Steps to add new L2 switch to Flex Linked switches

[sungpillhan]

Hello, 

First of all, I'm a system admin, but not so much experience on switching. 

I will add a 2nd switch to below diagram and have questions. 

1. Will it cause any downtime on SW1? sh int sum shows traffic on  SW1 port 1/0/50. Not sure what this traffic is even though the port is the backup port in SW1 Flex Link. 

2. Does configuration on SW1 and SW2 look good? 

3. How can loop be handled once SW2 is in the place? Any command to change port to forward, block?

4. Anything else to keep in mind? This is manufacturing factory, the RTO should be notified to customers, so I'm being very careful and accurate to estimate RTO. 

SW 1 and SW 2 are catalyst 2960x PoE

Current Configuration on SW1:

-----------------------------------

interface GigabitEthernet1/0/49
description LGE::LAN::To BB_A
switchport trunk allowed vlan 1,5,10,11
switchport mode trunk
switchport nonegotiate
switchport backup interface Gi1/0/50
mls qos trust cos
spanning-tree portfast trunk
!
interface GigabitEthernet1/0/50
description LGE::LAN::To BB_B
switchport trunk allowed vlan 1,5,10,11
switchport mode trunk
mls qos trust cos
spanning-tree portfast trunk

New Configration on SW1:

-------------------------------

No Change

New Configuration on SW2:

-------------------------------

interface GigabitEthernet1/0/49
description LGE::LAN::To BB_B
switchport trunk allowed vlan 1,5,10,11
switchport mode trunk
switchport nonegotiate
mls qos trust cos
spanning-tree portfast trunk
!
interface GigabitEthernet1/0/50
description LGE::LAN::To SW1
switchport trunk allowed vlan 1,5,10,11
switchport mode trunk
mls qos trust cos
spanning-tree portfast trunk

Steps to add SW2:

----------------

1) SW1 has flex link to Backbone switch A(SW1 local port: gi1/0/49) and B(SW1 local port: gi1/0/50)

2) I'm adding SW2 (new ) to the topology. 

3) Pulling out the Fiber2 cable from SW1 gi1/0/50 and connect to SW2 gi 1/0/49

4) Connect SW1(gi1/0/50) and SW2(gi1/0/50) using Fiber3 cable 

Thank you experts in advance. 

[paul driver]

Hello

Adding that 2nd switch is applicable how you have shown however you should NOT keep the flex link configuration on sw1 as flex link disables spanning-tree and with the 2nd switch being introduced and how the interconnects will be placed then you will need STP

So suggest remove the flex link config off sw1 and portfast trunk off all the trunks

You don’t mention what stp mode you are running and I assume those core switches are the stp primary and secondary roots if they are not then they should be!

however you shouldn’t incur downtime if you attach the sw2 to sw1 via sw1 secondary link and then attach sw2 second link up towards its core switch 

The end stp topology should be with sw1/sw2 uplinks to each core become root ports and one of the ports via the interconnect link between each other should become stp blocked 

[sungpillhan]

Paul, 

Thank you for reply. 

I reviewed my CCNA book again and found something in our current layout and it also refers to your comment, removing 'spanning-tree portfast trunk'. 

It seem as our current network has STP not configured correctly. Not sure adding SW2(new) is OK at this point or postpone the maintenance until this issue is resolved. 

Actually, our network is configured as below:

There are two switches connected to BB, SW0 and SW1. I'm introducing SW2(new). 

And SW0 an SW1 spanning-tree status shows as below. 

Question 1. SW0 and SW1, both are root bridge  for VLAN 1, 4,10 and 12. It seems like bridge id is constructed as default priority + vlan id. But how can one vlan have multiple root bridges? Is this because STP is not configured correctly? How to fix? Take out 'spanning-tree portfast trunk'?

Question 2. What happens  if I introduce SW2(new) at this network configuration? There will be interlink between SW1 and SW2. If STP is not configured correctly, I think it will create loop. What to do? 

Both 

========SW0====================

SW0#sh spanning-tree

VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 00b8.b3ce.1100
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 00b8.b3ce.1100
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/0/51 Desg FWD 4 128.51 P2p

SW0# sh run

spanning-tree mode pvst
spanning-tree extend system-id

interface GigabitEthernet1/0/49

description To_BB_A
switchport trunk allowed vlan 1,4,10,12
switchport mode trunk
switchport nonegotiate
switchport backup interface Gi1/0/50
mls qos trust cos
spanning-tree portfast trunk
!
interface GigabitEthernet1/0/50

description To_BB_B
switchport trunk allowed vlan 1,4,10,12
switchport mode trunk
mls qos trust cos
spanning-tree portfast trunk

============SW1================

SW1#sh spanning-tree

VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 00a5.bf65.1200
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 00a5.bf65.1200
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/0/51 Desg FWD 4 128.51 P2p

SW1# sh run

spanning-tree mode pvst
spanning-tree extend system-id

interface GigabitEthernet1/0/49

description To_BB_A
switchport trunk allowed vlan 1,4,10,12
switchport mode trunk
switchport nonegotiate
switchport backup interface Gi1/0/50
mls qos trust cos
spanning-tree portfast trunk
!
interface GigabitEthernet1/0/50

description To_BB_B
switchport trunk allowed vlan 1,4,10,12
switchport mode trunk
mls qos trust cos
spanning-tree portfast trunk