[c-nsp] Sup720-3BXL load balancing

[Bruce Pinsky]

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Kristofer Sigurdsson wrote:
> Hi list,
> 
> I was wondering how a 7600 series router with a Sup720-3BXL handles
> load balancing between equal BGP paths.
> 
> I'm experiencing undesired behaviour in this area from at least two
> routers, both running IOS 12.2(18)SXF.  They seem to load balance
> extremely unevenly.  Same behaviour for the global routing table and
> within a VRF.  I've got an example from within a VRF:
> 
> 
> router#sh ip bgp vpnv4 vrf FOO x.x.9.0
> BGP routing table entry for <ASN>:<RD>:x.x.9.0/24, version 1002918
> Paths: (2 available, best #1, table FOO, not advertised to EBGP peer)
> Multipath: eBGP iBGP
>   Advertised to update-groups:
>      3
>   <Remote ASN>
>     x.x.x.142 from x.x.x.142 (y.y.14.2)
>       Origin incomplete, localpref 100, valid, external, multipath, best
>       Community: no-export
>       Extended Community: RT:<ASN>:<RD>
>   <Remote ASN>
>     x.x.x.146 from x.x.x.146 (y.y.14.100)
>       Origin incomplete, localpref 100, valid, external, multipath
>       Community: no-export
>       Extended Community: RT:<ASN>:<RD>
> 
> As you can see, the two paths are equal, but still, one of them is
> considered "best", but I've got BGP multipath enabled.
> 
>>From checking the routing table:
> 
> router#show ip route vrf FOO x.x.9.0
> Routing entry for x.x.9.0/24
>   Known via "bgp <ASN>", distance 20, metric 0
>   Tag <Remote ASN>, type external
>   Redistributing via ospf 104, ospf 103
>   Advertised by ospf 104 metric 30 metric-type 1 subnets route-map <BAR>
>                ospf 103 metric 400 metric-type 1 subnets route-map <BAR>
>   Last update from x.x.x.142 02:56:45 ago
>   Routing Descriptor Blocks:
>   * x.x.x.146, from x.x.x.146, 1d07h ago
>       Route metric is 0, traffic share count is 1
>       AS Hops 1
>       Route tag <Remote ASN>
>     x.x.x.142, from x.x.x.142, 02:56:45 ago
>       Route metric is 0, traffic share count is 1
>       AS Hops 1
>       Route tag <Remote ASN>
> 
> Still, a vast majority of the outgoing traffic (from us to them) goes
> over the link with the x.x.x.142 IP address.
> 
> I've been experiementing a little with this, it seems the majority of
> the traffic will go over the link which has the latest routing
> information (i.e. I reset the BGP session on one link, all the traffic
> goes to the other one while the session comes back up, then it all
> moves to the one I reset).
> 

BGP and the main routing table are used to establish the CEF entries (FIB)
that are actually used for forwarding decisions.  FIB will share based on
src/dst address.   What does CEF show for that route?

- --
=========
bep

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