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New Member

STP Problem??

I have 4 switches, a 24 port gig dell switch (SwitchA) as the backbone and 3 2950T SwitchB, C & D as the workgroup switches. Switch A connects to gig0/1 on SwitchB gig0/2 on switchB connects to gig0/1 on C, gig0/2 on c to gig 0/1 on D and gig 0/2 on D back to the Dell SwitchA. I have set the SPT to be pvst on the cisco switches. Dell switch has priority of 16384, Switch B is the secondary root bridge (28673) and the others have 32769. My question is which which ports should be forwarding. Currently all ports are forwarding except gig0/2 on switchC Is this correct?. Also what is the difference between a designated root and a designated bridge? Sorry to be a bit long winded and maybe vague, it appears that all is working OK however if someone could explain the above points to me I would be very grateful.

Regards,

Richard

4 REPLIES

Re: STP Problem??

Hi,

this is correct.

A is the root, everything else (port costs and priorities) is left default. Switch C is receiving equal cost BPDUs from both sides so it has to break the loop. Port number is the tie-break when everything else is equal, so the port gi0/2 (higher number) is blocked.

AFAIK, designated root is the same as root.

Designated bridge is the one which is forwarding traffic to a shared segment (there is allways only one on each segment which has a forwarding port, all the others have their ports blocked).

I think everythging should work fine as far as you have only one VLAN in your network. BUT if you plan to use more VLANs you can get into troubles. Dell switch probably doesn't support PVSTP and there might be also problems with trunks between Dell and Ciscos.

Regards,

Milan

New Member

Re: STP Problem??

Milan,

Thanks very much for that, I did not understand however what you mean by...

Designated bridge is the one which is forwarding traffic to a shared segment (there is allways only one on each segment which has a forwarding port, all the others have their ports blocked).

What is the shared segment? Is this the link between 2 switches?

Cheers,

Richard.

Re: STP Problem??

Sorry, my mistake.

The correct definition is:

"For each LAN segment, the switches communicate with each other to determine which switch on that LAN segment is best to use for moving data from that segment to the root bridge. This switch is called the designated switch."

(See http://www.cisco.com/warp/customer/473/5.html.)

All the other switches have their ports blocked, unless the port is the root port of a particular switches.

The designated switch is also the only one sending BPDUs to the segment in a stable topology.

A link between 2 switches is also a LAN segment. One port on it is designated and the other one is either a root port or a blocked one.

Regards,

Milan

New Member

Re: STP Problem??

Milan,

Thanks very much for that!!

Cheers,

Rich.

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