[isp-bgp] Re: Outbound Balancing

[Joe Pruett <joey@...>]

you can also use med values as a way to make selections.  i use some magic 
with match lists to adjust values for just part of the prefixes.  
in the router bgp section i use:
 bgp always-compare-med

this is the access list:
ip access-list extended bgp-magic
 permit ip 0.1.0.0 255.254.255.255 any

this matches prefixes where the second byte is odd.  that kinda maps to
half of the prefixes accepted.

then my route maps look like this:

route-map l3-out permit 10
 match ip address bgp-magic
route-map l3-out permit 20
 set as-path prepend 27218

route-map l3-in permit 10
 set metric 2

route-map eli-in permit 10
 match ip address bgp-magic
 set metric 1
 set as-path prepend last-as 1
route-map eli-in permit 20
 set metric 3

so for the l3 link i do a prepend on outgoing traffic that doesn't match 
the access-list.  and for incoming i set the med to 2.  for eli, i don't 
do anything for outgoing, but  for incoming i set the med to 1 but add an 
extra hop to the as path for things that do match and set the med to 3 for 
things that don't match.

after all those convolutions, i get decent traffic balancing for my 
specific setup.  prefixes that match bgp-magic will look better for coming 
in on l3.  for outgoing, the ones that match are slightly more likely to 
go out l3 because of the extra path length on eli, but slightly better med 
there.

believe me, playing with bgp balancing is quite a dark art and can cause 
great amounts of frustration as you try to play with the very gross knobs 
that bgp gives you.  using access-lists like this was something i started 
doing a few years back and it has helped me tune things much more easily.







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