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New Member

STP and blocked ports


I have five 3550s daisy-chained as a core hosting 2950s where the 2950s' Gig port are attached to different 3550s. Using IOS 12.20. No vlans. I noticed that sometimes a blocked port connecting the 2950 and 3550 is blocked at the 3550 instead of the 2950 (G0/12). Is this normal? Thanks

New Member

Re: STP and blocked ports


This is all to do with the STP root bridge selection on your network.

The root will have all ports as Designated Ports. Access switche ports forwarding traffic towards the root will be Root Ports. Other ports will be in blocking mode.

Use the command show spanning-tree and try to figure out which is your root bridge and then u will be able to work it out. It is important that the root bridge is pretty much in the centre of your network. Try to avoid access switches becoming the root.



Re: STP and blocked ports

Follow the path to the root. The root port should not be constantly changing. Check if the root also changes when the root port changes. These blocking ports are known as Designated Port. These switches have a lower cost path to the Root that are called root port.

Take this topology for example:


g0/1 g0/2

| |

| |

g0/1 g0/1



The drawing above shows SwA being the root and SwB and SwC are connected to one another via g0/2. The SwA being the root will have its gi0/1 and gi0/2 in forwarding mode and are called Designated Port. Both SwB and SwC will have their g0/1 in forwarding mode and are called root port. Now, either SwB or SwC's port gig 0/2 will be DP and the other NDP, this will be based on which one have a lower BID. Let's say the SwB have the lower BID (Bridge Priority + mac-address) than SwC. This means SwB's gig 0/2 will be DP thus will be forwarding and SwC's port gig 0/2 will be NDP and blocking. The only way this will change is if the cost of the path change, the path to the root had changed thus changing the cost and if the BPDU from SwB is not being received on SwC's gig 0/2 which will cause it to transition to DP and forward.

New Member

Re: STP and blocked ports

I appreciate the replies very much.

In looking at the show spanning-tree command on my 3550s (IOS 12.20) I noticed there is a "Bridge Group 1" and a separate "VLAN0001", each with its own Root ID Priority and Bridge ID Priority. What is this? Also I have one 3550 as the Bridge Group 1 Root (says this bridge is root) and another 3550 as VLAN0001 Root (also says this bridge is root).

These two things, Bridge Group 1 and VLAN0001 have their own timers and MAC addresses too. I know this is probably getting pretty complicated, but I would like to understand these two things and what they're for. If someone has a link or an explanation I would be extremely grateful.

Re: STP and blocked ports

YOu must have fallback bridging enabled. Look for the following in the configurations:

bridge bridge-group protocol vlan-bridge


bridge-group 1 under the interface vlan.

IF you have fallback-bridging enabled what are you bridging?

New Member

Re: STP and blocked ports

Again, thanks for the help and links.

Here is a conceptual drawing of my problem:

3550 #1(FWD)------(FWD)3550 #2(FWD)------(FWD)3550 #3






Rt Prior:...1.......1.......1........1


Br Prior:....1....32767....8139....32769

The Gig connection between the 2950 and 3550 #3 is blocked at the 3550 end. Is this because the cost is lower for the 2950 than the 3550 #3? Is this really a problem or normal?

Re: STP and blocked ports

The gig connection from the 2950 to the 3550# is blocking on the 3550#3 side based on Desiganted Port election:

1. Path cost to the root bridge.

2. If 3550#3 have equal cost to the root bridge from 3550#2 and 2950, then it will make it's decision based on sender BID.

A detailed example is outlined on Cisco LAN Switching by Clark and Hamilton, Cisco Press. Chapter 6, p. 172.