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Spanning-tree question


I am in the process of implementing a redundant fast-Ethernet link on my LAN. Currently we have one Cisco Catalyst 4006 backbone switch and 3 “2950” Desktop switches, all are connected through point-to-point gigabit fibre trunk links. I would like to implement a fast-Ethernet backup trunk solution as a redundant link between the 4006 and the 2950's.

My concern is if I have both the links the fast-Ethernet and the gibic enabled will spanning tree work its magic and but the fa link into blocking state and the gibic in forwarding making it the Root Port or will it put both the fa & the gibic into forwarding state casuing a loop as theoretically they are both directly connected interfaces and should be both root port.

If spanning tree is intelligent enough and puts the fa interace into blocking will it bring the interface up when the gibic interface blows as a fall back redundant link. Currently all the gibic interfaces and fa interfaces on all the switches have a default cost of 4 and 19, but all the ports for all the VLANS have a prority of 128. All the interfaces on all the switches are a part of single VLAN 300. There are other VLANs but they only have the gibic modules as members. Would there be any problem having two point-to-point trunks enabled between the backbone 4006 and Desktop 2950’s.



Re: Spanning-tree question


This will work, as long as you leave the default path cost and have the 4006 configured as root bridge (lowest priority). The fastethernet will be in blocking state and go into forwarding in case the gigabit goes down.

In the design described I would prefer another solution though. I would configure fastethernet-channels between the 2950 desktop switches as redundant path to the core switch (if physically possible).

Kind regards,



Re: Spanning-tree question


I tried the setup but it did not work, connected the fa trunck links between the 2950 & 4006 but they came up as forwarding while the gibic was enabled and forwarding, ideally it should have blocked the fa link as it had a lower cost of 19 and kept the gibic forwarding as it has cost 4. How does spanning tree consider a preferred link, does it consider cost or port priority? or both? if it considers both which has higher prefrence?

Would it be that because the fastethernet interface is directly connected to the root switch that it decided to make it the root port as well and put it in forwarding state? any suggestions? The fa port is configured as a trunk with dot1q encap

Need help thanks


Re: Spanning-tree question


if you have your 2950 connected by Gigabit and FastEthernet to 4006 only, configure your 4006 as STP root (for all VLANS you use) and leave everything else default, it should work OK.

There is a lot of useful articles on





Re: Spanning-tree question


Sorry I forgot to mention previously that my 4006 is configured as a root for all VLAN's. I did not wait for very long around 10min after plugging in the fastethernet cable as i presumed it would take 15sec (forward delay time) to converage from forwarding to blocking. Why would it change to forwarding in the first place? the fa port has a lesser cost than the gigabit, is it because they are directly connected to the root switch and the 2950 decided to make both the ports (giga & fa) root ports as it is recieving CBPDU's on both ports? my netwrok is hub & spoke design so i have two cables (cat5 & gibic) running from 4006 to 2950 configured as trunk



Re: Spanning-tree question


Any chance you can post a "show spanning-tree" and "show interfaces trunk" when the FE and GE ports are enabled.

The 2950's should put the FE port in blocking mode and allow only the Gig E port to forward.

First thing the switch looks at first is Root Path Cost to reach the root, the port receiving the CBPDU w/the lowest cost which in this case the Gig E, should be put in the forwarding state.

If there is a tie in Root Path Cost for example two parallel FE or Gig E links, the switch would then look at the port receiving the CBPDU w/the lowest advertised Port Priority, if its default of 128. Then it moves to the CBPDU w/the lowest port number.


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