From: Victor Cappuccio (cvictor@protokolgroup.com)
Date: Sun Oct 02 2005 - 04:06:56 GMT-3
Thanks for that Henk!!
:D
Henk de Tombe wrote:
>Hi Victor,
>
>Link to configuring port-priority:
>http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/12225sec/3550scg/s
>wstp.htm#wp1020470
>
>Link to configuring cost:
>http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/12225sec/3550scg/s
>wstp.htm#wp1105354
>
>
>The cost and port-priority both have the same goal, make one link more
>attractive than the other(s). The thing you have to watch out for is where
>you use these commands.
>
>The cost command is used on NON ROOT bridges. If all interfaces have the
>same cost value, spanning tree puts the interface with the lowest interface
>number in the forwarding state and blocks the other interfaces.
>
>Look at the show commands below, port gi1/0/13 is in forwarding state. This
>switch is a secondary root switch because it has some ports in blocking
>state.
>
>Rack1SW2(config)#do sh span vl 3
>
>VLAN0003
> Spanning tree enabled protocol ieee
> Root ID Priority 32771
> Address 000d.295f.6380
> Cost 4
> Port 13 (GigabitEthernet1/0/13)
> Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
>
> Bridge ID Priority 32771 (priority 32768 sys-id-ext 3)
> Address 000d.edbc.5480
> Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
> Aging Time 15
>
>Interface Role Sts Cost Prio.Nbr Type
>---------------- ---- --- --------- --------
>--------------------------------
>Gi1/0/13 Root FWD 4 128.13 P2p
>Gi1/0/14 Altn BLK 4 128.14 P2p
>Gi1/0/15 Altn BLK 4 128.15 P2p
>
>
>
>To make e.g. port Gi1/0/15 forwarding frames for this VLAN, we have to
>change the port cost.
>
>Rack1SW2(config)#int gig 1/0/15
>Rack1SW2(config-if)#spanning-tree vlan 3 cost 3
>Rack1SW2(config-if)#
>Rack1SW2(config-if)#do sh span vl 3
>
>VLAN0003
> Spanning tree enabled protocol ieee
> Root ID Priority 32771
> Address 000d.295f.6380
> Cost 3
> Port 15 (GigabitEthernet1/0/15)
> Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
>
> Bridge ID Priority 32771 (priority 32768 sys-id-ext 3)
> Address 000d.edbc.5480
> Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
> Aging Time 15
>
>Interface Role Sts Cost Prio.Nbr Type
>---------------- ---- --- --------- --------
>--------------------------------
>Gi1/0/13 Altn BLK 4 128.13 P2p
>Gi1/0/14 Altn BLK 4 128.14 P2p
>Gi1/0/15 Root FWD 3 128.15 P2p
>
>
>
>You can see that port Gi1/0/15 is in forwarding state right now, and ports
>1/0/13 and 1/0/14 are in Alternate state. Changing the port-priority command
>at a NON ROOT switch doesn't have any effect because the port-priority value
>is advertised in BPDU packets. Since Rack1SW2 is a NON ROOT switch it
>doesn't send BPDU packets, the switch only receives them.
>
>
>
>Let's take a look at the port-priority command. Below is a show span vlan 3
>output at the ROOT switch. You can see that all ports are in forwarding
>state, this is expected behaviour for a root switch. I've changed the
>port-priority on port 1/0/15 from 128 to 96.
>
>Rack1SW1(config-if)#do sh span vl 3
>
>VLAN0003
> Spanning tree enabled protocol ieee
> Root ID Priority 32771
> Address 000d.295f.6380
> This bridge is the root
> Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
>
> Bridge ID Priority 32771 (priority 32768 sys-id-ext 3)
> Address 000d.295f.6380
> Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
> Aging Time 15
>
>Interface Role Sts Cost Prio.Nbr Type
>---------------- ---- --- --------- --------
>--------------------------------
>Gi1/0/3 Desg FWD 19 128.3 P2p
>Gi1/0/13 Desg FWD 4 128.13 P2p
>Gi1/0/14 Desg FWD 4 128.14 P2p
>Gi1/0/15 Desg FWD 4 96.15 P2p
>
>
>A "show span vlan 3" on the secondary root switch shows the following
>output.
>
>Rack1SW2(config-if)#do sh span vl 3
>
>VLAN0003
> Spanning tree enabled protocol ieee
> Root ID Priority 32771
> Address 000d.295f.6380
> Cost 4
> Port 15 (GigabitEthernet1/0/15)
> Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
>
> Bridge ID Priority 32771 (priority 32768 sys-id-ext 3)
> Address 000d.edbc.5480
> Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
> Aging Time 300
>
>Interface Role Sts Cost Prio.Nbr Type
>---------------- ---- --- --------- --------
>--------------------------------
>Gi1/0/13 Altn BLK 4 128.13 P2p
>Gi1/0/14 Altn BLK 4 128.14 P2p
>Gi1/0/15 Root FWD 4 128.15 P2p
>
>
>Port 1/0/15 is in forwarding state, because the ROOT switch "advertised" a
>lower port-priority values for port 1/0/15 than port 13 & 14.
>
>There is one more option to consider when we are talking about changing port
>and priority values on the switches.
>
>
>
>Regards,
>Henk
>
>
>
>
>
>
>-----Oorspronkelijk bericht-----
>Van: nobody@groupstudy.com [mailto:nobody@groupstudy.com] Namens Victor
>Cappuccio
>Verzonden: zondag 2 oktober 2005 00:59
>Aan: Cisco certification
>Onderwerp: Weakness in Spanning-Tree Port Costs and Priorities
>
>S: Weakness in Spanning-Tree Port Costs and Priorities
>
>Hello People..
>
>Please can one suggest me a good link to understand these 2 evils.
>
>Thanks
>Victor
>
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