TCAM

[vishalpatil86]

hi,

what is TCAM??? I found it very hard to understand...

pls help on this                  

[Edison Ortiz · Accepted solution]

Stands for Ternary Content Addressable Memory.

Think of it as a dedicated hardware used for a specific service or multiple services.

For instance, in the 6500 - ACLs are stored in TCAMs.

This allows the switch to perform faster because ACLs are not consuming shared

resources and have dedicated hardware for it.

The drawback from having TCAMs is their limits.

Once you go over the limit, the device will run those services in software causing high CPU conditions.

[vishalpatil86]

hi edison,

sorry for replying late... i have one more doubt...

ACcording to ur explanation about tcam, this is only for ACLs?

and which switches support this TCAM?

[Ivan Shirshin · Accepted solution]

Hi Vishal,

TCAM is used on all Catalyst switches + Nexus for L2 Forwarding, L3 Forwarding, QoS ACLs.

Kind Regards,
Ivan

**Please grade this post if you find it useful.

[vishalpatil86]

hi ivan,

can u explain with the help of example? if it is possibe for you...

i have 4506e switch.. how can see the tcam configuration?

[Ivan Shirshin · Accepted solution]

Hi Vishal,

CAM table is a special type of memory used by Cisco switches. In the case of ordinary RAM the IOS uses a memory address to get the data stored at this memory location, while with CAM the IOS does the inverse - it uses the data and the CAM returns the address where the data is stored. Also the CAM is considered to be faster than the RAM since the CAM searches the entire memory in one operation.

The problem with CAM is that it can only do exact matches on ones and zeros (binary CAMs), and here TCAM (Ternary CAM) comes, since it can match a third state which is any. This makes TCAM a very important component of Cisco layer 3 switches and modern routers, since they can store their routing table in the TCAMs, allowing for very fast lookups, which is considerably better than routing tables stored in ordinary RAM.

There is a good explanation of TCAM in the BSCN book (see extract from it in attached file).

PS: check the following guide if you suspect a problem with TCAM on 4500:

http://www.cisco.com/en/US/products/hw/switches/ps663/products_tech_note09186a008054a499.shtml#types

Kind Regards,
Ivan

**Please grade this post if you find it useful.

[vishalpatil86]

thanks ivan, this helped me a lot

[Ivan Shirshin]

Welcome, Vishal!

[Kishore Chennupati]

loved your explanation Ivan. 5+ to you

[Ivan Shirshin]

thanks :-)