Flexlink diagram // loop RPVST+

[sebastien3]

Hello,

I would like to set up flexlink on the SW4 switch and prioritize the SW4>SW3 link and put SW4>SW2 as a backup link.

Other switches use RPVST+. Does the FlexLink have to be set to SW4 or SW3 to not have a black hole with RPVST+ ?

[Reza Sharifi]

Hi,

You just configure it on SW4 if your IOS supports it. Flex links is an older protocol and is not support it on all IOS images.

HTH 

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3550/software/release/12-2_25_see/configuration/guide/swflink.html

HTH

[sebastien3]

Hi @Reza Sharifi 

You also confirm that the main node will be SW4 on which there must be Flexlink.

According to you, if the main link SW4>SW3 and the backup link is SW4>SW2 in Flexlink.

If the link between SW2 and SW3 fails, we agree that there is a problem ? SW4 won't be able to chat with SW1/SW/SW3 ?

[Reza Sharifi]

Hi,

If the link between SW2 and SW3 fails, we agree that there is a problem ? SW4 won't be able to chat with SW1/SW/SW3 ?

That is correct. If the primary link is sw4-sw3 and the backup is sw4-sw2 than if the link between sw3 and sw2 fails, there will be no communication from sw3 to the rest of the network.

HTH

[MHM Cisco World]

You can play with rstp link cost so no need for flexlink

Make cost of backup link higher than primary

[sebastien3]

@MHM Cisco World 

The cost should be done only on SW4 ?

interface GigabitEthernet1/0/1
description *** UPLINK to SW3 // PRIMARY ***
switchport trunk allowed vlan 10-15,20,35
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree vlan 10-15,20,35 cost 4

interface GigabitEthernet1/0/2
description *** UPLINK to SW2 // BACKUP ***
switchport trunk allowed vlan 10-15,20,35
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree vlan 10-15,20,35 cost 6

In fact I wanted to use Flexlink because it converges very quickly...

[MHM Cisco World]

Cost must config in SW2 and SW4. 

[sebastien3]

@MHM Cisco World I have a question about the right way to do it :

#### SW4
interface GigabitEthernet1/0/1
description *** UPLINK to SW3 G1/0/1 // PRIMARY ***
spanning-tree vlan 10-15,20,35 cost 4

interface GigabitEthernet1/0/2
description *** UPLINK to SW2 G1/0/2 // BACKUP ***
spanning-tree vlan 10-15,20,35 cost 6

#### SW2
interface GigabitEthernet1/0/3
description *** UPLINK to SW3 G1/0/3 // PRIMARY ***
spanning-tree vlan 10-15,20,35 cost 4

interface GigabitEthernet1/0/2
description *** UPLINK to SW4 G1/0/2 // BACKUP ***
spanning-tree vlan 10-15,20,35 cost 6

Is that how you have to do the COST on the 2  SW (2 and 4) ?

[MHM Cisco World]

[sebastien3]

It does not work ? What did you want to highlight, I didn't really understand...

[MHM Cisco World]

[MHM Cisco World]

two case before and after set Cost
the Cost between IOU2 and IOU4 must high than sum of Cost IOU2-IOU3-IOU4
you see each time the ping from R2 to R1 take path that we decide. 

[sebastien3]

Yes I now agree with these images. The configuration must only be done on SW4 with a very high COST.

What else do you see as a tuning trick in this scenario ?

[MHM Cisco World]

Points :-
1-the config MUST done in SW2 and SW4 
2- the SW2 SW3 or SW4 can not be root of any vlan, the root have all port FWD i.e. override Cost of Link. 
I mention where is Root in my lab 

[paul driver]

Hello
In that topology it would have to be set on SW4, as it the only switch that interconnects SW5-6 to the rest of the network.

Note: Flexlink doesnt partcipate in spanning-tree, it has no need to.

Example
Sw4
int x/x
Description link to sw2
backup interface <link to sw3>