From: Huan Pham (Huan.Pham@peopletelecom.com.au)
Date: Sun Jul 13 2008 - 22:14:07 ART
Hi GS,
I've just noticed that when we configure uplink fast on a switch, the
default costs of all ports may be raised by 3000.
Note that this does not happen all the time. For instance, if we have a
ring topology like below, and SW1 is the root
SW1 ---SW3
| |
| |
SW2----SW4
When you configure uplinkfast on SW2, SW3, SW4, the default port costs
only change on SW2 and SW3.
Cisco documentation on this topic is very limited.
It looks like the objective of raising the port costs is to minimize the
chance of looping, but doing this way may also change SPT path selection
(L2 traffic engineering),i.e. breaking the previous tasks.
I am curious to know based on what factors the port cost change, and the
purpose of raising port cost by 3000.
Looking forward to see your view on this topic.
SW2(config)#no spanning-tree uplinkfast
SW2(config)#do sh span vlan 1
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 001c.b0d9.0280
Cost 19
Port 16 (FastEthernet0/14)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0014.a86b.df00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- --------
--------------------------------
Fa0/4 Desg FWD 19 128.6 P2p
Fa0/6 Desg FWD 19 128.8 P2p
Fa0/13 Altn BLK 19 128.15 P2p
Fa0/14 Root FWD 19 128.16 P2p
Fa0/19 Desg FWD 19 128.21 P2p
Fa0/20 Desg FWD 19 128.22 P2p
Fa0/21 Desg FWD 19 128.23 P2p
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- --------
--------------------------------
Fa0/24 Desg FWD 19 128.26 P2p
SW2(config)#span uplinkfast
SW2(config)#do sh span vlan 1
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 001c.b0d9.0280
Cost 3019
Port 16 (FastEthernet0/14)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 49153 (priority 49152 sys-id-ext 1)
Address 0014.a86b.df00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15
Uplinkfast enabled
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- --------
--------------------------------
Fa0/4 Desg FWD 3019 128.6 P2p
Fa0/6 Desg FWD 3019 128.8 P2p
Fa0/13 Altn BLK 3019 128.15 P2p
Fa0/14 Root FWD 3019 128.16 P2p
Fa0/19 Desg FWD 3019 128.21 P2p
Fa0/20 Desg FWD 3019 128.22 P2p
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- --------
--------------------------------
Fa0/21 Desg FWD 3019 128.23 P2p
Fa0/24 Desg FWD 3019 128.26 P2p
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