Route server: Route-server.ip.att.net

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1

Hi,

Could you give me the relevant configs explaining why when I traceroute to
12.83.43.9 on route-server.ip.att.net, the first hop is "
j6300.cbbtier3.att.net (12.0.1.202)". However, when I type "show ip route
12.83.43.9", the RIB shows, "* 12.122.83.91, from 12.122.83.91, 7w0d ago".

I asked someone who is knowledgeable about the matter, and he seems to
think that you can change the interface which sends back ICMP unreachables,
but I don't know how to do this on my own simulated equipment.

Also, I have noticed that when I traceroute to any ip address on the
internet from my home connection, the last hop that's in common with all
traceroutes is 12.83.43.9. This is a hop after several hops which seem to
be filtered. What is the purpose of this IP?

Are there any publically available documentation that would help me
understand the process of aggregating multiple DSLAMs, etc on my at&t
u-verse connection?

I am a CCNA/CCNP student in college and this would help me understand WANs
better.

Thanks

Michael R. Sabino
michael.rocco.sabino@gmail.com

2

Did you do a show ip route for 12.122.83.91? It's probably a loopback of
the nearest BGP peer it may not be the actual next hop interface IP
though. Not sure about the blocked hops, but I can think of a few
explanations. Overall the point of that router is to provide a view of the
route table and not the physical hops from one point to another. Since
actual customer traffic wouldn't flow through the route server the first
few hops are probably irrelevant.

3

A couple things here:

12.122.83.91 is the BGP next-hop in the RIB. It needs to be resolved.
In this case it's being resolved via a /13 static route:

route-server>sho ip route 12.122.83.91
Routing entry for 12.120.0.0/13
  Known via "static", distance 1, metric 0
  Redistributing via bgp 65000
  Advertised by bgp 65000
  Routing Descriptor Blocks:
  * 12.0.1.1, via GigabitEthernet0/1
      Route metric is 0, traffic share count is 1

In real life it'd probably be resolved via an IGP such as OSPF or IS-IS,
but this is a route server, not a transit router.

So, the real next-hop is 12.0.1.1. You can also verify this with the
following, since it's a Cisco box:

route-server>show ip cef 12.83.43.9
12.0.0.0/9
  nexthop 12.0.1.1 GigabitEthernet0/1

However, you don't see 12.0.1.1 in the traceroute because it looks to be
the VRRP address of the Juniper J6300 upstream router (just judging by
the hostname):

route-server>sho arp 12.0.1.1
Protocol Address Age (min) Hardware Addr Type Interface
Internet 12.0.1.1 81 0000.5e00.0101 ARPA
GigabitEthernet0/1

The MAC address is a giveaway that it's VRRP, since 00-00-5E-00-01 is
reserved by IANA for VRRP (IPv4 only):

http://tools.ietf.org/html/rfc5798#section-7.3

The Juniper router will send back ICMP TTL-exceeded messages from the
real IP on its interface, which appears to be 12.0.1.202.

Hope this helps.

- Mark