Networking with Cisco

If you don’t pay attention to the CAM, your network could face serious problems.

What is the CAM and Why is it important?
The CAM is short for Content-Adressable Memory and is a type of memory for high speed searching applications. Other names are associative memory or when programming; associative arrays.

The CAM makes it possible to make routing decisions in hardware instead of bothering the CPU, routes are placed in the CAM so that the linecard ASIC or FPGA hardware can look up which interface to send the packet out on somewhat directly from the memory. This decreases routing latency drastically and makes wirespeed performance possible.

Imagine how your router would perform without this now..

OK, Why is it important?
Because every router have a limited amount of physical memory, and this memory space has to contain IPv4 routes, IPv6 routes and everything you are (or want to do) in hardware.
This makes partitioning of this memory important.

You have different ways of doing this, but it mostly involves a reload of the router.

CAM Profiles
On Foundry routers it’s called CAM profiles, here are the basics:
CAM partition profiles for Release 03.1.00:

Profile	IPv4	IPv6	MAC	VPLS MAC	IPv4 VPN	IPv4/L2 Inbound ACL	IPv6 Inbound ACL	IPv4/L2 Outbound ACL	IPv6 Outbound ACL
IPv4	1M	0	32K	0	0	112K	0	64K	0
IPv6	64K	240K	32K	0	0	16K	24K	16K	12K
mpls-l3vpn	256K	0	32K	0	480K	64K	0K	64K	0

CAM partition profiles for Release 03.2.00:

Profile	IPv4	IPv6	MAC	VPLS MAC	IPv4 VPN	IPv4/L2 Inbound ACL	IPv6 Inbound ACL	IPv4/L2 Outbound ACL	IPv6 Outbound ACL
Default	512K	64K	64K	64K	128K	48K	4K	48K	4K
IPv4	1M	0	32K	0	0	112K	0	64K	0
IPv6	64K	240K	32K	0	0	16K	24K	16K	12K
mpls-l3vpn	256K	0	32K	0	480K	64K	0K	64K	0
ipv4-vpn	320K	0	0	32K	448K	64K	0K	64K	0
mpls-l3vpn-2	128K	0	32K	0	544K	64K	0K	64K	0

The Internet Routing table now have about 260K prefixes, so you should worry.

To check my CAM usage I use:

show cam-partition usage

On a Cisco 6500/7600 switch, you could use

show tcam details

When there are no more CAM space for a route, it will become unreachable.
So pay attention to your CAM/TCAM.

Tags: cam, cisco, foundry, memory, routing, tcam, wirespeed

This entry was posted on Thursday, September 18th, 2008 at 8:25 pm and is filed under Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.