Cisco Support Community
Community Member

spanning tree , end port costs

Hi Guys,

I'm just wondering, the previous network administrator here has setup RSTP, and has devided half the ports (odds) and the other half (evens) into seperate costs. So odd ports get cost 19 and evens cost 100.

The ports all connect to voip phones (which have internal switching) and PCs (which are portfast enabled).

Why has this been done, I can't think of a reason at all.

The PCs with portfast enabled are just useless because they dont participate in cost decisions....

Is there something he knows, I dont?

VIP Super Bronze

spanning tree , end port costs

Hi Mina,

I don't see any reason for changing port cost on interfaces connecting to phones and PC. However you should always enable portfast on all your user interfaces.  If your ports are all fast Ethernet, the cost should be 19 for all.



Community Member

spanning tree , end port costs

you mentioned that portfast should be enabled on all ports facing users, this excludes phones as far as i know as phones contain switching ports.

Cisco Employee

spanning tree , end port costs

Just small guess.

Port not being configured with port fast - will send TCN BPDU on interface flap. Possibly assigning different corts old admin split ports in two group to locate the group on which port is flapping from the root bridge which will receive TCN.

Again - just a guess, never seen that before.


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