Inferring the location points of traffic exchange between two networks

Wed Jan 13 18:02:40 UTC 2016

On 1/13/16 9:36 AM, Reza Motamedi wrote:
> Hi NANOG,
> 
> I am researcher at the University of Oregon and my question is rather
> primitive. My research background is in networked systems and Internet
> measurement so I know how things work in theory.
> 
> My question is about BGP and what can be inferred from the output of
> different "show" commands, regarding the point of traffic exchange of two
> networks with different ASNs. I tried going through the some samples on
> Juniper and Cisco documentations but I did not get my answer.
> 
> Consider the following scenario; Say the point of traffic exchange between
> AS_a and AS_b is in San Francisco and we run "show bgp summary" 

show bgp summary just tells you about your bgp neighbors.

> and "show
> ip bgp <prefix>"on a BGP router of AS_a in LA. Do we see the peering
> between AS_a and AS_b in San Francisco using any of the two commands.

You see AS path, and the nexthop the route was learned from (which is
probably (nexthop self) the router on which the prefix is learned) in
san francisco. that route is probably resolved by your igp.

so in an extremely simple example

       Network             Next Hop         Metric  LocPref Weight Path
 * >   8.8.8.0/24          72.14.202.50     96      56      0       15169 i

the nexthop happens to be an attached google peer

the as path is
15169 i

> If
> yes is there a way to infer that in fact the traffic is not exchanged
> locally in LA? I think there should be a flag to differentiate records
> showing iBGP vs eBGP.

If the router in LA sees the path as being through a router in san
francisco that is the direction it will forward it in.

> On the same note, if we issue the commands on a router other than the
> border router in San Fran, is there any difference in the output of show
> commands?
> 
> Now how are things different if we actually run the commands on that
> gateway router in SF?
> 
> Best Regards
> Reza Motamedi (R.M)
> Graduate Research Fellow
> Oregon Network Research Group
> Computer and Information Science
> University of Oregon
> 

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