I'm trying to figure out how some of this works in the real world, so I'll
provide a real example to start with. I just now did this on our router
that is multihomed to Sprint and Verio:
OurRouter#sho ip bgp 192.146.214.0
BGP routing table entry for 192.146.214.0/24, version 1230975
Paths: (4 available, best #3, table Default-IP-Routing-Table)
Not advertised to any peer
1239 568 721
160.81.116.1 from 160.81.116.1 (144.228.242.88)
Origin IGP, localpref 100, valid, external
2914
199.239.118.185 from 199.239.118.185 (129.250.53.129)
Origin IGP, metric 100, localpref 100, valid, external, best
Community: 190972314
The way I see this, both AS2914 (Verio) and AS721 are claiming to originate
this route. The fact that both origin codes are IGP adds to my confusion.
Wouldn't that mean that both are explicitly advertising this prefix,
probably via a network statement?
My first thought was that Verio was advertising an aggregate, but that is
not the case. This is a specific /24 with no aggregation.
In my limited experience I just can't see how this sort of thing would
happen, unless it was an accident.
Hmm... maybe AS721 is connected to Verio but not running BGP with them. If
they are using Verio-assigned addresses, then Verio might be playing nice by
advertising the /24 and not aggregating it, allowing return traffic to that
prefix via both paths. Then, if AS721 decided to run BGP with AS568 only,
this would explain the inconsistent origin AS.
Am I on the right track?
Thanks,
John
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