MSTP Blocking port

jeffersoncbriones · ‎04-03-2009

Hi Gurus,

Im just wondering why my blocking port is still receiving input traffic. Is this normal? Should it have zero traffic since its blocking ?

tia

andrew.prince · ‎04-03-2009

No - the port is detected a loop on that port, i.e it knows that port leads to the MSTP root switch. That port will block all outgoing layer 2 traffic - but will still accept traffic.

HTH>

jeffersoncbriones · ‎04-03-2009

Hi Andrew,

So the blocking port only signifies no outgoing traffic and it still accepts traffic which shows on my mrtg. Are this vlan traffic? Coz its very alarming, like its eating up 60% of my link though its a blocking port.

Thanks.

jeff

andrew.prince · ‎04-04-2009

Jeff,

That is correct - the port must still accept traffic including BPDU's!

Yes it will be, they will be Broadcasts, unknown unicasts, unkown multicasts etc.

So now you need to think about your STP topology and more importantly VLAN Pruning! This will fix the issue you are seeing on your MRTG.

HTH>

Mohamed Sobair · ‎04-04-2009

Hi,

In its STP normal opertation, the port relies on continues stp bpdus to determine the port role. This applies to a port in Blocking state, the port in Blocking state relies in Continues STP BPDUs to remain at blocking state. As indicated by Andrew, you would notice incoming traffic but you wont see outgoing traffic sent out of the port.

HTH

Mohamed

jeffersoncbriones · ‎04-05-2009

Hi Andrew,

vlan pruning is for only for VTP? although only certain vlan are on the tagged on the trunk port.

Thanks.

jeff

andrew.prince · ‎04-05-2009

Jeff,

VTP ensures that all switches in the VTP domain are aware of all VLANs. However, there are occasions when VTP can create unnecessary traffic. All unknown unicasts and broadcasts in a VLAN are flooded over the entire VLAN. All switches in the network receive all broadcasts, even in situations in which few users are connected in that VLAN. VTP pruning is a feature that you use in order to eliminate or prune this unnecessary traffic.

When VTP pruning is enabled on a VTP server, pruning is enabled for the entire management domain. Making VLANs pruning-eligible or pruning-ineligible affects pruning eligibility for those VLANs on that trunk only (not on all switches in the VTP domain). VTP pruning takes effect several seconds after you enable it. VTP pruning does not prune traffic from VLANs that are pruning-ineligible. VLAN 1 and VLANs 1002 to 1005 are always pruning-ineligible; traffic from these VLANs cannot be pruned. Extended-range VLANs (VLAN IDs greater than 1005) are also pruning-ineligible.

HTH>

Giuseppe Larosa · ‎04-05-2009

Hello Jeff,

have you configured a list of allowed vlans on the port with

switchport trun allowed vlan

or all existing Vlans (as defined and propagated by VTP) are permitted on the port ?

This can make the difference.

VTP pruning allows to avoid unnecessary traffic over trunk links but still STP instances run for all the vlans (well if you have MST this is not a real issue)

Are you sure that the port is not in STP forwarding for some MST instance (and so for the associated Vlan set) ?

Hope to help

Giuseppe