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STP traffic flow

lerner cisco
Level 1
Level 1

All,

 

Got an below setup I am looking confirmation its works , your inputs please

 

 

   Sw1 - core  <------------------- trunk ----------- ----------------->sw2 core

   !                                                                                                          !

   !                                                                                                            !

  !                                                                                                             !

   Distribution L3 switch1   ---- no back to back connection --     Distribution L3 switch2

               !                                                               !

              trunk                                                trunk                                                      

                !                                                              !

                    access layer switch  sw1 - trunk connection to distribution switches

 

1)  I have core switches  ( SW1 and SW2 ) connected to distribution switches ( Distribution L3  SW1 and SW2 ) the port configured with trunk and L3 interface vlan 40  , the vlan 40 forming EIGRP neighbour ship with distribution switches , so port acting as L2 and L3 . also HSRP  vlan 40 configured  active ( sw1) standby ( sw2)

2)  distribution switches - connected to core switches with  trunk  & int vlan 40 forming EIGRP neighbour ship with both core switches - no hsrp vlan 40 configured on the distribution switches

 

4) access layer switch - trunk  connected to distribution switches and  L2  vlan 40 allowed . the gateway for this switch now is on the core switches  active HSRP vlan 40 IP

5) at access switch  the port connected Distribution sw1 - STP  FWD state , the other port in BLK state

 

would like to check with you , if connection made shut between access switch to distribution switch 1 , STP will remove seond port from BLK state and put in forwarding state &

traffic  will hit to the core sw2 and reach the gateway  HSRP active ip address at Core sw1

1 Accepted Solution

Accepted Solutions

Peter Paluch
Cisco Employee
Cisco Employee

Hello,

if connection made shut between access switch to distribution switch 1 , STP will remove seond port from BLK state and put in forwarding state & traffic  will hit to the core sw2 and reach the gateway  HSRP active ip address at Core sw1

Yes, that is correct. If the link between AccessSw and DistSw1 is removed or shut down, STP will unblock the link from AccessSw to DistSw2. Outbound traffic from VLAN, however, is still routed through the current HSRP Active switch, and that is CoreSw1. So the traffic will in fact need to travel through AccessSw, DistSw2, and CoreSw2 to CoreSw1 where it will be routed.

If the link between CoreSw1 and CoreSw2 was a pure routed link (i.e. no switchport on both ends), it would not exist in the switched topology. As a result, there would be no STP blocked port as there is no loop. CoreSw1 and CoreSw2's HSRP Hello packets traverse the full network diameter in this case. If, in this case, the link between AccessSw1 and DistSw1 went down, CoreSw1 and CoreSw2 would stop hearing each other's HSRP packets. As a result, CoreSw2 would become the Active router while CoreSw1 would remain the Active router - but it does not matter as the network is now partitioned and each partition does not clash with the other. Now, devices connected to AccessSw would only need to reach CoreSw2 to get routed off VLAN 40.

I do not quite understand why your DistSw1 and DistSw2 run EIGRP. Do they do any routing themselves? In your scenario, they seem to be used as plain Layer2 switches only.

Best regards,
Peter

View solution in original post

3 Replies 3

Peter Paluch
Cisco Employee
Cisco Employee

Hello,

if connection made shut between access switch to distribution switch 1 , STP will remove seond port from BLK state and put in forwarding state & traffic  will hit to the core sw2 and reach the gateway  HSRP active ip address at Core sw1

Yes, that is correct. If the link between AccessSw and DistSw1 is removed or shut down, STP will unblock the link from AccessSw to DistSw2. Outbound traffic from VLAN, however, is still routed through the current HSRP Active switch, and that is CoreSw1. So the traffic will in fact need to travel through AccessSw, DistSw2, and CoreSw2 to CoreSw1 where it will be routed.

If the link between CoreSw1 and CoreSw2 was a pure routed link (i.e. no switchport on both ends), it would not exist in the switched topology. As a result, there would be no STP blocked port as there is no loop. CoreSw1 and CoreSw2's HSRP Hello packets traverse the full network diameter in this case. If, in this case, the link between AccessSw1 and DistSw1 went down, CoreSw1 and CoreSw2 would stop hearing each other's HSRP packets. As a result, CoreSw2 would become the Active router while CoreSw1 would remain the Active router - but it does not matter as the network is now partitioned and each partition does not clash with the other. Now, devices connected to AccessSw would only need to reach CoreSw2 to get routed off VLAN 40.

I do not quite understand why your DistSw1 and DistSw2 run EIGRP. Do they do any routing themselves? In your scenario, they seem to be used as plain Layer2 switches only.

Best regards,
Peter

thank you peter , 

yes on the Dist SW1 and SW2  up links connected to downstream switches have both functionality some are L3 enabled and some  only L2 enabled ,

 

Thank again for your inputs , migration scheduled tomorrow I let you the outcome :)

 

thank you , its success

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