[187016] in North American Network Operators' Group
Re: Inferring the location points of traffic exchange between two
daemon@ATHENA.MIT.EDU (joel jaeggli)
Wed Jan 13 13:57:38 2016
X-Original-To: nanog@nanog.org
From: joel jaeggli <joelja@bogus.com>
To: Reza Motamedi <motamedi@cs.uoregon.edu>
Date: Wed, 13 Jan 2016 10:57:32 -0800
In-Reply-To: <CACGuEhG8JpneDc6VstMJx9D9qVorRCd42Tyhm6bpGoMgju-Nww@mail.gmail.com>
Cc: nanog@nanog.org
Errors-To: nanog-bounces@nanog.org
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On 1/13/16 10:15 AM, Reza Motamedi wrote:
> Thanks Joel. I like examples. :)
>=20
> So say I issue the command on a router that is not the gateway. Would I=
> get the following?
>=20
> Network Next Hop Metric LocPref Weight Pat=
h
> * > 8.8.8.0/24 <http://8.8.8.0/24> <IP in AS_a> 96 =20
> 56 0 <AS_a> 15169 i
It should be the nexthop self (loopback ip) of the originating router,
unless you don't do it that way and your provider numbered interfaces
are passively included in your igp.
> With respect to "show bgp summary", if I know the location of the route=
r
> and the router shows the BGP neighbor in the output, can I just rely on=
> this info and say the point of exchange is where the router is located?=
> For example the following show output from a router in city say "X"
if you elide the existence of long-haul-paths, distributed exchange
fabrics, ebgp multihop sessions, l2 vpn and so on. it is certainly not
the case with ibgp sessions which could include things like route
reflectors. topological adjacency might imply proximity but it's not an
assurance.
> BGP4 Summary=20
> Router ID: 192.65.184.1 Local AS Number: 513
> Confederation Identifier: not configured
> Confederation Peers:=20
> Cluster ID: 513
> Maximum Number of IP ECMP Paths Supported for Load Sharing: 4
> Number of Neighbors Configured: 18, UP: 18
> Number of Routes Installed: 997637, Uses 85796782 bytes
> Number of Routes Advertising to All Neighbors: 2196009 (569816 entrie=
s), Uses 27351168 bytes
> Number of Attribute Entries Installed: 305962, Uses 27536580 bytes
> Neighbor Address AS# State Time Rt:Accepted Filte=
red Sent ToSend
> 62.40.124.157 20965 ESTAB 76d23h58m 140497 0 =
28 0 =20
> 83.97.88.33 21320 ESTAB 49d 5h11m 0 0 =
28 0 =20
> 192.65.184.2 513 ESTAB 365d12h24m 243346 0 =
493626 0 =20
> 192.65.184.3 513 ESTAB 405d12h31m 7010 0 =
562695 0 =20
> 192.65.184.4 513 ESTAB 317d 9h 1m 0 0 =
569704 0 =20
> 192.65.184.24 513 ESTAB 54d16h26m 0 0 =
569704 0 =20
>=20
> tells me that 513 is peering with 20965 that city, right?
>=20
> Best Regards
> Reza Motamedi (R.M)
> Graduate Research Fellow
> Oregon Network Research Group
> Computer and Information Science
> University of Oregon
>=20
> On Wed, Jan 13, 2016 at 10:02 AM, joel jaeggli <joelja@bogus.com
> <mailto:joelja@bogus.com>> wrote:
>=20
> On 1/13/16 9:36 AM, Reza Motamedi wrote:
> > Hi NANOG,
> >
> > I am researcher at the University of Oregon and my question is ra=
ther
> > primitive. My research background is in networked systems and Int=
ernet
> > 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 exchang=
e of two
> > networks with different ASNs. I tried going through the some samp=
les on
> > Juniper and Cisco documentations but I did not get my answer.
> >
> > Consider the following scenario; Say the point of traffic exchang=
e between
> > AS_a and AS_b is in San Francisco and we run "show bgp summary"
>=20
> show bgp summary just tells you about your bgp neighbors.
>=20
> > and "show
> > ip bgp <prefix>"on a BGP router of AS_a in LA. Do we see the peer=
ing
> > between AS_a and AS_b in San Francisco using any of the two comma=
nds.
>=20
> 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.
>=20
> so in an extremely simple example
>=20
> Network Next Hop Metric LocPref Weight =
Path
> * > 8.8.8.0/24 <http://8.8.8.0/24> 72.14.202.50 96 =
> 56 0 15169 i
>=20
> the nexthop happens to be an attached google peer
>=20
> the as path is
> 15169 i
>=20
> > If
> > yes is there a way to infer that in fact the traffic is not excha=
nged
> > locally in LA? I think there should be a flag to differentiate re=
cords
> > showing iBGP vs eBGP.
>=20
> 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.
>=20
> > 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 t=
hat
> > gateway router in SF?
> >
> > Best Regards
> > Reza Motamedi (R.M)
> > Graduate Research Fellow
> > Oregon Network Research Group
> > Computer and Information Science
> > University of Oregon
> >
>=20
>=20
>=20
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