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) 126.96.36.199 from 188.8.131.52 (184.108.40.206) 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) 220.127.116.11 from 18.104.22.168 (22.214.171.124) 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?
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.
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