bgp routes

[paulkilcoyne]

Hi guys,

I'm after adding a third WAN Router to our campus network, I've added to my iBGP peer group also and increased the LAN switch bgp

maximum-paths from 2 to 3.

My question is, is there a way to see all thew possible routes to the same destination network in bgp?

I have 3 wan routers so in the CEF table should it not show these three routes via all three Routers?

Is there a command I can run to see all of these?

Hope this is clear.

Thanks,

Paul

[Edison Ortiz]

show ip route [ip subnet] should give you the information you are after.

If the destination is load balanced, it should show 3 next hops on the output above.

[paulkilcoyne]

Hi Edison,

Thanks for your reply. I tried that but it only showed me the one route in the routing table:

Show ip route 10.x.x.67

This only shows the preferred route bgp has chosen, but there should be other available routes to this destination that are in the forwarding table, is there any way of viewing the forwarding table?

I have "maximum-paths 3" set pin my bgp config.

Any idea?

Thanks,

Paul

[Edison Ortiz]

Can you post the output from show ip bgp 10.x.x.67?

show ip route is your forwarding table and perhaps the routes aren't equal metric on BGP.

In order to install 3 routes with the maximum-path command the 3 advertising routers must have equal metrics.

[paulkilcoyne]

Hello again Edison,

here's the output you requested:

S1-network2#sh ip bgp 10.x.x.67

BGP routing table entry for 10.0.0.0/8, version 10642

Paths: (3 available, best #3, table Default-IP-Routing-Table, not advertised outside local AS)

Multipath: eBGP

  Not advertised to any peer

  65000 65529 65099 65171

    10.x.x.35 (metric 20) from 10.x.x.35 (10.x.x.35)

      Origin incomplete, metric 0, localpref 50, valid, internal

      Community: local-AS

  65000 65529 65099 65171

    10.x.x.9 (metric 20) from 10.x.x.9 (10.x.x.9)

      Origin incomplete, metric 0, localpref 50, valid, internal

      Community: local-AS

  19855 65529 65099 65171

    10.x.x.1 from 10.x.x.1 (10.x.x.1)

      Origin incomplete, metric 0, localpref 50, valid, internal, best

      Community: local-AS

This seems to show all three routes out of our network to our three mpls connections.

But when I do a show ip route 10.x.x.67 we only see one route:

S1-network2#sh ip route 10.x.x.67

Routing entry for 10.0.0.0/8

  Known via "bgp 65301", distance 200, metric 0

  Tag 19855, type internal

  Redistributing via ospf 65505

  Last update from 10.x.x.1 3w0d ago

  Routing Descriptor Blocks:

  * 10.x.x.1, from 10.x.x.1, 3w0d ago

      Route metric is 0, traffic share count is 1

      AS Hops 4

      Route tag 19855

Can you explain this?

Are we truely load balancing over our 3 MPLS connections?

Thanks,

Paul

[Edison Ortiz · Accepted solution]

Paul,

To configure BGP equal-cost multipath load sharing, all path attributes must be the same. The path attributes include weight, local preference, autonomous system path (entire attribute and not just the length), origin code, Multi Exit Discriminator (MED), and Interior Gateway Protocol (IGP) distance.

Per your output the first 2 entries are the same but the 3rd entry is different in the AS-PATH.

This 3rd entry was chosen as the best path since it has a lower RID.

You need to force one of the other routers to be the best path then it can qualify as multi-path for 2 links, not 3.

The quickest way to make this work would be changing the RID on 10.x.x.1 to a value higher than the other peers.

Other options are changing the WEIGHT on received BGP routes from the first 2 entries

Regards,

Edison

[ameya_oke]

Hey Paul,

If you want to do an equal/unequal cost load balancing you can go for bgp dmzlink-bw feature.

You will find its explanation on below link.

Even if you dont opt this solution, i would urge you to try this on GNS, results are amazing.

http://cciethebeginning.wordpress.com/2009/01/19/bgp-link-bw-multipath-load-balancing/

Ameya

[paulkilcoyne]

Hey Guys,

thanks very much for all this info.

So all being equal in BGP it comes down to the lowest Router ID?

Many thanks,

Paul

[Edison Ortiz]

Yes,

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml

step 11.