[c-nsp] Outbound Load balancing using eBGP

[RAZ MUHAMMAD]

Hi Jay,

Many thanks for providing a practical example and a good piece of advice on
using default routes for dynamic load balancing.
Raz


On 22 December 2010 23:15, Jay Hennigan <jay at west.net> wrote:

> On 12/22/10 2:33 PM, RAZ MUHAMMAD wrote:
>
> > I would appreciate if someone can shed some further light on using the
> > default route or full routing table scenario while multi homed. In this
> case
> > hardware is not an issue, I am trying to assess the operational,
> > differences, or the outcome in terms of traffic patterns.
>
> Outbound is easier than inbound.  In general, use a route map to set
> local preference or another attribute based on as-path and apply to each
> neighbor.
>
> Say you're multi-homed to AS100 and AS200.
>
> You would do something like:
>
> ip as-path access-list 100 deny _200_
> ip as-path access-list 100 permit _100$
> ip as-path access-list 100 permit _100_[0-9]+$
> ip as-path access-list 100 permit _100_[0-9]+_[0-9]+$
>
> ip as-path access-list 200 deny _100_
> ip as-path access-list 200 permit _200$
> ip as-path access-list 200 permit _200_[0-9]+$
> ip as-path access-list 200 permit _200_[0-9]+_[0-9]+$
>
> Then towards your AS100 neighbor apply a route-map to bump local-pref to
> a value of 110 any inbound announcements matching as-path 100, likewise
> same on AS200 for as-path 200.  All else matches the default local-pref
> of 100.
>
> Other traffic will use the regular BGP metrics to choose a path.
>
> This sends your traffic to AS100 targets, its customers, and second
> level out the link to AS100 and likewise for AS200.  If you lose either
> link, the other will pick up all traffic.
>
> After a while you'll get a sense of how well balanced things are and you
> can tweak the lists to prefer one path or the other for portions of your
> outbound traffic to other networks.  For example, if AS200 is only
> taking 20% of your outbound traffic and you send quite a bit to AS300,
> then add a permit to as-path list 200 to prefer sending AS300 traffic
> out that path.
>
> Don't try to dynamically load-balance individual flows between your two
> neighbors.  You'll have horrible issues with packets out of order and
> things will get very ugly.
>
> You'll never get anywhere close to an exact 50-50 balance and it will
> vary a lot depending on what destinations become popular and unpopular
> with your customers at what time of day, etc.
>
> --
> Jay Hennigan - CCIE #7880 - Network Engineering - jay at impulse.net
> Impulse Internet Service  -  http://www.impulse.net/
> Your local telephone and internet company - 805 884-6323 - WB6RDV
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