Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

Spanning-tree port priority when more than 255 ports

Hi

Just rereading my spanning-tree stuff and I noticed that the port priority command is 8bits.8bits where the first 8 bits is priority and can be set and the second 8 bits is the position of the port on the switch i.e. fa0/10 would give 128.10 as the port priority, 128 being the default.

So far, so good. what happens when you have more than 255 ports in the system. E.g. a 6513 with dual supervisors and 10 48 port copper cards. How does the system work out port priority then?

Sorry I don't have one around to test it out myself. Would appreciate an answer though.

Thanks

Phil

1 REPLY

Re: Spanning-tree port priority when more than 255 ports

In that case we will steal some bits from the user configurable priority to encode more ports. I think it went into the standard as part of 802.1t (along with extended sysid). Right now, on switch ports for instance, we already allocated 4 bits to the port number (which is now 12 bits) and the configurable priority is by increments of 16 (because 4 bits were removed).

If we had to support more than 4k ports, we would have to reduce even further the number of bits left for the priority.

Regards,

Francois

196
Views
5
Helpful
1
Replies