Dual Homed BGP for failover

George Bonser gbonser at seven.com
Tue Jan 18 12:59:55 CST 2011

> From: Ahmed Yousuf 
> Sent: Tuesday, January 18, 2011 10:32 AM
> To: nanog at 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


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