From: Petr Lapukhov (petr@internetworkexpert.com)
Date: Mon Jul 14 2008 - 10:00:01 ART
Huan,
This is kind of weird - could you please look at "debug spanning-tree
uplinkfast" output on "misbehaving" switch.
Uplinkfast should always increase the switch priority to a high value along
with increasing port costs by 3000. The purpose is tied to the nature of
Uplikfast feature - the feature requires at least one alternative
(non-designated, blocking) on the switch. Essentially, by increasing bridge
priority, we ensure that a switch would not likely become a root bridge
(with all port being designated). And by increasing port costs, we ensure
that ports are unlikely to become designated on the respective conenctions.
Also, in the ring topology you demostrated (provided that each switch has
just one connection to it's peers), there is going to be just one blocked
port (if you are using multiple links between switches, you will see
additional blocked ports). Therefore, it makes sense to configure Uplinkfast
just on the switch with the blocked port. Obviously, if any of the links in
the ring is lost, the topology will no longer be redundant, and UplinkFast
would become ineffective, even if configured on all non-root switches in the
topology.
Due to these reasons, uplinkfast is the feature usually recommeded for
access-level switches with redundant uplink connections. However, in modern
days, it's usually not a good idea using STP for redundancy purposes in
enterprise networks :)
-- Petr Lapukhov, CCIE #16379 (R&S/Security/SP/Voice) petr@internetworkexpert.comInternetwork Expert, Inc. http://www.InternetworkExpert.com Toll Free: 877-224-8987 Outside US: 775-826-4344 Online Community: http://www.IEOC.com CCIE Blog: http://blog.internetworkexpert.com
2008/7/14 Huan Pham <Huan.Pham@peopletelecom.com.au>:
> 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 > > > _______________________________________________________________________ > Subscription information may be found at: > http://www.groupstudy.com/list/CCIELab.html
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