

This is our first time configuring these types of MPLS VPNs on IOS XR.

I've got a config that looks a little something like this:


route-policy BACKUP-IN
  # For backup, localpref = 350
  set local-preference 350
  set community BGP-SITECODE additive

route-policy BACKUP-OUT
  # For backup, prepend = 2
  # Announce only default route
  if destination in ( then
    prepend as-path MY-AS 2
    set community BGP-SITECODE

router bgp 53828
    neighbor THEIR-IP
      remote-as THEIR-AS
      address-family ipv4 unicast
        route-policy BACKUP-IN in
        route-policy BACKUP-OUT out
        soft-reconfig inbound
        maximum-prefix 500 80 restart 1


The config originates a default route towards THEIR-IP, which is expected.
But the default route that is generated is apparently not influenced by the
BACKUP-OUT policy (prepend MY-AS 2 times).

RP/0/8/CPU0:AR1.ATL1#sh bgp ipv4 u nei THEIR-IP adv
Wed Apr 20 01:00:39.545 CDT
Network            Next Hop        From            AS Path         Local           i

What's the simplest way to prepend AS path on a default route advertised to
a single peer?

A few sources suggest adding a default route to Null0 in our Internet
routing table and advertising it through redistribution, or enter a
"network" command in the BGP address-family, both of which can be
influenced by route policies.  Neither of those options sound particularly
simple to me.

I have also tried the following to no avail:


  prepend as-path 53828 2
  set community BGP-SITECODE

router bgp 53828
    neighbor THEIR-IP
      address-family ipv4 unicast
default-originate route-policy DEFAULT-BACKUP-OUT


I'd be grateful for any insight.


-Brian Knight

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