How to change blocking port

[ciscolover]

Hi all,

I have 3 switches. The root switch A and another 2 switches, B and C.

I have 2 wired cables that connect the B and C switches and I would like to change the switch that blocks the stp port. In that case is fast ethernet of switch B the blocking port. I would like the blocking port on switch C and not in switch B.

I need to change the priority or the cost on the ports?

Thanks ¡¡¡

[InayathUlla Sharieff]

Both parameters are used to determine which interface will be in forwarding state.

They have differences though:

If you want to affect how to the local switch elects the root port change the cost on the links.  Cost is cumulative throughout the STP domain. The higher cost is the less preferred.

If you want to affect how downstream switch elects its root port change the priority. This is only local significant between the two directly connected switches. Highest priority is less preferred.

Going away from the root of the tree use priority whereas, when going towards the root of the tree use cost.

Regards

Inayath

[ciscolover]

Ok,

If I increase the priority in switch B and I decrease the priority on switch C switch C will block one of the 2 ports?

Thanks ¡¡¡

[Jan Hrnko]

Hi,

I just want to point out that priority in STP is calculated in opposite manner as in other protocols. That means, that if you want to increase priority at switch B, you should LOWER the value. To decrease the priority on switch C, you should HIGHER the value.

From the information provided I am not sure what your topology looks like precisely.

Inayath is correct, however I think that you are talking about election of the designated ports and not the election of root ports. One thing though - keep in mind, that the cost parameter is added to the cummulative value upon reception of the BPDU and not upon its transmission. Moreover - designated port is that port which is SENDING the best BPDU and that these BPDUs between neighbor ports are compared IMMEDIATELY upon reception (thus not adding anything to the cost parameter).

Best regards,

Jan

[Jan Hrnko]

Hi,

in addition, I thought of this:

What will be the parameter that will be the tie breaker when comparing such BPDUs?

1. Root Bridge ID is the same (Switch A)

2. Root Path Cost (I assume that the links have same cost )

3. Sender BID (There is a difference for sure)

4. SPID doesn't matter here at all

5. RPID doesn't matter here at all

Referring to 2nd tie breaker:

You  could change cost of the link from B to A or C to A.  Therefore it will  matter when comparing the RPC of BPDUs between B and  C.

Referring to 3rd tie breaker:

You can change the priority of the switches.

Best regards,

Jan

[Suraj.Cisco Community]

Dear,

sir,

which commands are used in Cisco packet tracer to change manually forwarding & blocking ports?

Please, sir, share it.

                      Thank You

                      Regards,

                    Suraj Bhaskar

[ciscolover]

Ok I have switch A , switch B and switch C conected. Switch C has the lower mac and is the root.

Switch A with interface 1 and 2 forwarding.

Switch A mac address 001a-c197... default priority

Interface 1 designated port, priority 128, port cost 200000

Interface 2 designated port, priority 128, port cost 256

interface 3 root port....

Switch B with interface 1 forwarding and 2 BLOCKED

Switch B mac address 0022.557f......

interface 1 root port, path cost 19, Port priority 128, designated path cost 10000

interface 2 blocked port, path cost 19, priority 128, designated path cost 10000

How can I configure this scenario for Blocked interface are in switch A and not in switch B ??????

Thanks

[cadet alain]

Hi,

It would be better posting a diagram and also the configs of the switches along with the show spanning-tree vlan x on each one

Regards

Alain

Don't forget to rate helpful posts.

[Jan Hrnko]

Hi,

change the priority of the switches so that: Switch C will have the lowest priority, for example 0 (remember, lower is better in STP), Switch B will have the second lowest priority, for example 4096 and Switch A can keep its default priority (if it's higher than 4096).

You can change the Switch priority for a VLAN by issuing command:

Switch(config)#spanning-tree vlan [exact vlan or range of vlans] priority [priority number]

so, for all VLANs:

SwitchC(config)#spanning-tree vlan 1-4094 priority 0

SwitchB(config)#spanning-tree vlan 1-4094 priority 4096

optional, to make Switch A priority default if it is not SwitchA(config)#spanning-tree vlan 1-4094 priority 32768

Best regards,

Jan