Solved

spanning tree help

Posted on 2011-03-10
4
418 Views
Last Modified: 2012-05-11
Hi,

 i have uplinks from core switches that connect to Virgin WAN router configured as access ports and in vlan 999. By mistake a cable is plugged in thats configured in different vlan and has portfast set. This leads to error message of not in common subnet and all ports in vlan999 being blocked until reconvergence.

This seemed to take a long time and I want  to avoid it happening again. I think if i set all ports except uplinks with bpduguard so they error disable if a switch is plugged in, this would stop some problems. But would this help protect against the not in common subnet error and blockign affecting all switches on vlan 999?

Is spanning tree detecting a loop when it gives not in common subnet ?Thanks
0
Comment
Question by:jly999
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 2
4 Comments
 
LVL 34

Expert Comment

by:Istvan Kalmar
ID: 35095241
Hi,

You need to set bootg devices same native vlan

conf t
 int x/x
 switchport trunk native vlan 999
0
 
LVL 17

Expert Comment

by:MAG03
ID: 35127643
The not on common subnet error just means that there are two different subnets trying to use the same vlan.

BPDUguard will not stop the not on common subnet error.  It will just shutdown the port if someone tries to connect a switch to that access port. However, since the port that a switch was just connected is shut down when BPDUs are detected, the not on common subnet will not happen.
0
 

Author Comment

by:jly999
ID: 35146529
thanks a lot for the comments

i wonder is it good practice to have uplink ports set as access with bpduguard set?
or better to have trunk ports with native vlan, or it doesnt matter?

Is it standard for STP to block everything and reconverge if it sees two subnets trying to use the same vlan?
0
 
LVL 17

Accepted Solution

by:
MAG03 earned 500 total points
ID: 35146596
if by uplink port you mean access ports then yes, best practice is to set all non inter switch ports as access ports and configure bpduguard on those ports aswell.

It is also best practice to set an unused vlan to be the native vlan.

Spanning-tree only looks for and prevents loops, it doesn't see or care if two subnets are trying to use the same vlan.
0

Featured Post

On Demand Webinar: Networking for the Cloud Era

Did you know SD-WANs can improve network connectivity? Check out this webinar to learn how an SD-WAN simplified, one-click tool can help you migrate and manage data in the cloud.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Load balancing is the method of dividing the total amount of work performed by one computer between two or more computers. Its aim is to get more work done in the same amount of time, ensuring that all the users get served faster.
WARNING:   If you follow the instructions here, you will wipe out your VTP and VLAN configurations.  Make sure you have backed up your switch!!! I recently had some issues with a few low-end Cisco routers (RV325) and I opened a case with Cisco TA…
In this video we outline the Physical Segments view of NetCrunch network monitor. By following this brief how-to video, you will be able to learn how NetCrunch visualizes your network, how granular is the information collected, as well as where to f…
NetCrunch network monitor is a highly extensive platform for network monitoring and alert generation. In this video you'll see a live demo of NetCrunch with most notable features explained in a walk-through manner. You'll also get to know the philos…
Suggested Courses

615 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question