Re: One question about 3550 Port Priority?

From: stefan vogt (stefan-uwe_vogt@web.de)
Date: Sun Jan 05 2003 - 18:05:03 GMT-3

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Hello Richard,

Root Port Election:
A BPDU is better than another if it has:
1. a lower Root Bridge ID
2. a lower path cost to the Root
3. a lower sending Bridge ID
4. a lower sending port ID
I agree that it's better to adjust 2. than 4.

According Clark this has to be done on the downstream (non-Root Bridge). Let's prove it:

c3550-1#sh spanning vlan 1

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID Priority 10
             Address 0030.7170.0480
             Cost 38
             Port 1 (FastEthernet0/1)
             Hello Time 3 sec Max Age 40 sec Forward Delay 20 sec

  Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
             Address 000a.8a8b.e780
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
             Aging Time 300

Interface Port ID Designated Port ID
Name Prio.Nbr Cost Sts Cost Bridge ID Prio.Nbr
---------------- -------- --------- --- --------- -------------------- --------
Fa0/1 128.1 19 FWD 19 32768 00b0.c2b9.cc00 128.197
Fa0/47 128.47 19 FWD 38 32769 000a.8a8b.e780 128.47
Fa0/48 128.48 19 FWD 38 32769 000a.8a8b.e780 128.48

c3550-1#

c3550-2#sh spanning vlan 1

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID Priority 10
             Address 0030.7170.0480
             Cost 57
             Port 47 (FastEthernet0/47)
             Hello Time 3 sec Max Age 40 sec Forward Delay 20 sec

  Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
             Address 000a.8a73.dd00
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
             Aging Time 300

Interface Port ID Designated Port ID
Name Prio.Nbr Cost Sts Cost Bridge ID Prio.Nbr
---------------- -------- --------- --- --------- -------------------- --------
Fa0/47 128.47 19 FWD 38 32769 000a.8a8b.e780 128.47
Fa0/48 128.48 19 BLK 38 32769 000a.8a8b.e780 128.48

c3550-2#

c3550-2#conf t
Enter configuration commands, one per line. End with CNTL/Z.
c3550-2(config)#int fa 0/48
c3550-2(config-if)#spanning-tree vlan 1 cost 10
c3550-2(config-if)#end
c3550-2#

c3550-1#sh spanning vlan 1

  Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
             Address 000a.8a8b.e780
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
             Aging Time 300

c3550-1#

c3550-2#sh spanning vlan 1

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID Priority 10
             Address 0030.7170.0480
             Cost 48
             Port 48 (FastEthernet0/48)
             Hello Time 3 sec Max Age 40 sec Forward Delay 20 sec

  Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
             Address 000a.8a73.dd00
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
             Aging Time 20

Interface Port ID Designated Port ID
Name Prio.Nbr Cost Sts Cost Bridge ID Prio.Nbr
---------------- -------- --------- --- --------- -------------------- --------
Fa0/47 128.47 19 BLK 38 32769 000a.8a8b.e780 128.47
Fa0/48 128.48 10 FWD 38 32769 000a.8a8b.e780 128.48

c3550-2#

-Stefan

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Maybe in reply to: Richard Hanks: "One question about 3550 Port Priority?"
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