Solved: Role of 'root' in spantree?

iholdings · ‎04-21-2010

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

Giuseppe Larosa · ‎04-21-2010

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

View solution in original post

Giuseppe Larosa · ‎04-21-2010

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 · ‎04-21-2010

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 · ‎04-21-2010

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

visitor68 · ‎04-21-2010

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 · ‎04-21-2010

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 · ‎04-21-2010

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

visitor68 · ‎04-21-2010

Enter this on the 2948.

set spantree priority 4096 mst

visitor68 · ‎04-21-2010

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 · ‎04-21-2010

Thanks for everyone's help. I'll be doing some serious reading regarding spantree.