Switching

Hi there,

 

I had a difficult experience in my network recently, noticed high broadcast traffic (esacially DHCP) in two vlans, surprisingly the dhcp broadcast in one vlan can be seen in other vlan.Does this mean a loop, any bridging happening b.w two vlans ?., How we can troubeshoot this kind of issue. We use MSTP, I guess this should prevent all these ussues, rigt ?, if so how can this happen. Could help with some expert advise.

 

Regards

Can you give some more details ? about configs ? It looks like you have some kind of "bridge" between two vlans this can be different things (A real Bridge) some kind of patch between two ports in both vlans.

Thanks Marc, but if spanning tree is configured it should block the port , right ?. both vlans are in different instances of MSTP.

Yes that should be the case! also depending of the rest of the network setup and configuration.

 

 

I would first verify your MSTP configuration to make sure it really is configured properly.  MSTP can be tricky to set up and configurations between switches require certain things to match, such as the Name, Revision Level, and VLAN-to-Instance mappings.  One of those things not matching on any switch in the MSTP region will cause MSTP not to work properly.

 

Additionally, I would make sure that you don't have any switches on the network that might be using another form of spanning tree, such as PVST or RSTP.  Different versions of spanning tree on the network can cause strange problems like this, too, since they all have slightly different methods of transmitting BPDUs and/or different BPDU formats.

Spanning Tree knows nothing about VLANs - it only knows about physical network. If it is ONLY DHCP packets you are seeing on both, I would check your DHCP Relay settings as very likely you have a config in which DHCP requests (broadcast packet) are suppose to be sent to multiple VLANs, for which this is correct operation. Packets crossing VLA boundary situations were almost all solved many years ago.

I wouldn't say that spanning tree knows nothing about VLANs.  PVST, VSTP, and MSTP are all STP implementations that take into account multiple VLANs for topology decisions.

 

That said, rccpgm makes an excellent point - if it's just DHCP broadcasts you are seeing, it makes a lot more sense that it's a bad DHCP relay config somewhere than an STP issue. 

Thank you all, I will check this and get back.

Please post back your findings when you figure it out and make sure the right person gets the credit for answering your question if one of these answers turns out to be the solution.