Spanning tree root bridge issue

[dimitra]

Hi.

We have a network with a core layer switch 5406 and access switches.

Spanning tree root guard protection is in place on the links from Core to access.

However at some point I get this messsage in one of the access switches:

I 02/19/15 15:38:58 00839 stp: CST Root changed from 4096: a01d48-80cc00 to
            32768: (this device)
I 02/19/15 15:38:58 00839 stp: IST Root changed from 4096: a01d48-80cc00 to
            32768: (this device)
I 02/19/15 15:38:58 00077 ports: port 52 is now off-line

And at the same time on the Core:
Feb 19 15:39:52 192.168.200.254 00077 ports:  port B13 is now off-line
Feb 19 15:39:59 192.168.200.254 00435 ports:  port B13 is Blocked by STP
Feb 19 15:40:00 192.168.200.254 00076 ports:  port B13 is now on-line
Feb 19 15:40:17 192.168.200.254 00077 ports:  port B13 is now off-line
Feb 19 15:40:20 192.168.200.254 00435 ports:  port B13 is Blocked by STP
Feb 19 15:40:20 192.168.200.254 00076 ports:  port B13 is now on-line

After a while the logs on the access switch:

I 02/19/15 15:39:00 00435 ports: port 52 is Blocked by STP
I 02/19/15 15:39:01 00839 stp: CST Root changed from 32768: (this device) to
            4096: a01d48-80cc00
I 02/19/15 15:39:01 00839 stp: IST Root changed from 32768: (this device) to
            4096: a01d48-80cc00
I 02/19/15 15:39:01 00076 ports: port 52 is now on-line

Any ideas what sould possibly casue this since the root guard command is on place?

Regards,

Dimitra K.

[Vince-Whirlwind]

I would be looking at the connection between the two ports - it looks like it might be flapping.