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Q on Spanning-tree on Access Switch ...


My question pertains to Spanning Tree on an access switch.

This is how it looks like on my switch which is switch_A.

Switch_A#sh spanning-tree int gi1/1

Vlan Role Sts Cost Prio.Nbr Type

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

VLAN0112 Desg FWD 3004 128.1 P2p

VLAN0140 Root FWD 3004 128.1 P2p

VLAN0161 Root FWD 3004 128.1 P2p

VLAN0162 Root FWD 3004 128.1 P2p

VLAN0163 Root FWD 3004 128.1 P2p

VLAN0164 Root FWD 3004 128.1 P2p

My question is that vlan0112 does not work on this switch. If my understanding on SPT is correct, it needs to have a Root port for vlan 112 which it is not having. [1] My question is how do

I put int gi1/1 as root for vlan 112 ?

[2] Also what is the numbering 128.1 for ? Other switches have this same numbering 128.2 and

128.3 and so on ... What does this signify ? Also the cost here is the cost path to the root bridge right ?

[3] P2P means point-to-point which is a full/duplex connection and P2P Edge is a point-to-point to an end station and Shr is a half duplex connectoion right ? This is just to let us know what type of connection is this interface isn't it ?

[4] I have setup a trunk and notice that the trunk works even though the vlans on both end of the trunks are not the same !!! I thaught trunks won't form if the vlans are not the same in it .. I guess that I have learnt something new here.

[5] How can I add vlan 112 to an existing trunk on a 6500 running CatOS ? There is no downtime when I add the connection right ? Below is the existing configuration.

clear trunk 5/12 1-45,47-119,122-126,128-139,141-160,169-649,655-699,702-709,711-4094

set trunk 5/12 on dot1q 46,120-121,127,140,161-168,650-654,700-701,710

Sorry for the many parts to it, but it is all related to my first question.

Thanks a million,




Re: Q on Spanning-tree on Access Switch ...

1. Vlan 112 may have a root port on a different port. This bridge might be the root bridge for vlan 112, in which case it will have no root port at all (the root port is the port leading to the root bridge).

2. 128 is the port priority, .1 is the port number. You can tune the port priority if necessary. The port cost is what is used currently on the port. It's a local value, not the aggregated cost to the root that is shown in the global part of show spanning-tree. 3004 -> you probably have uplinkfast configured (the cost is bumped on the interfaces of a bridge configured for uplinkfast).

3. p2p vs shared is an important parameter that needs to be provided to the STP when in Rapid-PVST or MST mode. By default, STP assume that a full duplex link is p2p and a half duplex link is shared. If this is not the case, you must configure the correct value explicitly on the interface. That's why this p2p/shared flag is displayed in the show spanning-tree command output.

4. Yes, you can have a mismatch (channel ports on the same box must have the same vlan configured, that's probably why you are surprised). If a vlan exist on one side of the trunk but not on the other, there will be no connectivity for this vlan. This should not be a big deal (well, with MST, that could cause some black holing of traffic).

5. Can't remember catos. I think it's "set trunk x/y 112" (I don't think you have to repeat the trunk type in the command).