BGP peering strategies for smaller routers

James Milko jmilko at
Mon May 2 20:05:53 UTC 2016

On Mon, May 2, 2016 at 3:07 PM, Mike <mike-nanog at> wrote:

> Hello,
>     I have an ASR1000 router with 4gb of ram. The specs say I can get '1
> million routes' on it, but as far as I have been advised, a full table of
> internet routes numbers more than 530k by itself, so taking 2 full tables
> seems to be out of the question (?).
>      I am looking to connect to a second ip transit provider and I'm
> looking for any advice or strategies that would allow me to take advantage
> and make good forwarding decisions while not breaking the bank on bgp
> memory consumption. I simply don't understand how this would likely play
> out and what memory consumption mitigation steps may be necessary here. Im
> open to ideas... a pair of route reflectors? selective bgp download? static
> route filter maps?
> Thank you.
> Mike-
You have to keep in mind there are two pools of memory on the router.  The
RIB and the FIB.  The RIB contains all the routes that the router could
possibly load.  This includes all your BGP routes, even the ones not
selected as best, and all your IGP routes.  Whereas the FIB would only have
the best routes that the router is actively using to forward traffic.

FIB = 'sh ip route'
RIB = 'sh ip bgp' (or OSPF, or IS-IS, or RIP, etc, etc)

Generally FIB capacity is given as a number of routes and RIB capacity is
given as a memory amount.  Since the router manufacturer doesn't know what
protocols you'll be running or how many attributes you'll be storing they
don't really have a good idea of how many routes you'll get in the memory
the router has.

Now that you have a full table on the router, you won't see much growth on
the FIB.  For the most part every route you'll learn is already learned.
You'll have some growth because not every transit provider will advertise
the exact same table.  My router with 3 transits has 580k routes for

In short, you're fine.  Read up on RIB and FIB though so you have a good
handle on when you're about to start running into problems.


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