odd spanning tree issue

paul amaral · ‎08-17-2017

I'm having an odd issue where i have 2 switches connected together one link via comcast ethernet service and the other via a wireless link. I have the switches set to per vlan RSTP. Switch 1 is the root and switch 2 is the downstream switch.

On switch 2, the non root switch, the comcast ethernet link is delivered on a trunk the wireless link is a point to point interface.

if i leave to comcast ethernet line as an access port i get

Interface Role Sts Cost Prio.Nbr Type

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

Gi4/6 Root FWD 19 32.390 P2p

Gi9/46 Desg FWD 4 16.1070 P2p

Even though im telling this switch i want to use gig9/6 with the lower pri.

Note its not blocking either :(

if i change the comcast ethernet port to a trunk on the root switch i get the following which correct.

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi4/6 Altn BLK 200 32.390 P2p
Gi9/46 Root FWD 100 16.1070 P2p

Im not sure what is going on here its almost like if port 9/46 is getting the vlan tag stripped

below is a more technical explanation

if comcast ethernet link port is an access port:

switch2 non root #sh spanning-tree vlan 109

VLAN0109

Spanning tree enabled protocol rstp

Root ID Priority 4205

Address 00a2.89c9.6d80

Cost 19

Port 390 (GigabitEthernet4/6)

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 8192

Address 0009.12a7.386d

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 300

Interface Role Sts Cost Prio.Nbr Type

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

Gi4/6 Root FWD 19 32.390 P2p

Gi9/46 Desg FWD 4 16.1070 P2p

switch1 root switch#sh spanning-tree vlan 109

VLAN0109

Spanning tree enabled protocol rstp

Root ID Priority 4205

Address 00a2.89c9.6d80

This bridge is the root

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 4205 (priority 4096 sys-id-ext 109)

Address 00a2.89c9.6d80

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 300 sec

Interface Role Sts Cost Prio.Nbr Type

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

Gi1/0/22 Desg FWD 4 128.22 P2p Edge

Gi1/0/23 Desg FWD 200 32.23 P2p

Gi1/0/24 Desg FWD 100 16.24 P2p

After I switch the comcast link port to trunk

switch1 on root switch#sh spanning-tree vlan 109

VLAN0109

Spanning tree enabled protocol rstp

Root ID Priority 4205

Address 00a2.89c9.6d80

Cost 100

Port 1070 (GigabitEthernet9/46)

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 8192

Address 0009.12a7.386d

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 300

Interface Role Sts Cost Prio.Nbr Type

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

Gi4/6 Altn BLK 200 32.390 P2p

Gi9/46 Root FWD 100 16.1070 P2p

this is what i want port 9/46 to forward and 4/6 to blk i just can understand why this has to be changed to a trunk port and why an access port tagged with vlan 109 doesnt work.

any ideas,

paul

chrihussey · ‎08-22-2017

Which command did you use to set the port priority?

"Cisco IOS uses the port priority value when the LAN port is configured as an access port and uses VLAN port priority values when the LAN port is configured as a trunk port. "

Here is a link:

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/15-5SY/config_guide/sup2T/15_5_sy_swcg_2T/spanning_tree.html?dtid=osscdc000283#21334

paul amaral · ‎08-25-2017

after switching the link to a trunk on the root switch i do see

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi4/6 Altn BLK 200 32.390 P2p
Gi9/46 Root FWD 100 16.1070 P2p

which is correct and working, im just not sure why have the port an access port on root caused port 9/46 on the non root to be desg and 4/6 to be the root port. The port on the non root switch is a trunk but a trunk should be able to communicate with an access port if its tag with it was.

this is the trunk setup on the non root.

switchport
switchport trunk allowed vlan 109,400-499
switchport trunk pruning vlan 109,400-499
switchport mode trunk
load-interval 30
no cdp enable
spanning-tree link-type point-to-point
spanning-tree vlan 109 port-priority 16
spanning-tree vlan 109 cost 100
end

just curious on this why configuring the root port as and access port tagged with 109 doesnt work.

thanks,

P

chrihussey · ‎08-29-2017

"The port on the non root switch is a trunk but a trunk should be able to communicate with an access port if its tag with it was."

This is not a correct statement and may be your issue. I've read this several times now and assume you had the two switches connected with one side being an access port and the other a trunk. The side configured as an access port would not insert a tag to packets identifying it as VLAN 109. If you think about it, if there was PC connected to an access port in VLAN 109, do you think packets sent to the PC would have a VLAN tag? No, they would be regular ethernet frames.

With one switch configured as access and the other trunk, the side configured as access would send untagged frames to the trunk port and be assumed to be in VLAN 1 since that was the native VLAN of the trunk.

Only when both sides are configured the same, (access to access or trunk to trunk), would you be able to properly control and achieve the desired spanning tree topology.