Share this post on:

Introduction

Topology change notification is not generated when ALT/BLK port state changes.

Topology

S1 is a root bridge, all switches run STP 802.1D. On Switch 2, port eth0/0 is in blocking state.

Let’s enable debugging on Switch 3:

S3#debug spanning-tree events
Spanning Tree event debugging is on

Let’s disable port that is in blocking state:

S3#sh spanning-tree blockedports
 Name                 Blocked Interfaces List
 
 VLAN0001             Et0/0
 Number of blocked ports (segments) in the system : 1
Name Blocked Interfaces List

OK, let’s shutdown eth0/0 and hope for some debugging messages:

S3(config)#interface ethernet 0/0
S3(config-if)#shut
S3(config-if)#shutdown
S3(config-if)#
*Jan 19 12:23:17.329: %LINK-5-CHANGED: Interface Ethernet0/0, changed state to administratively down
*Jan 19 12:23:18.329: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/0, changed state to down

As we can see above, no topology change messages have been generated. This is the expected behaviour of the Spanning Tree.

We can test it by shutting down the other port that is not in blocked state:

S3(config)#int ethernet 0/3
S3(config-if)#sh
S3(config-if)#
*Jan 19 12:28:14.078: STP: VLAN0001 sent Topology Change Notice on Et0/1
*Jan 19 12:28:14.078: STP[1]: Generating TC trap for port Ethernet0/3
S3(config-if)#
*Jan 19 12:28:16.081: %LINK-5-CHANGED: Interface Ethernet0/3, changed state to administratively down
*Jan 19 12:28:17.082: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/3, changed state to down

Leave a Comment

Your email address will not be published. Required fields are marked *