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+ ?

[paul driver]

Hello

---

@sebastien3 wrote:
I have a question about the right way to do it

---

If you do not wish to unitilze flexlink then suggest you append stp port-priority on the  sw2-sw3 downstream links connecting to sw4, this way you don't not change the whole stp cost for the estate but just change sw4 selection root port

Sw3
int x/x
description link to sw4
spanning-tree vlan xx port-priority 240 ( higher value less preferred)

[sebastien3]

Hello @paul driver @MHM Cisco World 

My opinion is that I don't really want to force a root and secondary switch to use the COST port because the topology may change (addition of new link + switch)... I'm afraid that this is a source of problems in case of forgetting or removal a switch.

I am wrong ?

The idea of ​​port-priority is perhaps the simplest... The final configuration is as follows only on SW3 and SW2, right ?

SW3
int x/x
description link to SW4 / Primary
spanning-tree vlan xx port-priority 240

SW2
int x/x
description link to SW4 / Backup
spanning-tree vlan xx port-priority 300

[MHM Cisco World]

The STP port priority impacts which port is an alternate port when multiple links are used between switches. In our test topology, shutting down the link between SW3 and SW5 forces SW5 to choose one of the links connected to SW4 as a root port.

Only cost can help you in your case' 

https://www.ciscopress.com/articles/article.asp?p=2995351&seqNum=2

[sebastien3]

OK @MHM Cisco World ! Who do you think should be the root and secondary SW in this topology in order to use COST port ?

There will be a link between SW4 and SW6. Under consideration to secure...

[MHM Cisco World]

Any SW except three SW that form triangle sw2'sw3'sw4.

[sebastien3]

OK, SW1 is root primary and SW6 is root secondary ?

[MHM Cisco World]

You get excellent point, 
I mention before that three SW must not elect as root but you find exact way to do that. 
excellent friend 
thanks for point that 

[sebastien3]

Hi,

I tried with Packet Tracer to reproduce this. If I position SW1 in SPT root primary and SW6 in SPT root secondary it is not possible for me to privilege the link SW4--SW3, it will always be preferred SW4--SW2 !

[MHM Cisco World]

dont worry I will open my lab and check the primary and secondary root with cost config 
thanks 
MHM

[sebastien3]

OK @MHM Cisco World ! Did you reproduce my packet tracer lab with your GSN lab ?

Thanks

[MHM Cisco World]

IOU1 is primary root, 
I shut the e0/0 in IOU1

IOU5 is root secondary,
after I shut the e0/0 in IOU1 the IOU5 become the new root of domain 

the IOU4 is now change it Root from IOU1 to IOU5

[sebastien3]

Hi @MHM Cisco World 

On my LAB it doesn't work on SW2,SW3 and SW4... The cost doesn't matter much.

Can you show me exactly the SW2,SW3 and SW4 conf ?

THANKS

[MHM Cisco World]

Sure I will share Config for all SW's
MHM

[MHM Cisco World]

IOU5# show run
IOU5# show running-config
Building configuration...

Current configuration : 1391 bytes
!
! Last configuration change at 11:47:14 UTC Thu Apr 20 2023
!
version 15.2
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
service compress-config
!
hostname IOU5
!
boot-start-marker
boot-end-marker
!
!
logging discriminator EXCESS severity drops 6 msg-body drops EXCESSCOLL
logging buffered 50000
logging console discriminator EXCESS
!
no aaa new-model
!
!
!
!
!
no ip icmp rate-limit unreachable
!
!
!
no ip domain-lookup
ip cef
no ipv6 cef
!
!
!
spanning-tree mode pvst
spanning-tree extend system-id
spanning-tree vlan 1 priority 28672
!
vlan internal allocation policy ascending
!
ip tcp synwait-time 5
!
!
!
!
!
!
!
!
!
!
!
!
!
interface Ethernet0/0
!
interface Ethernet0/1
!
interface Ethernet0/2
!
interface Ethernet0/3
!
interface Ethernet1/0
!
interface Ethernet1/1
!
interface Ethernet1/2
!
interface Ethernet1/3
!
interface Ethernet2/0
!
interface Ethernet2/1
!
interface Ethernet2/2
!
interface Ethernet2/3
!
interface Ethernet3/0
!
interface Ethernet3/1
!
interface Ethernet3/2
!
interface Ethernet3/3
!
interface Vlan1
no ip address
shutdown
!
ip forward-protocol nd
!
!
no ip http server
no ip http secure-server
!
!
!
!
!
control-plane
!
!
line con 0
exec-timeout 0 0
privilege level 15
logging synchronous
line aux 0
exec-timeout 0 0
privilege level 15
logging synchronous
line vty 0 4
login
!
!
end

IOU1# show run
IOU1# show running-config
Building configuration...

Current configuration : 1400 bytes
!
! Last configuration change at 11:51:41 UTC Thu Apr 20 2023
!
version 15.2
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
service compress-config
!
hostname IOU1
!
boot-start-marker
boot-end-marker
!
!
logging discriminator EXCESS severity drops 6 msg-body drops EXCESSCOLL
logging buffered 50000
logging console discriminator EXCESS
!
no aaa new-model
!
!
!
!
!
no ip icmp rate-limit unreachable
!
!
!
no ip domain-lookup
ip cef
no ipv6 cef
!
!
!
spanning-tree mode pvst
spanning-tree extend system-id
spanning-tree vlan 1 priority 4096
!
vlan internal allocation policy ascending
!
ip tcp synwait-time 5
!
!
!
!
!
!
!
!
!
!
!
!
!
interface Ethernet0/0
shutdown
!
interface Ethernet0/1
!
interface Ethernet0/2
!
interface Ethernet0/3
!
interface Ethernet1/0
!
interface Ethernet1/1
!
interface Ethernet1/2
!
interface Ethernet1/3
!
interface Ethernet2/0
!
interface Ethernet2/1
!
interface Ethernet2/2
!
interface Ethernet2/3
!
interface Ethernet3/0
!
interface Ethernet3/1
!
interface Ethernet3/2
!
interface Ethernet3/3
!
interface Vlan1
no ip address
shutdown
!
ip forward-protocol nd
!
!
no ip http server
no ip http secure-server
!
!
!
!
!
control-plane
!
!
line con 0
exec-timeout 0 0
privilege level 15
logging synchronous
line aux 0
exec-timeout 0 0
privilege level 15
logging synchronous
line vty 0 4
login

[sebastien3]

@MHM Cisco World OK for SW1 and SW5 but the problem is rather on SW2/SW3/SW4 on which I would like to see the config on the COST part.