[c-nsp] ebgp load balancing using maxiumu-paths TCAM impact on Sup720-3BXL?

Kevin Hodle kevin.hodle at gmail.com
Thu May 21 02:20:43 EDT 2009

Hi Peter,

  Another option for load balancing outbound traffic in your scenario
would be to do some netflow analysis on your upstream ports and have a
look at what the top destination ASNs your outbound traffic is flowing
toward. Using this data, you can construct as-path ACLs which you can
utilize in your inbound route-map on each upstream BGP session to set
a higher local-preference  for 'preferred' routes on each session (ie
routes from ASXXX get a local-preference 1 higher than your standard
upstream route local-preference), and accept the rest of the full
table on each session with your normal local-preference. Using your
netflow analysis you should be able to achieve a fairly equal traffic
split (as you will be able to see what % of your total outbound
traffic is going to which ASNs, use this data to come up with an
approximated 50/50 outbound traffic split) and you will still have
redundancy in place for all routes if one of the sessions drop. It
would take a little more effort than simply turning on multi-pathing,
but in your scenario it might be more ideal as you won't have to worry
about 3bxl TCAM constraints with this method.

Kevin Hodle

On Wed, May 20, 2009 at 9:40 PM, Peter Kranz <pkranz at unwiredltd.com> wrote:
> Setup is as follows; 2 edge routers, each with a BGP session receiving full
> routes to the same provider router. The provider is load balancing inbound
> traffic to our AS nicely, 50/50 between the edge routers.. I would also like
> to load balance the outbound traffic.. I've considered adding 'maximum-paths
> 2' to install the two equal paths, but an concerned about FIB TCAM impacts.
> Will adding this command cause each equal cost route to take one additional
> TCAM entry, i.e. full routing table x 2 > 524k TCAM limit = EPIC meltdown?
> Current FIB TCAM:
> L3 Forwarding Resources
>             FIB TCAM usage:                     Total        Used
> %Used
>                  72 bits (IPv4, MPLS, EoM)     524288      285506
> 54%
>                 144 bits (IP mcast, IPv6)      262144           5
> 1%
> Peter Kranz
>  <http://www.UnwiredLtd.com> www.UnwiredLtd.com
> Desk: 510-868-1614 x100
> Mobile: 510-207-0000
>  <mailto:pkranz at unwiredltd.com> pkranz at unwiredltd.com
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