Solved: STP port-priority on interface feature problem

blue phoenix · ‎05-11-2015

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

Jon Marshall · ‎05-12-2015

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

View solution in original post

Bilal Nawaz · ‎05-11-2015

.

Please rate useful posts & remember to mark any solved questions as answered. Thank you.

blue phoenix · ‎05-11-2015

Sorry, you were saying?

Rolf Fischer · ‎05-11-2015

Hi,

I don't understand exactly what you're trying to achieve but remember that the port-priority is the very last tie-breaker when BPDUs are compared in order to elect the root port or the designated ports.

Prior their costs are compared, and in case of equal costs the (sender) Bridge-IDs are compared.

So depending on what you want to achieve you may want to change costs rather than priorities.

HTH

Rolf

blue phoenix · ‎05-11-2015

yes, I know it's supposed to be a tie breaker, but I believe that the spanning-tree vlan 3 port-priority that is configured on the interface level basically trumps up or can manipulate what interface to forward bpdu or or vlan traffic. At least that's what I think the INE ccnp switch workbook lab is trying to achieve...

SW1------SW3 (3 trunk interfaces) ----|

| |

Po1 (3 phy interfaces) |

| |

SW2 Po2 (3 phy interfaces) ------------|

Jon Marshall · ‎05-11-2015

I agree with Rolf, this is a very confusing post ie.

you show the output of vlan 3 but don't say which switch it is off and then you talk about vlan 4 ?

Jon

blue phoenix · ‎05-11-2015

It's about VLAN3 ,typo error now corrected.

Jon Marshall · ‎05-11-2015

Okay but it's still not clear what you are trying to do and which switch the above output is from.

Jon

blue phoenix · ‎05-11-2015

I have corrected it, SW2 is directly connected to SW1. Using port-priority on that connected link to trick that the best path is go via SW3 then SW1...

Jon Marshall · ‎05-11-2015

As Rolf has said the priority of the sending port is only considered if the costs to the root switch are the same via both paths.

If the costs are not the same then it doesn't matter what you have done with port priorities because it never gets to those.

If the costs are the same via both paths then it comes down to what Rolf has outlined.

So if you want to manipulate the path selection based on port priority you need to make sure everything else used before it are equal.

Are they ?

Jon

Jon Marshall · ‎05-11-2015

Still not sure I follow ie.

I am trying to trick ROOT SW2 into believing that the directly connected SW2 should not be used for VLAN3

the above doesn't really make sense.

You also say that SW2 is the root for vlan 3 but not from the output you posted it isn't.

Jon

blue phoenix · ‎05-11-2015

Sorry, SW1 is the Root Switch for VLAN3...

Rolf Fischer · ‎05-12-2015

!!! 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

Sw1 and Sw3 do not have the same bridge-IDs, they just have the same bridge priorities.
The bridge priority is just the configurable part of the bridge-ID, followed by the MAC-Address, which is the non-configurable part and ensures uniqueness among different switches even when their priorities are the same.

[edited]

The topology is a triangle, the port-priority doesn't come into play here unless you have more than one connection between two switches. Without such a connection, the election process is done based on path-costs and, as a tie-breaker, the sender bridge-IDs.

As far as I see, you don't have a portchannel between Sw1 and Sw3. So between those two switches you can actually change the topology by STP port-priority but first you'll need to increase a portchannel's cost in order to move it into the blocking state.

blue phoenix · ‎05-12-2015

If I lower the cost on Po1, Po1 will immediately go to ALT/BLK mode and 1 of the 3 trunk links will become a RP and a DP at the same time?. Also no need to do a port-priority configuration then is that right?

blue phoenix · ‎05-12-2015

remove duplicate reply