STP port-priority on interface feature problem

[blue phoenix]

SW1#sh spanning-tree vlan 3

VLAN0003
  Spanning tree enabled protocol ieee
  Root ID    Priority    8195
             Address     dc7b.9489.cf00
             Cost        3
             Port        56 (Port-channel1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32771  (priority 32768 sys-id-ext 3)
             Address     dc7b.94b3.0d80
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/1               Desg FWD 19        128.1    P2p Edge
Gi0/16              Altn BLK 4          16.16   P2p
Gi0/17              Altn BLK 4          16.17   P2p
Gi0/18              Altn BLK 4          16.18   P2p
Po1                 Root FWD 3         128.56   P2p

!!! HERE YOU WILL SEE I HAVE LOWER THE PRIORITY TO 16 MAKE THE 3 PHYSICAL TRUNK LINKS THE PREFERRED PATH GOING TO SW2 VIA SW3 INSTEAD OF USING THE DIRECTLY CONNECTED etherchannelo po1 to SW2.  TAKE NOTE BRIDGE ID IS SAME AS WITH SW3 327771 and the port priority of Po1 is now 128 which is higher...

SW1#sh run int g0/16
Building configuration...

Current configuration : 236 bytes
!
interface GigabitEthernet0/16
 description to SW3 Gi1/0/16
 switchport access vlan 2
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 3
 switchport mode dynamic desirable
 spanning-tree vlan 3 port-priority 16
end

SW1#sh run int g0/17
Building configuration...

Current configuration : 236 bytes
!
interface GigabitEthernet0/17
 description to SW3 Gi1/0/17
 switchport access vlan 2
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 3
 switchport mode dynamic desirable
 spanning-tree vlan 3 port-priority 16
end

SW1#sh run int g0/18
Building configuration...

Current configuration : 236 bytes
!
interface GigabitEthernet0/18
 description to SW3 Gi1/0/18
 switchport access vlan 2
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 3
 switchport mode dynamic desirable
 spanning-tree vlan 3 port-priority 16
end

!!! HERE IS THE STP CONFIGURATION ON THE ROOT SWITCH SW2
SW2#sh spanning-tree vlan 3

VLAN0003
  Spanning tree enabled protocol ieee
  Root ID    Priority    8195
             Address     dc7b.9489.cf00
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    8195   (priority 8192 sys-id-ext 3)
             Address     dc7b.9489.cf00
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/2               Desg FWD 19        128.2    P2p
Po1                 Desg FWD 3         128.56   P2p
Po2                 Desg FWD 3         128.64   P2p

!!! HERE IS THE STP CONFIGURATION ON SW3, AS YOU CAN SEE SW1 AND SW3 HAVE THE SAME BRIDGE ID 32771, SO THE TIE-BREAKER SHOULD BE THE spanning-tree vlan 3 port-priority, the priority here 
SW3#sh spanning-tree vlan 3

VLAN0003
  Spanning tree enabled protocol ieee
  Root ID    Priority    8195
             Address     dc7b.9489.cf00
             Cost        3
             Port        496 (Port-channel2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32771  (priority 32768 sys-id-ext 3)
             Address     0018.b969.db80
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/0/3             Desg FWD 100       128.3    Shr Edge
Gi1/0/13            Desg FWD 4         128.13   P2p
Gi1/0/14            Desg FWD 4         128.14   P2p
Gi1/0/15            Desg FWD 4         128.15   P2p
Po2                 Root FWD 3         128.496  P2p

Please let me know if anything here is still unclear...

Connections are below...

SW1#sh cdp neigh
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone,
                  D - Remote, C - CVTA, M - Two-port Mac Relay

Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
SW2              Gig 0/15          129              S I   WS-C3560G Gig 0/15
SW2              Gig 0/14          129              S I   WS-C3560G Gig 0/14
SW2              Gig 0/13          129              S I   WS-C3560G Gig 0/13
SW3              Gig 0/18          172              S I   WS-C3750G Gig 1/0/15
SW3              Gig 0/17          172              S I   WS-C3750G Gig 1/0/14
SW3              Gig 0/16          172              S I   WS-C3750G Gig 1/0/13
R1               Gig 0/1           161             R S I  2611XM    Fas 0/0

SW2#sh cdp neigh
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone,
                  D - Remote, C - CVTA, M - Two-port Mac Relay

Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
SW1              Gig 0/14          179              S I   WS-C3560G Gig 0/14
SW1              Gig 0/15          179              S I   WS-C3560G Gig 0/15
SW1              Gig 0/13          179              S I   WS-C3560G Gig 0/13
SW3              Gig 0/18          121              S I   WS-C3750G Gig 1/0/18
SW3              Gig 0/17          121              S I   WS-C3750G Gig 1/0/17
SW3              Gig 0/16          179              S I   WS-C3750G Gig 1/0/16
R2               Gig 0/2           151             R S I  2610XM    Fas 0/0
R6               Gig 0/6           150             R S I  1841      Fas 0/0

SW3#sh cdp neigh
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone,
                  D - Remote, C - CVTA, M - Two-port Mac Relay

Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
SW1              Gig 1/0/15        152              S I   WS-C3560G Gig 0/18
SW1              Gig 1/0/14        152              S I   WS-C3560G Gig 0/17
SW1              Gig 1/0/13        152              S I   WS-C3560G Gig 0/16
SW2              Gig 1/0/18        147              S I   WS-C3560G Gig 0/18
SW2              Gig 1/0/17        147              S I   WS-C3560G Gig 0/17
SW2              Gig 1/0/16        145              S I   WS-C3560G Gig 0/16
R3               Gig 1/0/3         147             R S I  3640      Eth 2/1
R5               Gig 1/0/5         169             R S I  1841      Fas 0/1

[Rolf Fischer]

The idea is to increase the portchannel's cost in order to unblock Gi0/16-18 (only one of the three as the other two would form a loop too). Which one of those then results in the forwarding-state can be influenced by

• costs on the local switch
• port-priority on the remote switch (if local costs are equal)

P.S.: This is a lab, right?
 

[blue phoenix]

yeah, it's a workbook lab for CCNP.... so basically I need to do 2 configurations in order for the port-priority to work?  If that is the case then port-priority only works on redundant links going to the same switch... and cannot manipulate switching path to a different switch is that it?

[Jon Marshall · Accepted solution]

If that is the case then port-priority only works on redundant links going to the same switch... and cannot manipulate switching path to a different switch is that it?

Yes, because by definition if the switch was connected to different switches then it would be the switch with the lowest ID that was chosen and it would never get to the port priorities.

Jon

[paul driver]

Hello

Just to clarify if you have two paths to the same root switch, the difference between these two commands are:

stp vlan x port- priority cost = Affecting neighboring switches ONLY
changes made are only seen on the switch you apply the port priority command to.



stp vlan x cost -Affecting all switches in path to root birdge - Applied for a total stp path cost via other switches to root bridge

Presently if the stp cost to the root bridge on its selected interface  is lost then the alternate interface to the root is preferred 

stp cost can simulate this behavior.

The new cost value on the primary stp interface will need to be of an higher value than the present total alternate root path cost or vice versa

This will also change the total stp cost of the other switches in this path to the root bridge -

res

Paul

[blue phoenix]

I think what I learned from this lab work is that spanning-tree port-priority x can only affect path on redundant links going to the SAME switch.

spanning-tree vlan x cost can change traffic direction from one switch to the other...

thanks all for the replies and comments!