[c-nsp] Load balancing outbound traffic with BGP

[Jeff Chan]

Quoting Matyas Koszik <koszik at atw.hu>:

> You may want to try
>
> bgp bestpath as-path multipath-relax
>
> to achieve load-sharing accross the providers, with different (but equal
> length) as-paths. (Works for me like a charm in a situation similar to
> yours.)


Thanks, but it didn't seem to work for me.  I really should read the  
only paper I could find on the Cisco site mentioning it:

   http://www.cisco.com/univercd/cc/td/doc/solution/tdprpdgx.pdf


but perhaps I'm using it in an incompatible way:

router bgp xxxx
  no synchronization
  bgp always-compare-med
  bgp log-neighbor-changes
  bgp bestpath as-path multipath-relax
  bgp bestpath compare-routerid
  bgp dampening


Gert is right, I wasn't actually using localpref, but MED, since MED  
is considered after path length whereas localpref before.  localpref  
overrides path length, which indeed is way too powerful.  The shorter  
path should usually win.

Cheers,

Jeff C.


> On Tue, 26 Feb 2008, Jeff Chan wrote:
>
>> Hi All,
>> Given multiple, roughly equal upstreams (Sprint, ATT, Level3)
>> providing full BGP tables to a small ISP, what's the best way to
>> balance the outbound traffic?  The problem is that all else being
>> equal (path length, local pref, etc.) BGP decides to take the one with
>> the lowest peering IP address.  Given that the upstreams have
>> many/most of the same customers and peers, the peer with the lowest IP
>> address seems to win too often, meaning it does too much outbound
>> compared to the others.
>>
>> I asked the same question some time ago and the common practice answer
>> seemed to be prefer traffic for some other large networks (UUNet,
>> Qwest, AOL, etc.) over the peers with higher IP address.  Is this
>> still the case?  Seems kind of an ugly hack, but it works.
>>
>> Are there any other approaches?  How about jumbling up or staggering
>> the local preferences:
>>
>> ISP S:
>>
>> customers: localpref 120
>> peers:     localpref 110
>> others:    localpref 100
>>
>> ISP A:
>>
>> customers: localpref 110
>> peers:     localpref 100
>> others:    localpref  90
>>
>> ISP L:
>>
>> customers: localpref 100
>> peers:     localpref  90
>> others:    localpref  80
>>
>> Where S has the highest IP address, A next highest, L lowest.  Haven't
>> tried this; just a thought to try to compensate for the IP address
>> decision.
>>
>> Cheers,
>>
>> Jeff C.
>>
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>