mst instances

Answered Question
Aug 20th, 2008
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hi everybody!

let say we have only two possible stp instances: ints1,inst2

we have mapped v1 to inst1,v2 to inst2 where "v" stands for vlan.

if a switch receives a frame for v1 but due to physical link failure inst1 no longer exists what will switch do with that frame destined for v1?

thanks a lot!

Correct Answer by Giuseppe Larosa about 8 years 8 months ago

Hello Sarah,

actually the MST instances haven't their own separated BDPUs : information for all existing MST instances are carried within a single BPDU that of instance 0 the infrastructure IST.

So in a MST region a failure event will:

a) be recovered by link redundancy

b) or the region will be divided in two parts


but this will happen for all MST instances for the fact they share the BPDUs propagation.


As explained by Tim if a frame has an unknown unicast destination frame it is flooded out all ports in forwarding state for that vlan.

Notice the STP deals only with links to manage redundancy not with user MAC addresses.

MAC address learning is a different functionality.


Hope to help

Giuseppe

Correct Answer by tdrais about 8 years 8 months ago

Not sure what you are asking. Spanning tree is only somewhat related to the traffic flow. If you lose a path to the root bridge spanning tree will unblock one of the blocked ports if its available.


A packet that a switch does not know where to send the destination is broadcast out all ports that are not blocked on the vlan including trunk ports except the port it was received on.


You can disable spanning tree completely on a switch and the traffic will still flow between switches correctly as long there is only a single path. Its when you have multiple paths between switches that they need the help of spanning tree

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Correct Answer
tdrais Wed, 08/20/2008 - 21:02
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Not sure what you are asking. Spanning tree is only somewhat related to the traffic flow. If you lose a path to the root bridge spanning tree will unblock one of the blocked ports if its available.


A packet that a switch does not know where to send the destination is broadcast out all ports that are not blocked on the vlan including trunk ports except the port it was received on.


You can disable spanning tree completely on a switch and the traffic will still flow between switches correctly as long there is only a single path. Its when you have multiple paths between switches that they need the help of spanning tree

Correct Answer
Giuseppe Larosa Sat, 08/23/2008 - 09:40
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Hello Sarah,

actually the MST instances haven't their own separated BDPUs : information for all existing MST instances are carried within a single BPDU that of instance 0 the infrastructure IST.

So in a MST region a failure event will:

a) be recovered by link redundancy

b) or the region will be divided in two parts


but this will happen for all MST instances for the fact they share the BPDUs propagation.


As explained by Tim if a frame has an unknown unicast destination frame it is flooded out all ports in forwarding state for that vlan.

Notice the STP deals only with links to manage redundancy not with user MAC addresses.

MAC address learning is a different functionality.


Hope to help

Giuseppe

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