> From: Ahmed Yousuf > Sent: Tuesday, January 18, 2011 10:32 AM > To: nanog@nanog.org > Subject: Dual Homed BGP for failover > > > > - Is this really a good idea, as the BGP process won't care > what > the utilisation of the links are and you will see situations where the > lower > speed link gets used even though the high speed link utilisation is 0?
It is possible. But one thing, and I know it is a semantics nit but it is really important. There is no difference in the "speed" of the links. There is a difference in the capacity of the two but the traffic flows at the same "speed" across both. That said, have you actually tried seeing what the "natural" breakdown of the traffic is? Without any AS prepend or local pref adjustment, what is the natural ratio of traffic on the two links? Generally different ISPs have different connectivity and some destinations will be favored via one path and others via the other path. It might be useful to determine how BGP naturally routes things first and then you can get an idea of what needs adjusting. > > > - If we are doing this, I don't want to take a full routing > table, > I would rather just take the ISPs routes and perhaps their connected > customers. One ISP has said they will only provide full routing table > or > default. I really don't want to take a full table, is receiving > default > only going to be a problem for my setup? Interesting. Most ISPs offer "default", "full", or "customer routes". You can take a full table but simply filter out any that aren't from your ISPs ASN or within one hop of it and only install the routes that meet those criteria. In addition to using AS prepending, your providers might offer communities that allow you to control redistribution of your routing information to their peers. You might want to tell the ISP on the smaller link not to announce your routes to a major peer. That major peer will now find its path to you via the larger pipe. > > > - Any advice on how to avoid situations where the low > bandwidth > link is being used even though there is 0 utilisation on the high > bandwidth > link? If that happens, it would mean that the world does not see your path via the high bandwidth pipe as being an attractive path. As mentioned above, you might be able to append communities to your routes to the lower bandwidth ISP that control how they redistribute your routes. One example might be something like "don't redistribute my routes if you see them coming from another source" in which case that ISP only redistributes your routes when they don't see the announcement via the high bandwidth provider and effectively acts as a backup outside of their own AS but you would still receive traffic originated within their AS over the low bandwidth connection. > Ahmed G
