Multihomed to 2 ISPs - Load Balance?

Mon Jun 26 06:28:41 UTC 2006

> you could leak from BGP to 'igp' and make sure you have both paths in the
> IGP. NOTE: this is possibly very dangerous... you've been warned sorta :)
> (why dangerous? something breaks in your leak mechanism and you drop 'full
> internet routes' on ospf/eigrp/isis... network go boom! it is fun to
> watch though.)

Well leaking BGP into IGP is oooooooouutttta question for obvious reasons !!! :-)

> >> install EBGP ECMP routes, how do we advertise this information to our
> >> downstream peers? As far as my working knowledge of BGP4 goes, it wouldnt
> >> let me do this.

> correct... BGP selects 'best path' and sends that along to it's neighbors.
> There is a flag on one vendor I believe to force it to send 'all paths',
> but this is also dangerous, or could be if misused. Perhaps someone who's
> used that feature could speak up?

I would too be interested in this!

>> >> I wish to understand how other network operators do this?
>>
>> > You don't, not usually anyway.  You advertise the best path to your
>> > downstreams.  If you want to 'load balance' per packet or otherwise to one
>> > or more upstreams that's an internal/your AS decision only.  There's
>> > nothing to tell the downstreams about from BGP's point of view.
>> >
>>  I think there is a need to tell my downstream peers about ASes the
>> traffic is gonna go through.

> There isn't a facility in bgp to tell a neighbor more than one possible
> aspath... or not one that most network folk use currently.
> 
> I suppose for a subset of routes you might hack up some community based
> solution, but it'd be a horrible hack, and it'd cause you to keep churning
> your router configs on a very regular basis as things up stream changed.
> 
> If the downstream has a connection only to you does it matter where they
> send packets? everything has to go through your AS to get anywhere...
> right? If they have a multihomed solution (you and another isp) they are
> going to have to decide on some other internal metric (interal to them
> based perhaps on non-routing-table information, like 'john has a oc-12 to
> provider-Y, Jim only has a T1.... send to John!') whete to send traffic.

I dont think its as simple as this. In the simplest case, assume that my downstream peer has a policy to reject all routes that traverse AS 20. Now i am splitting all load across AS 10 and AS 20, while i tell him that i'll be only sending the traffic through AS 10. This can create problems for my downstream peers, and in the worst case can lead to blackholes/loops.

>>
>>  I'm thinking wildly, and it may not make a lot of sense but heres the
>> scenario i have in mind: You load balance (per stream which is usually

> per flow... is the normal terminology I think, but sure.

Blame it on the lack of coffee and early morning blues .. it happens sometimes ! :-)

>> what most of the vendors do) and you distribute your traffic through
>> ASes 10 and 20. Now you are advertising only one BGP path, say the one
>> through AS 10. Isnt this a problem? Isnt "Advertise what you yourself
>> use" one of the basic shibboleths of BGP or routing for that matter?
>

> BGP will only pick 'one best path', So, unless you did some local static
> or IGP based thing (see the leak suggestion above) you'll only really be
> using one path to AS10 or AS20, and only be sending internally (then
> externally on the other side of the network) one path.
> 
> If you were sending to AS10 initially and that link failed or otherwise
> became 'worst path' you churn on your edge then ship an update with new
> path info along to your ebgp peers... They have to then churn and decide
> which path is 'best' and move forward. What benefit is there in sending
> them 2 paths? They still must remove a path and re-converge, eh? (if you
> could even send them 2 paths of course)

I dont mean to send them 2 paths! Its just a question that i am posing to the network operators and/or the vendors in this list !!

Cheers,
John

> 
> Oh, and to throw in another monkey wrench... if you really wanted to do
> this for some reason you COULD provide ebgp-multihop peers to your border
> routers to all customers (ebgp neighbors) that wanted this 'service'...
> again, this is messy and ugly, but it'd get them multiple copies of the
> same route, they could then decide on 'best path' based on this
> information. (this also is not recommended, just a thought)

> (glad someone atleast replied offline :) )

-Chris