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BGP question, prepending route and results

I have an MPLS connection from HQ to 10 branches via BGP.

Our DR site is one of the MPLS branch links. DR also has a point to point connection to HQ via BGP.

I am curious about some results with prepending a default into MPLS from the HQ site,

Under normal conditions without prepending, the remote branch (not DR) shows the path to default route as

Remote Branch---> MPLS---> HQ site

sh ip bgp 0.0.0.0
BGP routing table entry for 0.0.0.0/0, version 19245
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
  Not advertised to any peer
  65000 65001, (received & used)
    6.1.1.1 from 6.1.1.1 (5.2.2.2)
      Origin IGP, localpref 100, valid, external, best

When I prepend the defaul route:

ip prefix-list only-default permit 0.0.0.0/0

route-map PREPEND permit 10
match ip address prefix default-only
set as-path prepend 65001 65001 65001

router bgp 65001
neighbor (PE router) remote-as 65000
neighbor (PE Router) route-map PREPEND out

The result becomes:

Remote Branch--->MPLS---> DR Site--->HQ Site

sh ip bgp 0.0.0.0
BGP routing table entry for 0.0.0.0/0, version 19243
Paths: (1 available, best #1, table Default-IP-Routing-Table)
  Not advertised to any peer
  65000 65002 65001, (received & used)
    6.1.1.1 from 6.1.1.1 (5.2.2.2)
      Origin IGP, localpref 100, valid, external, best

My question is:

If both paths are valid, why doesnt both of them show up with one of them being the best path and the other being the prepended path?

1 REPLY
Hall of Fame Super Silver

Re: BGP question, prepending route and results

Hello Richard,

if this is a L3 MPLS VPN only the PE nodes of service provider sees the prepended path, they do a best path choice on behalf of your CE routers and propagate only best path to your CE nodes at your VRF sites.

You can see the prepended path only if the primary path disappears, you are not peering BGP with your own routers but with PE nodes so you cannot demonstrate the existance of the backup prepedended path from your routers.

in this case you are prepending on HQ eBGP session with SP but not on BGP session with DR. By the fact you are using different AS numbers at DR ad HQ, so DR site is allowed to advertise what it receives on eBGP session with HQ on eBGP session with SP PE node.

Other sites see the effect of the change you did but indirectly.

Hope to help

Giuseppe

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