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

[brad dreisbach]

On Wed, Apr 20, 2016 at 01:50:44AM -0500, Brian Knight wrote:
>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
>  apply IPV4-RESERVED-DENY
>  apply IPV4-DEFAULT-DENY
>  # For backup, localpref = 350
>  #
>  set local-preference 350
>  set community BGP-SITECODE additive
>end-policy
>
>route-policy BACKUP-OUT
>  # For backup, prepend = 2
>  #
>  # Announce only default route
>  if destination in (0.0.0.0/0) then
>    prepend as-path MY-AS 2
>    set community BGP-SITECODE
>  else
>    drop
>  endif
>end-policy
>
>router bgp 53828
>    neighbor THEIR-IP
>      remote-as THEIR-AS
>      address-family ipv4 unicast
>default-originate
>        route-policy BACKUP-IN in
>        route-policy BACKUP-OUT out
>        send-community-ebgp
>        send-extended-community-ebgp
>        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
>0.0.0.0/0          0.0.0.0         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 0.0.0.0/0" 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:
>
>---------
>
>route-policy DEFAULT-BACKUP-OUT
>  prepend as-path 53828 2
>  set community BGP-SITECODE
>end-policy
>
>router bgp 53828
>    neighbor THEIR-IP
>      address-family ipv4 unicast
>default-originate route-policy DEFAULT-BACKUP-OUT
>
>---------
>
>I'd be grateful for any insight.
>

you need to apply a policy at the default-originate statement to do
stuff to the originated default on the outbound:

default-originate route-policy blah