Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

spanning tree

Can anyone answer me this, If I have 2 links between 2 switches, if the speed of the link is the same i.e same cost, which port will it block ?

  • Other Network Infrastructure Subjects
8 REPLIES
New Member

Re: spanning tree

Hi ,

To decide which port to block it will be decided like this

1) Root Bridge ID

2) Path Cost

3) Sender Bridge ID ( Bridge Priority + Bridge MAC add)

4) Port ID ( Port Priority + Port No.)

regards

Neo

Re: spanning tree

Hi Carl,

If 2 switches are connected via parallel link keeping everything on default settings path cost, bridge id, bridge priority will remain same for both the links only thing which will differ is port id.

So it will check the port id of the uplink switch and will block the port for the downstream switch which is connected to higher port id of the uplink switch.

As an example

switch A connected via port fa0/1 and fa0/2 to switch B to fa0/8 and fa0/7 of the downstream switch.

So port fa0/7 of the downstream switch will be blocked because the uplink switch A fa0/2 has higher port id.

HTH, if yes please rate the post.

Ankur

New Member

Re: spanning tree

thanks for that, how would i check port id ?

Re: spanning tree

HI

Port ID refers to the position of port in switch.

i.e f0/1 has higher port id than f0/2 it depends on the port number in the switch.think it as 1,2,3 so 1 is lower then 2.

Thanks

Mahmood

Re: spanning tree

Hi Carl,

Port ID is nothing but the port number.

If I say fa0/1 ,fa0/22 ...1,22 are the port IDs and it again goes from lower to higher number like fa0/1 has a lower port id then compared to fa0/2.

HTH, if yes please rate all post.

Ankur

New Member

Re: spanning tree

hi there, so does it block the lower port or higer port ?

thanks

Re: spanning tree

Hi

It blocks the higher port.

Thanks

Mahmood

Re: spanning tree

Hi Carl,

Please do not mind but if you read the first post you will get all things cleared.

It will check the port id of the uplink switch and will block the port for the downstream switch which is connected to higher port id of the uplink switch.

I have also explain taking an example.

Regards,

Ankur

106
Views
10
Helpful
8
Replies
This widget could not be displayed.