Question regarding designated port in STP

[chandra_rc16]

Can someone please let me know how can i change the designated port to Fa0/2 on switch2 instead of Fa0/2 on switch1 ? Without changing the priority on Switch3. Since switch3 is the ROOT switch.

Regards,

Chandu

[John Blakley]

Chandu,

Can you post a diagram of how the switches are laid out?

HTH,
John

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[Rolf Fischer]

I guess the topology is a triangle and you want to influence which port becomes the designated port for the segment between the non-root bridges?

You can achive a change by

• increasing/decreasing a root-port's cost on switch1/switch2
• increasing/decreasing a bridge priority on switch1/switch2 when costs are equal

When costs and priorities are equal, the MAC-address (as part of the bridge ID) is the tie-breaker.

HTH

Rolf

[Joseph W. Doherty]

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Posting

You could change either STP port cost or STP port priority on your non-root switches.

[chandra_rc16]

I have attached the topology.

Regards,

Chandu

[johnlloyd_13 · Accepted solution]

hi,

issue in global config the spanning-tree vlan x root secondary on Switch2.

[Rolf Fischer · Accepted solution]

So the assumption in my first post was right, you want to change the DP of the segment between Switch1 and Switch2.

Generally, the port with the better BPDU will be elected as the DP and the non-designated port will result in blocking state.

As you most probably know, in order to build a loop-free spanning-tree, this decision sequence is processed:

• Lowest Bridge-ID (becomes root bridge; already done)
• Lowest Cost to Root-Bridge
• Lowest Sender-ID
• Lowest Port-ID (doesn't play a role in this topology)

Both BPDUs (Switch1 Fa0/2 and Switch2 Fa0/2) have by default the same cost to the root bridge (19).

The  sender bridge ID has 2 parts: A configurable priority (Cisco default: 0x8000 / 32.768) and the MAC-Address. As the priority is the same on both switches, the MAC-Adress of Switch 1 wins because it is the lower one (last tie-breaker in this topology).

Changing the cost of the root-ports (Switch1 Fa0/1 "more expensive" or Switch2 Fa0/1 "cheaper") is the first possibility, changing the priority (Switch1 > 0x8000 or Switch2 < 0x8000) the second. John showed a very elegant way to achieve that on Switch2 with the spanning-tree root secondary macro.

I hope this helps, feel free to ask for more details

Regards

Rolf

[chandra_rc16]

Hi Roff,

---------------------------------

Changing the cost of the root-ports (Switch1 Fa0/1 "more expensive" or  Switch2 Fa0/1 "cheaper") is the first possibility, changing the priority  (Switch1 > 0x8000 or Switch2 < 0x8000) the second.

--------------------------------

If i change the priority to > or < than 8000, what happens is either of the switches will become ROOT switch. I want only switch3 to be ROOT switch.

I hope this will work:

spanning-tree vlan x root secondary

on Switch2.

Since i'm using PT i may not be able to do that i guess, because i never came across that parameter in PT.

Any way thanks for both of you.

Regards,

Chandu

[johnlloyd_13 · Accepted solution]

hi,

i've checked in PT and the command is supported on a 2960. give it a try.

[Rolf Fischer · Accepted solution]

Chandu,

Btw: 0x8000 is hex, I used that because it's easier to learn than the decimal value of 32.768.

However, for the configuration you have to use the decimal values, so perhaps I confused you a little bit.

I thought you've changed the priority on switch3 to make it the root bridge, but you didn't, right?

In this case you could simply configure Switch 1 with a priority greater than 32.768 (decimal).

The root secondary macro is no option when you didn't run the root primay on Switch3 first.

HTH

Rolf

[chandra_rc16]

And. Yes i didn't changed the priority on switch3.

[chandra_rc16]

Thanks Roff,

Really you're awsome.. for sure i'll try it.

And the way you imagined the topolgoy before i posted is simply superb.

How long are you into this field (Networking) ? I'm new here.

Regards,

Chandu

[Rolf Fischer · Accepted solution]

Chandu,

you're welcome! Thanks for the great feedback!

I started with LANs about 10 years ago and at this time I had no idea how all this stuff worked, so I know the value of  getting some help while starting to learn from my own experience at that time.

One little hint: When you start a new conversation and explain that you're learning and give some overview about your knowledge so far (regarding the topic), it's much easier to provide you with the adequate answer. This forum is really a great place to improve you knowledge and after some time you'll be able to give something back if you like to.

Best regards

Rolf