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STP selection Processes

I have misconception in the selection process of:

. Root bridge --> Lowest BID / Lowest Cost / Lowest Sender ID / Lowest port ID

. Root Port --> Lowest cost / Lowest port ID

. Designated Port --> Lowest cost / Lowest port ID

. Designated Switch (for a network segment) --> Lowest BID

. Best path to the root bridge --> Lowest Cost / Lowest Sender ID / Lowest port ID

Can anybody correct my info.

Thanks in advance.

3 REPLIES

Re: STP selection Processes

Feel free to increase your knowledge of spanning tree with the following:

http://www.cisco.com/en/US/tech/tk389/tk621/tsd_technology_support_protocol_home.html

Regards,

Leo

Re: STP selection Processes

I found these pdfs previously, but i needed to confirm what i've understood.

Thanks.

New Member

Re: STP selection Processes

Hi,

First the root bridge will be selected based on the lowest Bridge ID (made of 2bytes priority and MAC address of the bridge). Once the root is slected other switches have to decide how they will get to the bridge. The parameters used are in the order:

1. Port-cost (lowest better) - This is as seen from the access switches towards the root.

2. Port-priority (lowest better) - Advertised by the root and affects downstream switches.

3. Port-ID (lowest better)- When you look at the output of show spann, for f0/4 you will see something like 128.4. 128 is the port priority and 4 is the port ID. See output below.

Interface Role Sts Cost Prio.Nbr Type

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

Fa0/4 Root FWD 19 128.4 P2p

Fa0/5 Altn BLK 19 128.5 P2p

Fa0/6 Altn BLK 19 128.6 P2p

Port types:

Designated Port - Is the port which advertises superior BPDUs on its segment.

Root Port - Is the port used to access the root.

Blocked port/Alternate port - is one that is neigher a DR or RP.

Form the above it can be concluded that all ports on the root are DPs.Other switches ports not connecting to the root could be DP if transmitting superior BPDUs.

HTH

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