(Span-Tree) Question

[Mohamed Sobair]

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..

[Roberto Salazar]

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

also what is the default priority?

>> The bridge priority what a switch uses to elect the root for that spanning-tree instance. You should never configure a switch to have a higher priority, that does not accomplich anything, and you should lower a particular switch to a certain value if you want to ensure that switch will always be root. The default is 32768 on all switches.

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?

>> The spanning-tree cose is used by the switches to determine the optimum path to the root switch. You lower or increase the cost depending on how you want the STP topology to look like, there may some link that you do not want use in place of another link, in this case you would give the other link lower cost and the one you do not prefer a higher cost.

Default value depends on the interface type

10 Mbps - 100

100 Mbps - 19

1 Gig - 4

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/sw_8_5/confg_gd/spantree.htm#wp1020675

Please rate helpful posts.

[Mohamed Sobair]

Dear Roberto,

Thanks for the Post, its extremely helpful,

[Mohamed Sobair]

Hi Roberto,

I am just have one query,

What is root ID & the root Bridge??

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

Appreciate your feedback,

[Mohamed Sobair]

Hi Roberto,

I forgot to post an example:

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

Omam# sh span tree vlan 805

MST07

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

what is the root ID above & What is the Bridge ID??

Appreciate your feedback,

[farkascsgy]

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

bye

FCS

Please rate me if I helped.

[Mohamed Sobair]

Hi Roberto,

Could you please explain to me the bellow output of (sh span tree vlan 805#) from my switch as follows:

Desg FWD

Boun FWD

Root FWD

Altn BLK

could you explain the meaning of (Desg,Root,Boun,Altn) ports,

Appreciate your feedback,

[farkascsgy]

Designated port - it means that this port has best root path cost from the segment.

..............RootBridge

......cost2/......\ cost19

............../........\

.........RA-------X-RB

...............seg C.....

The designated port is for segment C on RA router because it has better cost to the RootBridge and the other router (RB) is in blocking state.

Root Port - the best path to the root, on every switch there is only one root port. There is one on RA and RB - which are directly connected to the RootBridge.

Alternate - If there is another link between the Root and RA, one of them are better (has lower interface number) and prefered, The prefered is the root port and the other is the alternate port. In case of failure of the root port it can function immediately as root port.

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml#states

bye

FCS

Please rate me if I helped.