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Rapid-PVST: Alternate Bloking Decision

philipp.misch
Level 1
Level 1

‎10-16-2010 02:31 AM - edited ‎03-06-2019 01:33 PM

Hi all,

I have a situation in my network which slightly confuses me!

The design is a typical 3 layer model. Backbone connects via L3 to distribution connects via L2 to access. Each access-switch has only one vlan.

Now my problem:

Why is the 'alternate blocked'-port in the c45/tengigabit scenario on the distribution switch while it is on the access-switch in the gigabit construct?

Perhaps I only misunderstood something within r-pvst, but in my opinion the 'ab'-port should be located on the access-switch like in the gig scenario.

I have already tried to reboot all switches to see what will happen to r-pvst.

The stp port cost of the port-channel interfaces is 3 for both (tengig or gig channels).

vln584.PNG

Vlan 584 (TenGig)

Distri1#sh spanning-tree vlan 584

VLAN0584
  Spanning tree enabled protocol rstp
  Root ID    Priority    8776
             Address     c84c.7581.a0c0
             Cost        3
             Port        1665 (Port-channel1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    16968  (priority 16384 sys-id-ext 584)
             Address     c84c.756a.8040
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 480

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Te6/3               Altn BLK 2         128.643  P2p
Po1                 Root FWD 3         128.1665 P2p


Distri2#sh spanning-tree vlan 584

VLAN0584
  Spanning tree enabled protocol rstp
  Root ID    Priority    8776
             Address     c84c.7581.a0c0
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    8776   (priority 8192 sys-id-ext 584)
             Address     c84c.7581.a0c0
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 480

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Te6/3               Desg FWD 2         128.643  P2p
Po1                 Desg FWD 3         128.1665 P2p

ASW#sh spanning-tree vlan 584

VLAN0584
  Spanning tree enabled protocol rstp
  Root ID    Priority    8776
             Address     c84c.7581.a0c0
             Cost        2
             Port        2 (TenGigabitEthernet1/2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    33352  (priority 32768 sys-id-ext 584)
             Address     f866.f27d.9100
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Te1/1               Desg FWD 2         128.1    P2p
Te1/2               Root FWD 2         128.2    P2p

vln472.PNG

Vlan 472 (Gig)

Dist1#sh spanning-tree vlan 472

VLAN0472
  Spanning tree enabled protocol ieee
  Root ID    Priority    8664
             Address     c84c.7581.9b80
             Cost        3
             Port        1665 (Port-channel1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    16856  (priority 16384 sys-id-ext 472)
             Address     5475.d07f.87c0
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 480

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/6               Desg FWD 4         128.6    P2p
Po1                 Root FWD 3         128.1665 P2p

Dist2#sh spanning-tree vlan 472

VLAN0472
  Spanning tree enabled protocol ieee
  Root ID    Priority    8664
             Address     c84c.7581.9b80
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    8664   (priority 8192 sys-id-ext 472)
             Address     c84c.7581.9b80
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 480

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/6               Desg FWD 4         128.6    P2p
Po1                 Desg FWD 3         128.1665 P2p


ASW#sh spanning-tree vlan 472

VLAN0472
  Spanning tree enabled protocol rstp
  Root ID    Priority    8664
             Address     c84c.7581.9b80
             Cost        4
             Port        50 (GigabitEthernet0/2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    33240  (priority 32768 sys-id-ext 472)
             Address     58bc.279b.3a80
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Desg FWD 100       128.1    P2p Edge
Fa0/2               Desg FWD 100       128.2    P2p Edge
Fa0/11              Desg FWD 19        128.11   P2p Edge
Fa0/19              Desg FWD 19        128.19   P2p Edge
Fa0/24              Desg FWD 100       128.24   Shr Edge
Fa0/28              Desg FWD 19        128.28   P2p Edge
Fa0/29              Desg FWD 19        128.29   P2p Edge

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------

Fa0/39              Desg FWD 19        128.39   P2p Edge
Fa0/41              Desg FWD 19        128.41   Shr Edge
Fa0/42              Desg FWD 100       128.42   Shr Edge
Fa0/43              Desg FWD 19        128.43   P2p Edge
Fa0/44              Desg FWD 19        128.44   P2p Edge
Fa0/45              Desg FWD 19        128.45   P2p Edge
Gi0/1               Altn BLK 4         128.49   P2p Peer(STP)
Gi0/2               Root FWD 4         128.50   P2p Peer(STP)

Perhaps I just found the mistake I made in my mind:


The sum of costs on the ASW is 2+2 (twice 10G) and on Distri1 2+3 (10G+10GChannel but not root-bridge) so the alternate blocking moves on it's position.

In the other design the ASW has 4+4 (twice 1G) and on Distri1 4+3 (1G+1GChannel but not root-bridge) so the alternate blocking is on the ASW.

Is this correct?

Regards

Philipp

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0 Helpful
2 Replies 2

philipp.misch
Level 1
Level 1

‎10-16-2010 03:19 AM

Changed port cost on TenGig Portchannel to 1 and this solved my confusion

0 Helpful

andtoth
Level 4
Level 4

‎10-16-2010 04:18 AM

Hi,

This is a normal behavior and has nothing to do with RSTP. After the Root Ports have been selected, Dist1 and ASW need to decide who will be the designated bridge on the segment between them.

Dist1 hears the root with a path cost of 3 because Po1 cost is 3. ASW hears the root with a cost of 2 because of T1/2.

Dist1 will send a BPDU towards ASW with root path cost 3. ASW has cost 2 on T1/2 so it will send a BPDU to Dist1 with a root path cost 2.

Therefore ASW assumes a designated role because it is sending a lower (better) root path cost BPDU to the segment than Dist1 (cost 2 versus 3) and ASW T1/1 will be designated and Dist1 T6/3 will be alternate blocking.

The lower link cost means a better path to the root. When you change the cost of Po1 to 1, Dist1 will advertise a better root path cost than ASW and it will be a designated bridge on the segment between them. IOS by default also applies a lower (better) cost to port channels as their bandwidth is higher than the single link.

Andras

0 Helpful
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