BGP "newbie" question(s)

ardealului · ‎11-20-2008

hi guys, this is my first post with my first question:

Basically, can someone explain to me the bellow `sh ip bgp sum`s ?

The setup is quite simple: 2x 2811 routers that connect to 2x ISPs and that have themselves (the routers) set as iBGP Peers. Filters are only set on the out to isp (so that i don't become a transit AS), not between the iBGP peers. HSRP enabled on both internal int of the routers (BGP shouldn't care though about it)

Question1: x.x.x.76 gets 268025 prefixes (subnets) from neighbor r.r.r.r & x.x.x.96 gets 268323 prefixes (subnets) from s.s.s.s. Why does x.x.x.96 say that he gets only 119006 prefixes (routes) from x.x.x.76 and not the 268025 that x.x.x.76 from r.r.r.r ? Same for the other Router (x.x.x.96).

Question 2: shouldn't the maximum of bgp activity paths equal (example for x.x.x.96) to 268323+119006+1(=287330) and not the 4391402 that it reports ?

Question 3: A prefix is a subnet in the `sh ip bgp` table right ?

Question 4: A path is every row in the `sh ip bgp` table for a distinct prefix/subnet, right ?

Question 5: What are (also example for x.x.x.96) 316682/46726 & 4778733/4391402 ?

Question 6(most important): If these two routers are connected to 2 ISPs via 2 different bandwidth connections (let's say x.x.x.96 has more bandwith to the metropolitan network), how can i tweak bgp so that all traffic (incoming&outgoing) from/to metro IPs comes/goes via him and notx.x.x.76 ?

On x.x.x.76 i get:

BGP activity 280508/10567 prefixes, 887554/386303 paths, scan interval 60 secs

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd

x.x.x.96 4 <AS> 102061 89429 1454987 0 0 17:00:10 233225

r.r.r.r 4 <AS> 102290 57 1454938 0 0 00:28:27 268025

on x.x.x.96 i get:

BGP activity 316682/46726 prefixes, 4778733/4391402 paths, scan interval 60 secs

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd

y.y.y.y 4 <AS> 1683 69378 7085866 0 0 20:14:22 1

x.x.x.76 4 <AS> 802772 726567 7085920 0 0 16:58:53 119006

s.s.s.s 4 <AS> 238859 69264 7085866 0 0 20:14:20 268323

Giuseppe Larosa · ‎11-20-2008

Hello Ardealul,

Q1) some form of route filtering can be in action here outbound and/or inbound and the result is a lower number of prefixes received.

q2) multiple paths for a single prefix can exist the ratio paths / prefixes shows how much redundancy is in your BGP internet table.

q3) yes

q4) yes and see answer to point q2: multiple As paths can exist in the table for the same prefix

q5)

BGP activity...

Displays the number of times that memory has been allocated or released for a path or prefix.

see command reference

http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_bgp5.html#wp1016342

q6)

increase local-preference on router connected to higher speed link.

You can combine this with the sending of a better default route in your internal IGP.

if the ISP are different you can only influence return path by your own AS number prepending to ISP with slower link

if the ISP is the same you can use a lower MED at router with higher speed link.

Be aware that if ISPs are different return path traffic will not be zero in less preferred path

Hope to help

Giuseppe