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SpanTree (Question)

Hi All,

I would like to know 2 things related to Span-tree, as we deployed it on our switches:

1- what is spantree priority? and when should I higher or lower the priority?

also what is the default priority?

2- what is spantree cost? and when should I lower or higher the spantree cost on an interface? what is the default spantree cost for a vlan?

Please let me know your feedback since I have lots of doubts about spantree & BPDU?

Appreciate your Help..


Re: SpanTree (Question)


1) Priority is used to elect root bridge; brdigeID=2byte (Priority)+ 6byte (MAC). Default is 32768, the lower value is more prefered in the root bridge election. If there is tie in priority the lower MAC address is prefered.

2) Cost is a kind of distance from the root bridge, cost is depend on the port speed: 10G -> 2; 1G ->4; 100M ->19; 10M ->100..., with cost you can control on which switch which port become designated port for the segment...



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Re: SpanTree (Question)


I just have one question to clarify 2 things:

What is the root ID & the Bridge ID??

Is it the port ID it self on the swith or the Switch ID??

FOR example: the bellow is an output from one of our backbone switches:

Omam# sh span tree vlan 805


Spanning tree enabled protocol mstp

Root ID Priority 32775

Address 0015.c6c0.8800

Cost 60000

Port 20 (GigabitEthernet0/20)

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

Bridge ID Priority 32775 (priority 32768 sys-id-ext 7)

Address 0016.c70d.7880

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

Interface Role Sts Cost Prio.Nbr Type

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

Gi0/1 Boun FWD 200000 128.1 P2p Bound(PVST)

Gi0/3 Boun FWD 200000 128.3 P2p Bound(PVST)

Gi0/4 Desg FWD 200000 128.4 P2p

Gi0/19 Boun FWD 200000 128.19 P2p Bound(PVST)

Gi0/20 Root FWD 20000 128.20 P2p

Po1 Altn BLK 100000 128.616 P2p

Appreciate your feedback,


Re: SpanTree (Question)

Root Bridge is a switch who wins the root election in SPT, it has the best RootBridgeID (best means lowest). RootBridegID is 8bytes ID, include 2 bytes prioriry and 6 byes MAC address.

This is your RootBridgeID:

Root ID Priority 32775 + Address 0015.c6c0.8800



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