From: Henk de Tombe (henk.de.tombe@qi.nl)
Date: Sun Oct 02 2005 - 03:58:45 GMT-3
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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