[c-nsp] Limits on dynamically learned MAC addresses per vlan

[Phil Mayers]

On 26/01/10 16:34, Steven Pfister wrote:
> I'm trying to troubleshoot connectivity problems between a virtual
> server at a central site and PCs in the same vlan at a remote site.
> At the central site is several VMWare servers connected to a 3560
> switch. The PCs at the remote site need to reach this virtual server,
> and while most do, some have trouble seeing it.
>
> A common thread seems to be whether the 3560 has been able to learn
> the MAC address of the PC. If it hasn't, we can put in a static
> address and everything is OK.
>
> I'm wondering... is there a limit to the number of dynamic MAC

Yes. Obviously all devices have a hard limit based on RAM or hardware 
CAM size. Your actual limit may be lower than the theoretical max - see 
below.

> addresses that a switch like the 3560 can learn? If so, can that
> limit be changed, and is the situation the same for static

Maybe

> addresses?

Maybe.

I've not used the 3560, but on 3550 and 3750 there is a concept of a 
thing called an "SDM" template, which partitions the hardware resources 
into certain sized buckets depending on the use of the switch.

e.g. one of ours says:

rt1#sh sdm prefer
  The current template is "aggregate IPv4 and IPv6 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:                  2K
   number of IPv4 IGMP groups + multicast routes:    1K

I'm not sure how static MAC entries are handled, but I'm willing to bet 
they'll take precedence over dynamic ones, so adding a static might 
"push" someone else out.

Other templates have numbers like:

rt1#sh sdm prefer access
  "aggregator access IPv4" 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

...or:

rt1#sh sdm prefer vlan
  "aggregate vlan" 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:                  12K


What does:

sh mac address-table count

...say?

If your device does need the SDM template changing, it will need a 
reboot to take effect I'm afraid.