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Webcast-Catalyst9k
New Member

Spanning trees question

Could someone explain to me what is the different between the Port # and Fastinterface in the output below? This is the output generated after executing the sho spanningtree detail.

Thanks in advance,

sK

=========================================

Port 50 (FastEthernet0/46) of VLAN0001 is forwarding

Port path cost 19, Port priority 128, Port Identifier 128.50.

Designated root has priority 32768, address 0007.eb78.e390

Designated bridge has priority 32769, address 0014.1cec.3f80

Designated port id is 128.50, designated path cost 19

Timers: message age 0, forward delay 0, hold 0

Number of transitions to forwarding state: 1

The port is in the portfast mode

Link type is shared by default

BPDU: sent 7393757, received 0

1 ACCEPTED SOLUTION

Accepted Solutions

Re: Spanning trees question

Hi Sadik,

Port 50 is the port's spanning-tree identifier.

You can see it in the second row as well: Port identifier 128.50

128 is the port priority, 50 is the port identifier.

The port identifier is not necessarily tied to the port number (Fastethernet0/46 in this case). You don't have to worry about it.

It is used by the spanning-tree algorithm in specific cases as a tiebreaker, if multiple ports are candidates for the designated port role:

The port that receives the BPDU with the lowest sender port ID, will become the designated port, if all other prior decisive factors (root bridge ID, root path cost, sender bridge ID and sender port priority) are the same.

Cheers:

Istvan

5 REPLIES

Re: Spanning trees question

Hi Sadik,

Port 50 is the port's spanning-tree identifier.

You can see it in the second row as well: Port identifier 128.50

128 is the port priority, 50 is the port identifier.

The port identifier is not necessarily tied to the port number (Fastethernet0/46 in this case). You don't have to worry about it.

It is used by the spanning-tree algorithm in specific cases as a tiebreaker, if multiple ports are candidates for the designated port role:

The port that receives the BPDU with the lowest sender port ID, will become the designated port, if all other prior decisive factors (root bridge ID, root path cost, sender bridge ID and sender port priority) are the same.

Cheers:

Istvan

Hall of Fame Super Blue

Re: Spanning trees question

Agree with Kevin on this, can't see a lot wrong with the answer.

Jon

Re: Spanning trees question

Sadik,

Just a small point: I think you may have misunderstood the rating system. A "1" means the reply was unhelpful, and a 5 means it was extremely helpful. Looking at the content of the reply, I am sure you did not mean to give a "1".

Kevin Dorrell

Luxembourg

New Member

Re: Spanning trees question

LOL.... I don't that I have rated the answers yet. I know how the rating system works and I will certainly go ahead and rate the responses.

Thanks to all who replied and provided helpful material.

sK

Re: Spanning trees question

Hi Guys,

Thank you very much for the ratings and for your good intentions.

However, I would like to ask you to correct any of my posts if you find they need correction from technological point of view.

I find it very rewarding to post my answers here. When I can help someone solve his/her problem, I think I am just as happy as he/she is.

The rating itself is an acknowledgment of the pleasure we can give.

Cheers:

Istvan

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