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Antonio Macia

TCAM capacity on stacked switches

Hi there,

My organization have 3850 and 3750X switches in a stack fashion. Is the TCAM capacity of the master switch shared among all the switches in the stack or each one uses its own TCAM having more resources then?

Thanks in advance.


I have the same question


I believe the stack master.  Easy way to confirm is to issue the "show sdm prefer" command on stacks with various amounts of members.  As long as the sdm template is the same you will see the same output.  Also be sure that the master switches are the same model and code.

Here is an example of a 3750 switch stack with six switches.  The numbers below did not change when i ran the same command on a stack of two switches.

#sh sdm prefer

The current template is "desktop default" template.

The selected template optimizes the resources in

the switch to support this level of features for

8 routed interfaces and 1024 VLANs.

  number of unicast mac addresses:                  6K

  number of IPv4 IGMP groups + multicast routes:    1K

  number of IPv4 unicast routes:                    8K

    number of directly-connected IPv4 hosts:        6K

    number of indirect IPv4 routes:                 2K

  number of IPv4 policy based routing aces:         0

  number of IPv4/MAC qos aces:                      0.5K

  number of IPv4/MAC security aces:                 1K

David Hild

Here's the way I understand it:

The Active (master) switch in a stack takes on the same role as a SUP in a modular chassis, with the member switches acting as line cards. The stacking protocol functions in the same way as the backplane of a chassis, separating Control plane functions from the data plane.  As such, it is the Active switch which manages the resources for the entire stack. This much you probably already knew.

In the case of specific resources such as TCAM, the Active switch can choose to distribute this function across the other switch members as it sees fit in an effort to make sure no single switch becomes overburdened. The stacking protocol makes this possible by allowing the switch chips and CPUs on the member switches to be bound together with the Active switch through the control plane. I believe this distribution function was introduced with StackWise-480 on the 3850, by the way, so the 3750X stack will not function the same way, as Daryl DeAngelis illustrated for us.

It's a high-level explanation, but I hope you find it helpful. I also welcome anyone from Cisco BU to fill in holes or make corrections as necessary.

Antonio Macia

Thanks guys for the replies. It makes sense. Let's see how the ACL TCAM performs on each model.