Slow RSTP Convergence (~30 Seconds)

[jradke]

I have a 7609 with two LAG groups (Etherchannel not LACP) going to two separate devices that DO NOT participate in spanning-tree (Occam gear if you must know). I'm running 802.1w across the LAG groups but the convergence time is terrible! In essence, the 7609 is running spanning tree against itself (between the two blades). What can I do to fix my configuration?

Topology:

7609 Blade 1 ----1G----> Occam 6450 VPT1

                                              /\

                                               | 10G

                                              \/

7609 Blade 2 ----1G----> Occam 6450 VPT2

Testing:

If I disable the ports on the equipment connected to g1/17 & g1/18, it takes ~30 seconds for spanning tree to start forwarding on g2/17 & g2/18.

When I bring the ports back up on the other side of g1/17 & g1/18 which are in a lag group, g2/17 & g2/18  immediately go into block mode while g1/17 & g1/18 start learning for another ~30 seconds!!

Cisco IOS Software Version 12.2(33)SRD5, RELEASE SOFTWARE (fc2)

7609#sho span int g1/17

Vlan                Role Sts Cost      Prio.Nbr Type

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

VLAN0081            Desg LRN 3         128.3329 P2p

7609#sho span int g1/18

Vlan                Role Sts Cost      Prio.Nbr Type

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

VLAN0081            Desg LRN 3         128.3329 P2p

7609#sho span int g2/17

Vlan                Role Sts Cost      Prio.Nbr Type

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

VLAN0081            Back BLK 3         128.3330 P2p

7609#sho span int g2/18

Vlan                Role Sts Cost      Prio.Nbr Type

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

VLAN0081            Back BLK 3         128.3330 P2p

sho spanning-tree vlan 81 detail

VLAN0081 is executing the rstp compatible Spanning Tree protocol

  Bridge Identifier has priority 32768, sysid 81, address 0027.0dfd.b600

  Configured hello time 2, max age 20, forward delay 15, tranmsit hold-count 6

  We are the root of the spanning tree

  Topology change flag not set, detected flag not set

  Number of topology changes 16 last change occurred 19:28:34 ago

          from Port-channel81

  Times:  hold 1, topology change 35, notification 2

          hello 2, max age 20, forward delay 15

  Timers: hello 0, topology change 0, notification 0, aging 300

Port 3329 (Port-channel81) of VLAN0081 is designated forwarding

   Port path cost 3, Port priority 128, Port Identifier 128.3329.

   Designated root has priority 32849, address 0027.0dfd.b600

   Designated bridge has priority 32849, address 0027.0dfd.b600

   Designated port id is 128.3329, designated path cost 0

   Timers: message age 0, forward delay 0, hold 0

   Number of transitions to forwarding state: 1

   Link type is point-to-point by default

   BPDU: sent 34958, received 1

Port 3330 (Port-channel82) of VLAN0081 is backup blocking

   Port path cost 3, Port priority 128, Port Identifier 128.3330.

   Designated root has priority 32849, address 0027.0dfd.b600

   Designated bridge has priority 32849, address 0027.0dfd.b600

   Designated port id is 128.3329, designated path cost 0

   Timers: message age 16, forward delay 0, hold 0

   Number of transitions to forwarding state: 1

   Link type is point-to-point by default

   BPDU: sent 92, received 35337

Configuration:

interface Port-channel81

description Network-8-VPTs

switchport

switchport access vlan 81

switchport trunk encapsulation dot1q

switchport trunk native vlan 80

switchport trunk allowed vlan 81

switchport mode trunk

load-interval 30

!

interface GigabitEthernet1/17

description Occam-VPT1-2

switchport

switchport access vlan 81

switchport trunk encapsulation dot1q

switchport trunk native vlan 80

switchport trunk allowed vlan 81

switchport mode trunk

logging event link-status

load-interval 30

channel-group 81 mode on

!

interface GigabitEthernet1/18

description Occam-VPT2-1

switchport

switchport access vlan 81

switchport trunk encapsulation dot1q

switchport trunk native vlan 80

switchport trunk allowed vlan 81

switchport mode trunk

logging event link-status

load-interval 30

channel-group 81 mode on

!

interface Port-channel82

description Network-8-VPTs

switchport

switchport access vlan 81

switchport trunk encapsulation dot1q

switchport trunk native vlan 80

switchport trunk allowed vlan 81

switchport mode trunk

!

interface GigabitEthernet2/17

description Occam-VPT2-2

switchport

switchport access vlan 81

switchport trunk encapsulation dot1q

switchport trunk native vlan 80

switchport trunk allowed vlan 81

switchport mode trunk

logging event link-status

load-interval 30

channel-group 82 mode on

!

interface GigabitEthernet2/18

description Occam-VPT2

switchport

switchport access vlan 81

switchport trunk encapsulation dot1q

switchport trunk native vlan 80

switchport trunk allowed vlan 81

switchport mode trunk

logging event link-status

load-interval 30

channel-group 82 mode on

end

Thanks for any help you can offer!

-=JGR

[Peter Paluch · Accepted solution]

Hello,

The behavior you are seeing may appear confusing but it is actually expectable.

What you have done is, exactly as you have noted yourself, essentially connected your 7609 back to itself. On this link, one of the ports is elected as Designated Forwarding, the other as Backup Discarding. The Backup port is defined as a port that provides a backup of Designated port - both these ports have to be located on the same switch and connected to the same segment. This is clearly the case with your link, as also evidenced by some of your attached output:

7609#sho span int g2/17

Vlan                Role Sts Cost      Prio.Nbr Type

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

VLAN0081            Back BLK 3         128.3330 P2p

7609#sho span int g2/18

Vlan                Role Sts Cost      Prio.Nbr Type

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

VLAN0081            Back BLK 3         128.3330 P2p

Now, Cisco switches do not put the Backup port rapidly into Designated Forwarding role/state. If the Designated port fails, the Backup ports has to go over the RSTP sequence of Discard -> Learning -> Forwarding states driven by timers, hence the 30s delay you are experiencing. I am not sure if this is mandated by the RSTP standard (I do believe it is but I am not 100% sure here) or if this is just a peculiarity of Cisco RSTP implementation but the bottom line is - RSTP will not rapidly unblock a Backup Discarding port.

What I suggest using here is the Flex Link feature, i.e. defining a pair of ports that actually do not use STP but that provide a mutual backup for each other - if one works, the other is automatically put to a blocking state, and if the primary port fails, the other is automatically and immediately put to forwarding state. Read more about the feature here:

http://www.cisco.com/en/US/docs/routers/7600/ios/12.2SR/configuration/guide/flexlink.html

Also, this documentation seems to be very limited on the additional features related to the Flex Links (preemption of the ports), so I suggest extending the knowledge about the feature using this documentation for a different Catalyst product line (3560):

http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_58_se/configuration/guide/swflink.html

In your case, the configuration should look as follows:

interface Po81
 switchport backup interface Po82
 switchport backup interface Po82 preemption mode forced

Please test this in a maintenance window and let us know if it worked!

Best regards,

Peter

[jradke]

Thanks Peter for the quick (and accurate) response!

I've tested this and it works well. My best results are 15-200ms and my worse 3-5 seconds all depending on where I shutdown or pull a link. This was tested across port-channels with details below.

Configuration:

interface Port-channel81

description Network-8-VPTs

switchport

switchport access vlan 81

switchport trunk encapsulation dot1q

switchport trunk native vlan 80

switchport trunk allowed vlan 81

switchport mode trunk

switchport backup interface Port-channel82

load-interval 30

!

interface Port-channel82

description Network-8-VPTs

switchport

switchport access vlan 81

switchport trunk encapsulation dot1q

switchport trunk native vlan 80

switchport trunk allowed vlan 81

switchport mode trunk

end

Traffic Test:

7609#sho int port-channel 81 | i second

  30 second input rate 337000 bits/sec, 539 packets/sec

  30 second output rate 18110000 bits/sec, 1501 packets/sec

7609#sho int port-channel 82 | i second

  30 second input rate 0 bits/sec, 0 packets/sec

  30 second output rate 0 bits/sec, 0 packets/sec

7609#sho int switchport backup

Switch Backup Interface Pairs:

Active Interface        Backup Interface        State

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

Port-channel81          Port-channel82          Active Up/Backup Standby

7609(config)#int port-channel 81

7609(config-if)#shut

7609#sho int port-channel 81 | i second

  30 second input rate 0 bits/sec, 0 packets/sec

  30 second output rate 0 bits/sec, 0 packets/sec

7609#sho int port-channel 82 | i second

  30 second input rate 109000 bits/sec, 173 packets/sec

  30 second output rate 6573000 bits/sec, 545 packets/sec

7609#sho int switchport backup

Switch Backup Interface Pairs:

Active Interface        Backup Interface        State

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

Port-channel81          Port-channel82          Active Down/Backup Up