[c-nsp] Outbound Load balancing using eBGP

[Jay Hennigan]

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