spanning tree

[carl_townshend]

Does spanning tree primarily use the bandwidth to achieve its goals, will spanning tree block the slowest link back to the root bridge ? and also what is a designated switch ?

[ankurbhasin]

Hi Carl,

Designated Switch is the switch closest to the root switch through which frames will be forwarded to the root.

STP does not directly use bandwidth to achieve its goal. There are certail election criteria which goes on.

Incase of parallel link between 2 switches STP uses path cost which is a value depending upon type of interface like whether it is 10 MBPS port, 100MBPS port or GIG interface. If the 2 parallel links have same path cost then port id is used to find the fwding and blocking link.

Have a look at this links

http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/sw_ntman/cwsimain/cwsi2/cwsiug2/vlan2/stpapp.htm#xtocid69350

HTH, if yes please rate the post.

Ankur

[carl_townshend]

Hi Ankur, So is the designated switch the one with the best connection to the root bridge ? also what values does the cost have on the port ? and I always thought that if there were 2 links on being faster it would pick the fastest one ?

[ankurbhasin]

Hi Carl,

No designated switch is not the one with the best connection to the root bridge but it is the switch which is directly connected to the root bridge. The first switch going down from the root switch is the designated switch.

Switch1(root)--switch A---switchB---switch C

In this topology switch A is the designated switch.

Cost of port differs depending upon its bandwidth. If the port is ethernet port 10MBPS the cost will be 100 if the port is fast ethernet the cost will be 19 and if the port is 1 gig etherner the cost will be 4.

Check port cost values from this link

http://www.hojmark.net/stp-port-cost.html

Now as you said 2 links on being faster it would pick the fastest one you can relate this way for supopse there are 2 parallel links 10 MBPS and 100 MBPS so cost will be 100 for 10MBPS port and 19 for 100 MBPS port so lower the cost will be a preferred link which indirectly is related as faster the link preferred link because the more bandwidth link has low cost and low cost to root bridge is the fwding link.

You can anytime change the default cost values of any port though cisco does not suggest doing that till the time you are aware of what you are doing keeping in mind of whole big network.

HTH, if yes please rate the post.

Ankur