Friday, 5 February 2010

Cisco Gotchas - 1800 Virtual Ports

This is a (very late) addition to the article here.

Cisco 6500 doesn't directly limit the number of spanning-tree instances. It has limits on the number of virtual ports per line card.

A virtual port is a VLAN being forwarded on a trunk. So if you have 10 VLANs and 5 active trunks with no pruning then you have 50 virtual ports.

A 6500 is limited to 1800 virtual ports per line-card (with caveats, see below!).

If you have a full 48-port linecard and use PVST then you can only create 37 VLANs before you hit the limit. If you intend to go over this then either manually prune some VLANs or upgrade.

Some of the SX IOS versions have removed the virtual port limit on the newer linecards (6500 and 6700 series), however the per-chassis limits still apply.

The per-chassis limits are shown here.

They are 10000 for RPVST+ and 13000 for PVST+, updated in IOS 12.2(33)SXI to be 12000/15000 respectively.

If you want to get extra lifetime out of older linecards then look into using MST as it drastically simplifies the forwarding. Unless you have a need for a different STP topology for every VLAN then it's a great idea.

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