On Fri, Nov 04, 2011 at 03:39:43PM -0500, Michael Sabino wrote:
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".
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 -- Mark Kamichoff prox@prolixium.com http://www.prolixium.com/