BGP Memory Usage

Unanswered Question
Feb 6th, 2008
User Badges:

I am sure this has been discussed many times, but I will ask again. I have two 3725 routers which comprise my network edge. The two routers are running an iBGP session with a common ASN. Each router is connected to a different ISP and each is accepting a full route feed. As I understand it, the current memory recommendation for a single full BGP feed is a router with at least 256MB of memory. My first question is this. Each router has 256MB of memory. Since they are trading routes via iBGP, is there a need to have enough memory available to handle a third feed? Better put, since each router has two BGP neighbors, does each router need to have enough memory to support dual BGP feeds? In my estimation that would be more like 512MB or memory to do this. These routers have run well for a long time but have recently started to have memory allocation errors. I think things are changing such that the route tables are just not being handled by 256MB of memory anymore.

I know you can implement AS path filters inbound to only accept your ISPs routes and the routes of their directly connected ASs along with a default route to reduce the size of the BGP/forwarding tables. One thing about this approach is that one router is set up really like a backup as it does not have the outbound BW to handle a ton of traffic. The primary router has a DS3 link so obviously thats the one I want traffic to move through. My second question is this. By going with the filtered approach, it seems to me that since both routers no longer have a full feed and just a subset of that feed, much of my outbound traffic will be sent over the minimal BW link and I will not be able to control it with any success. Is this correct? And is there a better way to reduce BGP memory usage? BTW I am using AS path pre-pending to wieght the path through my secondary router so that majority of my traffic is sent through the primary inbound from the Internet. Just looking for some advice on the best way to reduce the BGP memory usage so I can avoid buying new gear.

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Joseph W. Doherty Wed, 02/06/2008 - 18:53
User Badges:
  • Super Bronze, 10000 points or more

Assuming the primary consideration is link bandwidth rather than optimal AS hops, you might want to look at OER/PfR doing just dynamic link balancing.

Lars Persson Tue, 02/19/2008 - 01:05
User Badges:


In your case you could make it really easy.

Just accept a default from each connection and use local preference to set the primary path as preferred. This will make it a primary/fail over link.

If you want to use your fail over link you can choose to accept some prefixes over this link.

/best regs Lars

Danilo Dy Tue, 02/19/2008 - 02:38
User Badges:
  • Blue, 1500 points or more


WAN links are expensive (and its a recurring cost) putting the second feed as a full backup by receiving only default routes is not recommended. Increasing the memory or even replacing the router with a higher memory is only a one-time cost.

The number of prefix over the years increase to 240+K and so the IO Memory as more modules/interfaces are added, definitely 256MB base memory is not enough. 512MB (the maximum for 3700 series routers) is more decent.

You need to upgrade your 3725 ROMMON to 12.3(6r) to be able to detect/support 512MB.

Take note of the 3700 maximum WAN throughput though. There's no point upgrading the memory if your total bandwidth is more than the router can support without dropping packets. I think MIERCOM have a benchmark for the WAN throughput

Just keep this in mind, one or more feed with full BGP route, better use 512MB as minimum nowadays (the 256MB minimum is a thing in the past). So you don't have to compute IO, IGP, Prefixes, and other processing who need a share of the Memory pie :).



rfranzke Tue, 02/19/2008 - 06:54
User Badges:

Thanks for your reply. This has been the issue for me as I think more memory is the way to go. BGP tricks, or acceping defaults from your provider is fine, but in the end you will eventually need more memory. Frees you to truly optimize your local routing. I was unaware there was a 512MB memory upgrade available for the 3725. I have seen documentation for 3745 512MB upgrades but cannot find any to support that there is one available for the 3725. Can you confirm that there is such an upgrade available? New routers are on the books but for now this would buy me some time until we get them in. I have had to turn off IP CEF for the time being just to get the boxes to run. As far as our throughput, its pretty minimal. We have a DS3 but are only using 10MB of it, so I don't think we will have issues there. Thanks for the reply and any confirmation on the 512MB upgrade would be much appreciated. Thanks again


Danilo Dy Tue, 02/19/2008 - 07:04
User Badges:
  • Blue, 1500 points or more

Oops. Yeah, its 3745 that supports 512MB not 3725. My apology.




This Discussion