show spanning-tree vlan command

Answered Question
Jul 13th, 2008

hi everybody

could any body help explain the following:

show spanning-tree vlan 4 commands show alot of information such as designated cost. what is designated cost?

thanks a lot !

I have this problem too.
0 votes
Correct Answer by jamesl0112 about 8 years 6 months ago

It is the cost to reach root from this bridge. The root bridge transmits a BDPU with a cost of zero. It is only incremented when it is received and sent on by the next switch.

For instance, if the switch that receives the BPDU is connected directly to the root bridge, the designated cost will be zero:

Port Designated

Name Port ID Prio Cost Sts Cost Bridge ID Port ID

------- ------- ---- ---- --- ---- -------------- -------

Fa0/22 128.1 128 3010 FWD 0 000a.4191.7900 128.65

If you have a designated cost of 12, then perhaps there is another switch in between your switch and the root switch, that is connected to the root via a fastetherchannel.

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bvsnarayana03 Mon, 07/14/2008 - 03:25

Cost is one of the metrics used in determining the root port. there are predefined values of cost based on port speed. for e.g. 100mbps link has a cost of 19, 1 Gbps link has a cost of 4. Spanning tree choses the port with lowest cost as root port, in the event when BPDU's arive on multiple ports from a single switch.

sarahr202 Mon, 07/14/2008 - 08:40

thanks for your reply! when i use the command show spanning-tree vlan 3, it shows the interface associated cost and state for example: port id cost

fastethernet 0/1 128.1 19

designated cost bridge id

12 32180009.b7ee.9800128

what is designated cost here?

thanks a a lot !

Giuseppe Larosa Mon, 07/14/2008 - 11:45

Hello Sarah,

an STP port can be in forwarding state if:

a) it is the root port (the port to the root bridge)

b) it is the designated port for the network segment to which it is connected

The designated port is elected using the same rules that are used to choice the root port.

So the designated cost should be the lowest cost heard or sent out of an STP port.

It is so, see the following link:

http://www.cisco.com/en/US/docs/ios/12_3t/lanswitch/command/reference/lan_s1gt.html#wp1022593

Designated Cost

Designated cost for the path

Designated Bridge ID

Bridge identifier of the bridge assumed to be the designated bridge for the LAN associated with the port

Hope to help

Giuseppe

sarahr202 Mon, 07/14/2008 - 13:49

Thanks for your reply Giuseppe! The link you forwarded explain "designated cost" as designated cost for the path .

what kind cost is that?

show spanning-tree vlan #

shows the stp cost associated with interface, but also show designated cost.

for example int f0/2 128.1(port id) 19(stp cost for 100 megabit sec link)

fwd (stp port state) 12(desinated cost)

5666.5666.8990.0000(bridge id).

what is designated cost of 12 here?

thanks a lot ! I would really appreciate if anyone could explainit.

Correct Answer
jamesl0112 Tue, 07/15/2008 - 01:20

It is the cost to reach root from this bridge. The root bridge transmits a BDPU with a cost of zero. It is only incremented when it is received and sent on by the next switch.

For instance, if the switch that receives the BPDU is connected directly to the root bridge, the designated cost will be zero:

Port Designated

Name Port ID Prio Cost Sts Cost Bridge ID Port ID

------- ------- ---- ---- --- ---- -------------- -------

Fa0/22 128.1 128 3010 FWD 0 000a.4191.7900 128.65

If you have a designated cost of 12, then perhaps there is another switch in between your switch and the root switch, that is connected to the root via a fastetherchannel.

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