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Question about spanning-tree recalc

I have a trunk that has all possible vlans allowed on one end (the core switch) and a subset of those vlans allowed on the other end (access switch). If that link goes down, are all the vlans on the core going to go into a spanning-tree recalculation even though only a few of the vlans are actually in use on the downstream switch?

Scenario details are... Two core switches connected to each other, one access switch with a connection to each core switch. The downlink ports at the cores allow all vlans and are forwarding all vlans. The uplink ports at the access switch allow only vlan 4,5,7,8.  If my access switch reboots or the links go down for some reason is every vlan on the core going to stop forwarding traffic while spanning tree recalcs?

So even though the core is forwarding vlans 2 and 3 on the trunk link, there is no path back to either core switch for those vlans since there is no port on the access switch configured for vlan 2 or 3.

Thoughts?

Thanks in advance.

2 REPLIES
Cisco Employee

Question about spanning-tree recalc

Hello,

The uplink ports at the access switch allow only vlan 4,5,7,8.  If my  access switch reboots or the links go down for some reason is every vlan  on the core going to stop forwarding traffic while spanning tree  recalcs?

No, the VLANs on the core should not be affected at all. Regarding the loss of a port in particular, recalculation in STP/RSTP is performed only for a lost Root port. If any other port goes down (Designated, Alternate, Backup), STP/RSTP won't react in any way. Therefore, as the ports from core switches towards the access switch surely aren't Root ports in a sensibly configured network, rebooting the access switch or the links going down should not affect the VLANs in the core network.

In general, if only a subset of VLANs is allowed on a trunk, then for a switch, the up or down transition of this trunk potentially affects only the STP/RSTP running in this subset of VLANs (the word "potentially" stressing the fact that even STP/RSTP does not care about each and every event in the network). If the two ends of a single trunk use different subsets of allowed VLANs, the up or down transition of this trunk will potentially affect different STP/RSTP instances on each of these switches.

Please feel welcome to ask further!

Best regards,

Peter

New Member

Question about spanning-tree recalc

Thanks Peter! That's exactly what I needed to know.

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