Quick STP Question

Answered Question
Mar 8th, 2010

Drawing2.vsd

Switches 1 and 2 are L2 and running rstp - both with vlan 10 on them

switches 3 and 4 are routed with SVIs for vlan 10 on both of them - HSRP

all uplinks are dot1q 10 Gig that allow vlan 10

Switch 1 is the root bridge for vlan 10.

Given this, which ports will be STP blocking in the topology and WHY?

Please, IGNORE the numbers in parantheses....they mean nothing...

Thanks

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Correct Answer by Jon Marshall about 6 years 9 months ago

ex-engineer wrote:

Switches 1 and 2 are L2 and running rstp - both with vlan 10 on them

switches 3 and 4 are routed with SVIs for vlan 10 on both of them - HSRP

all uplinks are dot1q 10 Gig

Switch 1 is the root bridge for vlan 10.

Given this, which ports will be STP blocking in the topology and WHY?

Please, IGNORE the numbers in parantheses....they mean nothing...

Thanks

Joe

Can you post the .vsd as a .jpg

What is the output of "sh spanning-tree vlan 10" on all switches ?

Jon

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Correct Answer
Jon Marshall Mon, 03/08/2010 - 14:58

ex-engineer wrote:

Switches 1 and 2 are L2 and running rstp - both with vlan 10 on them

switches 3 and 4 are routed with SVIs for vlan 10 on both of them - HSRP

all uplinks are dot1q 10 Gig

Switch 1 is the root bridge for vlan 10.

Given this, which ports will be STP blocking in the topology and WHY?

Please, IGNORE the numbers in parantheses....they mean nothing...

Thanks

Joe

Can you post the .vsd as a .jpg

What is the output of "sh spanning-tree vlan 10" on all switches ?

Jon

Jon Marshall Mon, 03/08/2010 - 16:07

Ahh okay, so all links are the same speed and therefore the same cost

Sw1 will have all of it's ports forwarding so no blocking on that switch.

Next all other switches must elect a root port ie. the port with the lowest cost path to sw1. Looking at your diagram -

sw2 root port = 7/3
sw3 root port = 7/1
sw4 root port = 7/2

then each switch must elect a designated port for each segment

So we are left with these ports -

sw2 ->  7/2 & 7/1
sw3 ->  7/2 & 7/3
sw4 ->  7/3 & 7/1

On the root bridge all ports are designated and forwarding.

For the rest if there is a tie on path costs which there will be in your case then STP uses the following tie breakers to elect a designated port

1) lowest forwarder's bridge ID
2) lowest forwarder's port priority
3) lowest forwarder's port number

So it all comes down to the bridge ID in your example because the path costs are the same. All i can say at the moment is that the 3 links which would have blocked ports would be -

sw3 -> sw4
sw4 -> sw2
sw3 -> sw2

The above is standard STP. RSTP has the concept of alternate and backup ports as well so it is a bit more complex.

Jon

ex-engineer Mon, 03/08/2010 - 16:39

Jon, sorry, but I messed up., Switch 3 is the root....

anyway, let me be more specific....

Lets focus on switch 1...

port 7/1 will be the RP

ports 7/2 and 7/3 will be part of two inferior paths...blocked paths..

But the question is which port on those segments will be blocked??

As for the switch 1/switch 4 segment which port will be blocking? switch 1 port 7/2 or switch 4 port 7/2?

As for the switch 1 and switch 2segment, which will be blocked?  switch 1 port 7/3 or switch 2 port 7/3?

Jon Marshall Mon, 03/08/2010 - 17:02

ex-engineer wrote:

Jon, sorry, but I messed up., Switch 3 is the root....

anyway, let me be more specific....

Lets focus on switch 1...

port 7/1 will be the RP

ports 7/2 and 7/3 will be part of two inferior paths...blocked paths..

But the question is which port on those segments will be blocked??

As for the switch 1/switch 4 segment which port will be blocking? switch 1 port 7/2 or switch 4 port 7/2?

As for the switch 1 and switch 2segment, which will be blocked?  switch 1 port 7/3 or switch 2 port 7/3?

Joe

It will come down to the bridge id ie. the combination of the mac-address and STP priority. Assuming all priorities are the same then it comes down to which bridge has the lower mac-address.

So lets take the connection between sw1 and sw2. Because the links are all the same we will treat the cost of each link as 1.

For a port to become the designated port on a segment it must send the lowest hello onto that segment. The hellos are received from the root bridge, the cost on the incoming port is added to the existing  cost and then it is advertised out onto the segment.

So sw1 receives a hello from sw3 on port 7/1. The cost of the path to root on that port is 1 so sw1 now advertises out the hello onto the sw1 -> sw2 link with a cost of 1.

sw2 receives a hello from sw3 on 7/2. The cost again is 1 so sw2 advertises out the hello onto the sw2 -> sw1 link with a cost of 1.

So the costs are tied. When there is a tie the following tiebreakers are used -

1) lowest value of forwarding switches bridge ID

2) lowest port priority of the forwarding switch

3) the lowest port number of forwarding switch.

It will therefore be decided on 1). Assuming you have not changed the STP priority on either sw1 or sw2 it will be the switch with the lowest mac-address that wins and the other switch will place it's port into blocking state.

Jon

ex-engineer Mon, 03/08/2010 - 17:19

OK, so the same set of tiebreakers is used to determine the RP, DP and which port will be blocking...

Imagine that the switch 1 and switch 2 segment were part of the root path, then the 2 switches will not be trying to decide who should block their port (as  we are doing in our example), but instead competing for who is going to be the RP.

Yes?

Jon Marshall Tue, 03/09/2010 - 04:23

ex-engineer wrote:

OK, so the same set of tiebreakers is used to determine the RP, DP and which port will be blocking...

Imagine that the switch 1 and switch 2 segment were part of the root path, then the 2 switches will not be trying to decide who should block their port (as  we are doing in our example), but instead competing for who is going to be the RP.

Yes?

Joe

If the connection between sw1 -> sw2 was part of the shortest path to root then yes the 2 switches would be deciding which port was RP ie. which port has the lowest cost path to the root bridge.

Jon

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