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New Member

Cat 3550 with strange STP errors

Bought a cheap 3550 (well, cheap is relative - layer 2 only - WS-C3550-24-SMI), put a host name, IP address, and snmp community names on it and plugged it in to replace a much older 2900.

Spanning tree went nuts and set the fiber gig port I used to blocking mode. There is no way for a duplicate connection to exist - only have one pair of multmode fiber trenched between buildings as a connection back to the server room.

The errors were:

00:37:16: %LINK-3-UPDOWN: Interface GigabitEthernet0/1, changed state to up

00:37:18: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1, changed state to up

00:37:18: %SPANTREE-7-RECV_1Q_NON_TRUNK: Received 802.1Q BPDU on non trunk GigabitEthernet0/1 VLAN1.

00:37:18: %SPANTREE-7-BLOCK_PORT_TYPE: Blocking GigabitEthernet0/1 on VLAN0001. Inconsistent port type.

00:37:32: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1, changed state to down

00:37:35: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1, changed state to up

which repeated until I shut the port.

This box is connected to a Catalyst 4900 (WS-C4912) with all fiber ports. Nothing fancy set up on it either. Mostly factory defaults except for minimal changes for management.

To finaly get things to settle down, I turned off trunking on the 4900 port connecting to the 3550 (set trunk 2/6 off dot1q 1-1005). All ports were set to auto, which should have prevented any attempt at trunking.

Any idea why the 3550 and 4900 switch plugged together over one multimode fiber connection should have a problem? The fiber has been working just fine for several years. I only moved the GBIC, diden't even unplug the fiber.

Are there new secrets to setting up a 3550 out of the box? Or, did my changes only keep me occupied and not really have anything to do with the errors?

My Cisco rep suggested turning on UDLD which I did on the 3550, but the 4900 does not support it. Did not help anyway.

Any suggestions are greatly appreciated.

1 ACCEPTED SOLUTION

Accepted Solutions

Re: Cat 3550 with strange STP errors

Hi,

according to the info provided the only problem is trunking.

The error message shows that you are receiving 802.1Q BPDU form 4900 on your 3550 GigabitEthernet0/1 port which is not configured for trunking.

The difference between the old Cat2900 and the new Cat3550 is that 3550 is blocking inconsistently configured ports while 2900 leaves them up (while trunking not running correctly).

It has nothing to do with fiber, UDLD will not fix the problem.

You need to configure the ports on 3550 and 4900 consistently: either trunking or not trunking. The best way is to configure set trunk 2/6 nonegotiate on 4900 and switchport mode trunk on 3550 int G0/1 (don't forget switchport trunk encapsulation dot1q).

Regards,

Milan

3 REPLIES
xge
New Member

Re: Cat 3550 with strange STP errors

Remember check if there are BPDU guard enabled in the switch. For more info, check the URL:

http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/12113ea1/3550scg/swstpopt.htm#1031380

Also if you disable the trunk in 4900 can fix the problem, you need check the trunk mode in both ends.

Re: Cat 3550 with strange STP errors

Hi,

according to the info provided the only problem is trunking.

The error message shows that you are receiving 802.1Q BPDU form 4900 on your 3550 GigabitEthernet0/1 port which is not configured for trunking.

The difference between the old Cat2900 and the new Cat3550 is that 3550 is blocking inconsistently configured ports while 2900 leaves them up (while trunking not running correctly).

It has nothing to do with fiber, UDLD will not fix the problem.

You need to configure the ports on 3550 and 4900 consistently: either trunking or not trunking. The best way is to configure set trunk 2/6 nonegotiate on 4900 and switchport mode trunk on 3550 int G0/1 (don't forget switchport trunk encapsulation dot1q).

Regards,

Milan

New Member

Re: Cat 3550 with strange STP errors

I just wanted to point out that your 3550 will perform Layer-3 switching. The SMI image will allow for static routing and RIPv1/2.

Here is the link to the 3550 FAQs:

http://www.cisco.com/warp/public/cc/pd/si/casi/ca3550/prodlit/c3550_qp.htm

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