Role of 'root' in spantree?

Answered Question
Apr 21st, 2010
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Greetings,


We have a C2948G CatOS 8.3 switch.  When we plug in an AP controller into an open port - and then unplug that port the switch drops all traffic.


When I did a "show spantree" the port's role is ROOT (the only role of that kind on the switch:



2/28                    not-connected  -         100   32

2/29                    not-connected  -         100   32

2/30                    forwarding    ROOT         4   32 P2P

2/31                    forwarding    DESG         4   32 P2P

2/32                    forwarding    DESG        19   32 Shared

2/33                    not-connected  -         100   32

2/34                    not-connected  -         100   32

2/35                    not-connected  -         100   32

2/36                    not-connected  -         100   32

2/37                    not-connected  -         100   32

2/38                    not-connected  -         100   32

2/39                    forwarding    DESG         4   32 P2P

2/40                    not-connected  -         100   32

2/41                    not-connected  -         100   32

2/42                    not-connected  -         100   32

2/43                    forwarding    DESG         4   32 P2P

2/44                    not-connected  -         100   32

2/45                    forwarding    DESG         4   32 P2P

2/46                    not-connected  -         100   32

2/47                    forwarding    DESG        19   32 P2P

2/48                    forwarding    DESG        19   32 P2P

2/49                    forwarding    DESG         4   32 P2P

2/50                    forwarding    DESG         4   32 P2P, PEER(STP)

2/51                    not-connected  -           4   32

2/52                    not-connected  -           4   32


I don't know what that means.  Can anyone help?  Thanks

Correct Answer by Giuseppe Larosa about 7 years 1 week ago

Hello  Iholdings,

root means STP root port this means that when the AP is connected is seen as the root bridge


in STP the root bridge is the port to the root bridge


For this reason when you disconnect the AP you see problems


you should decrease the spanning-tree priority with


set spantree priority vlan#


it is not correct to have an AP as root bridge in STP it should be a regular LAN Swtich


STP priority defaults to 32768 and the lower is the better.


if priorities  are equal the lowest MAC wins


Bridge-ID = STP-priority + MAC address


lowest wins


Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Wed, 04/21/2010 - 07:09
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Hello  Iholdings,

root means STP root port this means that when the AP is connected is seen as the root bridge


in STP the root bridge is the port to the root bridge


For this reason when you disconnect the AP you see problems


you should decrease the spanning-tree priority with


set spantree priority vlan#


it is not correct to have an AP as root bridge in STP it should be a regular LAN Swtich


STP priority defaults to 32768 and the lower is the better.


if priorities  are equal the lowest MAC wins


Bridge-ID = STP-priority + MAC address


lowest wins


Hope to help

Giuseppe

iholdings Wed, 04/21/2010 - 08:04
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The device is not an AP but an AP controller.  Is it acting in the same manner?


So I need to change the spantree priority to a much lower number (default = 32768)?


Thanks for your help.

Jon Marshall Wed, 04/21/2010 - 08:13
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iholdings wrote:


The device is not an AP but an AP controller.  Is it acting in the same manner?


So I need to change the spantree priority to a much lower number (default = 32768)?


Thanks for your help.


Yes it is acting the same whether it is AP or AP controller.


Yes you need to set the priority on the switch to be lower than that of the AP controller.  Assuming the AP controller has not had it's STP priority modified you need to make the STP priority on the switch lower than 32768. See Giuseppe's command example for how to do it.


Jon

ex-engineer Wed, 04/21/2010 - 08:18
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Hi -- I think what Giuseppe is trying to say is that the AP controller is participating in the root bridge election for the particular vlan it is on -- and it is winning. This is why the port goes to "root." A port status of "root" means that this port has the lowest cost to the root bridge, which is your AP controller.


In PVST+ and other versions of it, the root bridge is the bridge with the lowest bridge ID FOR THAT VLAN.


The bridge ID is a combination of the bridge priority and the MAC address -- again for a particular vlan. In other words, each vlan in PVST+ on a particular switch has an associated bridge ID.


So, on the AP controller, change the priority setting for that vlan to a HIGHER value than the value that is normally used for the root bridge when the AP controller is not connected.

iholdings Wed, 04/21/2010 - 08:20
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Having zip experience configuring CatOs - any chance I could get some help with applying the correct set command?  Is this port specific?


Config. attached.


thanks.

Jon Marshall Wed, 04/21/2010 - 08:22
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iholdings wrote:


Having zip experience configuring CatOs - any chance I could get some help with applying the correct set command?  Is this port specific?


Config. attached.


thanks.


No it's not port specific, it is a global command. Giuseppe gave you the command ie.


set spantree priority vlan#


so use a priority of 8192 to make it the root bridge. If you have multiple vlans you need to enter that command for each vlan - so for example to set it for vlan 1 -


set spantree priority 8192 vlan 1


Do this out of hours as there may be an STP reconvergence.


Jon

ex-engineer Wed, 04/21/2010 - 08:27
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By the way, if you really know "zip" about Spanning Tree, you need to learn it fast or find someone who does to manage your switched environment. This is fundamental to magaing a switched network.

iholdings Wed, 04/21/2010 - 08:47
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Thanks for everyone's help.  I'll be doing some serious reading regarding spantree

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