Spanning-tree port priority when more than 255 ports

[rtwwpad]

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

[Francois Tallet]

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