Multihomed BGP on one router causing memory probs?

I have a multihomed BGP setup to two different ISPs. It is running on
my cisco 3640, which gets the full BGP routing tables. Lately, the
router has been having serious memory problems due to the size of the
BGP tables, and I would like to find a way to setup the following, if
possible:
Use ISP A for all traffic, and only download partial tables from ISP
A. If ISP A fails, then bring up ISP B and use that. I would like to
do this temporarily on one router until we can get a second one. Is
this possible? I've tried to conjur up any way to handle this, but in
all cases it seems that I need to have the full BGP tables from both
ISPs because I need both interfaces up.

I have one solution that I do NOT want to implement, because it seems
like a very poor solution, but it's all I can think of: issue a
shutdown to ISP B's serial interface. Manually monitor the connection
to ISP A, and if it fails, manually shutdown ISP A and no shut ISP B.
I could even have a program do this for me from within my network, but
again, this seems like a poor solution when I have a nice Cisco router
that seemingly should be able to handle doing this for me... maybe
not?

ANY help is GREATLY appreciated, since my company's internet drops out
about 5 times a day due to the router reloading from memory problems.

PS: I've got all the ram I can throw in this thing, and we can't
afford to buy totally new routers (nor think we need to)

 

Hi,

Any reason you need the full routing tables from your isps? Since you can
live with shutting down one of the interfaces, it seems to me the only thing
you're interested in to be always connected to the internet. If this is the
case, setup BGP to only get a default route from your provider. If one fails
the second will still be there and you'll be fine. Also, 2 default routes
will certainly not give you any memory problems.
If you also planned on doing some kind of load balancing, try to get partial
routes from both providers, but not the full routing table.

Erik

 

Their is obviously some specific reason that you are loading the full
Internet routing table in the first place. Can you advise the group to your
reason behind this and then we may be able to help in a way that achieves
your goal.

The only reason I can think of is that when certain routes are available
from one ISP only then this is the ISP you would use for that particular
path. In which case I feel you will need to cough up and upgrade.

Toby

 

There are numerous alternatives to running defaultless with BGP.
Depending upon the ISPs, you may find it effective to just take
local routes from each and load share the default route between
the two (but be careful, because some ISPs have huge local tables).

My favorite approach is to set my default route to a few key prefixes
upstream of both providers. That way, even if the POP stays up,
I'm protected against a ISP which gets disconnected from the rest
of the world.

Since you feel you can live with just one link working at a time,
your solutions can be much more flexible. Getting reliable failover
and load balancing at the same time can be tricky.

There is a brief white paper on my website which describes some of
the many options for multihoming with less than monster routers.

Good luck and have fun!