Root ID and Bridge ID

Unanswered Question
Aug 14th, 2007

Hi Experts,

I am trying to understand if Bridge ID referring to the so called Blocked Port?

VLAN5

Spanning tree enabled protocol ieee

Root ID Priority 32768

Address 0000.Ac00.3298

Cost 23

Port 2(GigabitEthernet0/1)

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32777 (priority 32768 sys-id-ext 9)

Address 00A5.A255.e500

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 300

Thanks in advanced,

I have this problem too.
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Jon Marshall Tue, 08/14/2007 - 22:34

Hi Cindy

The bridge ID is the mac-address of the switch you are on.

The root ID is the mac-address of the switch that is the root bridge for that vlan.

So if the bridge ID and root ID are the same then you are on the root bridge for that vlan.

HTH

Jon

mohammedmahmoud Tue, 08/14/2007 - 22:40

Hi Cindy,

Just to add a small point to Jon's reply (if i may please Jon :) ), regarding the port status, all the root bridge ports are designated ports, and if loops occur then the ports of the switches connected to the root bridge are blocked and not the root bridge ports itself.

HTH,

Mohammed Mahmoud.

cindylee27 Tue, 08/14/2007 - 22:57

Thanks Mohammed for the explaination. :)

What about the Blocked Port, how does the blocked port's behaviour come into picture with the root port?

What i understand is blocked port will be root port when the original root port is death,meaning disconnected.

Thanks in advanced.

mohammedmahmoud Tue, 08/14/2007 - 23:11

Hi Cindy,

You are very welcomed :), a non-root bridge has only one root port, if this port fails STP will try to find another blocked port that can be the root port and make it the new root port, and this behavior can be enhanced via UplinkFast feature.

HTH,

Mohammed Mahmoud.

cindylee27 Wed, 08/15/2007 - 00:42

Hi Mohammed,

Last question..

Is root bridge port = root port ?

Or root bridge port is a diff meaning which is existed from the spanning tree parameters?

Thanks again..

mohammedmahmoud Wed, 08/15/2007 - 01:09

Hi Cindy,

The answer is NO, root bridge port, is a port on the root bridge, which as i told you is always a designated port (all root bridge ports are designated ports), while a root port, is a port on a non-root bridge that has the best cost to the root bridge (a non-root bridge has only 1 root port).

I hope that i've been informative, and you are very welcomed.

HTH,

Mohammed Mahmoud.

cindylee27 Wed, 08/15/2007 - 01:26

Mohammed,I am about there.. :)

I am trying to understand what i understand ya..So, i will give the example below if i really understood..:)

Let's say, there is 1 switch 24 ports..

So I would say that 1 port is Root Port, another port is Blocked Port, and the balanced 22 ports are called root bridge ports which are also called designated ports.

Am I right?

Jon Marshall Wed, 08/15/2007 - 01:29

Cindy

Not necessarily. The switch will have one root port which is the port that leads to the root bridge.

The switch can have any number of blocked ports depending on how may many links you have connected to other switches.

Designated ports are ports that the switch uses to forward vlan traffic on for that subnet.

HTH

Jon

mohammedmahmoud Wed, 08/15/2007 - 01:36

Cindy no problems :)

No, lets take the example this way, we have switch 1 and switch 2 they are connected to each other via 2 ports, lets assume that switch 1 is the root switch (won the STP root bridge election) thus all ports on switch 1 are designated ports, as for switch 2, one of the 2 ports is a root port and the other is blocking, and if the root port fails the blocked port is going to be the root port.

HTH,

Mohammed Mahmoud.

cindylee27 Thu, 08/16/2007 - 07:55

Sorry for the late reply..Thanks guys for the explaination. Understand it better now..

Time to rate..:)

cindylee27 Thu, 08/16/2007 - 07:57

Sorry for the late reply..Thanks guys for the explaination. Understand it better now..

Time to rate..:)

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