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Re: Multihomed to 2 ISPs - Load Balance?
> 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