Solved: How to choose which mac-address has lower value in STP

mahesh18 · ‎04-05-2010

Hi all,

i was reading in cisco books that for selecting root bridge for STP we choose bridge id with lower value.

i know bridge is made of priority + mac addess .

Lower is better for priority and mac address.

i just need to know while looking at following mac address how do we decide which has lower value

1>0010.7bcc.733a and 0010.7bcc.7347 which has lower value ?

2>0010.7bcc.7320 and 0010.7bcc.732d which mac address is lower?

3>0013.8039.9500

0013.80d7.d580

0013.8030.5e80

0013.80c7.9700 which is lower from these 4 and why?

Also want to make sure if switch A fastethernet is connected to switch b gigabit port then bandwidth cost will be 19 of that link or sum of 19 + 1 =20?

many thanks

mahesh

Reza Sharifi · ‎04-05-2010

Hi Mahesh,

Just compare the numbers together until you get to the first number that are not alike, whichever is lower that is the lower MAC address

0010.7bcc.733a

0010.7bcc.7347

3 is lower then 4 so the first one is lower

0010.7bcc.7320

0010.7bcc.732d

0 is lower than d so the first one is lower.

It will negotiate the link to 100 so the link cost should be 19

Gig is 4

HTH

Reza

View solution in original post

Ganesh Hariharan · ‎04-05-2010

Hi,

Thanks for reply

so in this case which we will use

0013.8039.9500

0013.80d7.d580

0013.8030.5e80

0013.80c7.9700 as per your note 0013.8030.5e80is lower so that we choose?

thanks

Hi Mahesh,

You are right the lowest mac among the above is 0013.8030.5e80 just check from left to right with decimal value for each bit you will end in lower mac-address and in STP the bridge ID is used to elect the root bridge in the STP domain as well as to determine the root port for each of the remaining devices in the STP domain.it will elected automatically as per the election and exchange of BPDU packets.

If you want to manually wants to configure a switch as root bridge as per your environment then change the pirority of the switch.

Hope to Help !!

Ganesh.H

Remember to rate the helpful post

View solution in original post

Reza Sharifi · ‎04-05-2010

Hi Mahesh,

Just compare the numbers together until you get to the first number that are not alike, whichever is lower that is the lower MAC address

0010.7bcc.733a

0010.7bcc.7347

3 is lower then 4 so the first one is lower

0010.7bcc.7320

0010.7bcc.732d

0 is lower than d so the first one is lower.

It will negotiate the link to 100 so the link cost should be 19

Gig is 4

HTH

Reza

mahesh18 · ‎04-05-2010

Hi,

Thanks for reply

so in this case which we will use

0013.8039.9500

0013.80d7.d580

0013.8030.5e80

0013.80c7.9700 as per your note 0013.8030.5e80is lower so that we choose?

thanks

Ganesh Hariharan · ‎04-05-2010

Hi,

Thanks for reply

so in this case which we will use

0013.8039.9500

0013.80d7.d580

0013.8030.5e80

0013.80c7.9700 as per your note 0013.8030.5e80is lower so that we choose?

thanks

Hi Mahesh,

You are right the lowest mac among the above is 0013.8030.5e80 just check from left to right with decimal value for each bit you will end in lower mac-address and in STP the bridge ID is used to elect the root bridge in the STP domain as well as to determine the root port for each of the remaining devices in the STP domain.it will elected automatically as per the election and exchange of BPDU packets.

If you want to manually wants to configure a switch as root bridge as per your environment then change the pirority of the switch.

Hope to Help !!

Ganesh.H

Remember to rate the helpful post

mahesh18 · ‎04-05-2010

Hi Reza and Ganesh.

thanks a lot for your reply

mahesh