Two Blocking Ports in SpaningTree

bohra_ajay

i m having a ring of 5 Switches.

----SW1

---

SW2

---

SW3

---

SW4----SW5----

in here the Spannign Tree is configured on SW4.

When i check the Spanning Tree Blocked Port Status by running

SW4#sh sp block

I get the below output with showing 2 Ports in the Blocking Mode.

Name Blocked Interfaces List

---

---

VLAN0001 Gi1/0/2
VLAN0005 Gi1/0/2
VLAN0010 Gi1/0/2
VLAN0015 Gi1/0/2
VLAN0017 Gi1/0/2
VLAN0023 Gi1/0/2
VLAN0024 Gi1/0/2
VLAN0027 Gi1/0/2
VLAN0028 Gi1/0/2
VLAN0029 Gi1/0/2
VLAN0041 Gi1/0/2
VLAN0206 Gi1/0/2
VLAN0221 Gi1/0/2
VLAN0230 Gi1/0/2
VLAN0262 Gi1/0/2
VLAN0264 Gi1/0/2
VLAN0271 Gi1/0/2
VLAN0285 Gi1/0/2
VLAN0302 Gi1/0/2
VLAN0306 Gi1/0/1
VLAN0311 Gi1/0/1
VLAN0319 Gi1/0/2
VLAN0320 Gi1/0/2
VLAN0346 Gi1/0/2
VLAN0347 Gi1/0/2
VLAN0366 Gi1/0/2
VLAN0385 Gi1/0/2
VLAN0392 Gi1/0/2
VLAN0404 Gi1/0/2
VLAN0418 Gi1/0/2
VLAN0421 Gi1/0/2
VLAN0444 Gi1/0/2
VLAN0475 Gi1/0/2
VLAN0476 Gi1/0/2
VLAN0479 Gi1/0/2
VLAN0484 Gi1/0/1
VLAN0489 Gi1/0/2
VLAN0490 Gi1/0/2
VLAN0493 Gi1/0/2
VLAN0497 Gi1/0/2
VLAN0512 Gi1/0/2
VLAN0541 Gi1/0/2
VLAN0545 Gi1/0/2
VLAN0557 Gi1/0/2
VLAN0558 Gi1/0/2
VLAN0574 Gi1/0/2
VLAN0611 Gi1/0/2
VLAN0616 Gi1/0/2
VLAN0628 Gi1/0/2
VLAN0656 Gi1/0/2
VLAN0657 Gi1/0/2
VLAN0681 Gi1/0/2
VLAN0682 Gi1/0/1
VLAN0683 Gi1/0/1
VLAN0687 Gi1/0/1
VLAN0698 Gi1/0/1
VLAN0699 Gi1/0/1
VLAN0702 Gi1/0/1
VLAN0703 Gi1/0/1
VLAN0756 Gi1/0/1
VLAN0776 Gi1/0/1
VLAN0781 Gi1/0/1
VLAN0783 Gi1/0/1
VLAN0785 Gi1/0/1
VLAN0791 Gi1/0/1
VLAN0802 Gi1/0/1
VLAN0832 Gi1/0/1
VLAN0863 Gi1/0/1
VLAN0873 Gi1/0/1
VLAN0884 Gi1/0/1
VLAN0886 Gi1/0/1
VLAN0901 Gi1/0/2
VLAN0902 Gi1/0/1
VLAN0917 Gi1/0/1
VLAN0922 Gi1/0/1
VLAN0924 Gi1/0/1
VLAN0925 Gi1/0/1
VLAN0926 Gi1/0/1
VLAN0938 Gi1/0/1
VLAN0947 Gi1/0/1
VLAN0974 Gi1/0/1
VLAN0978 Gi1/0/1
VLAN0996 Gi1/0/1
Number of blocked ports (segments) in the system : 83


Though this is not causing any problem for me but still i would like to know y is this happening. And what is the solution to get rid of this as if in case this thing creates problem in future.

dtlokee

I am not sure I understand the question, but it would appear interface Gi1/0/2 is a redundant connection, and since you're using a form of PVST it shows each VLAN seperately.

bohra_ajay

Ok. but is it possible to have 2 Blocked ports int Spanning Tree. Already this stuff have made me confused a lto on the terms of Spanning Tree.

larkspur

i would be curious to see the config on sw4?

bohra_ajay

u mean running Config ????...

Also, when i check the SW2 for the Vlans those are blocked on Gi1/0/1, no Spanning Tree Instances are created.

The Vlans which are Blocked on the Gi1/0/2 have the instances created on SW2.

Y so????

larkspur

yes to the running-config

There are a lot questions that need to be answered before anyone can honestly answer your questions. viewing the runnign config and a sh spanning-tree would helpout alot,

The reason I asked to see teh runnign config is to check port-channel configs. If trunking on the po channel than what vlans are allowed, are you using vlan 1 for management, who is the root bridge,etc....

dtlokee

Ah, I missed the fact that some VLANS are blocked on GI1/0/1 and others on GI1/0/2, this is due to spanning tree and using PVST (or RPVST or MST) and having configured 1 switch as the root for some of the vlans, and another root for other vlans.

like this:

SW1

spanning-tree vlan 1-100 root primary
spanning tree vlan 101-200 root secondary

SW2

spanning-tree vlan 1-100 root secondary
spanning-tree clan 101-200 root primary

This means spanning tree will build a different topology (due to different roots) for VLANS 1-100 and 100-200. This is typically done to add some load balancing to the uplinks between switches.