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Spanning Tree Confusion

Hi,

I want to setup a simple Looped configuration using 3 switches 2x 2980 and 1x 3560.

Both the Routers are having HSRP configured and terminate on both the switches and connected to the Routers using VLAN -1.

Also to make everything smooth I created VTP Domain and made everything transparent across the network.

However to my surprize the moment I connect all the 3 switches the Switch2980A and Switch3560A looses the connectivity to HSRP.

Any suggestion?

Regards,

Wilson Samuel

7 REPLIES

Re: Spanning Tree Confusion

hi sam,

jus configure the below said one in both client & router switch, it works as you asked for a simple & easy STP.

spanning-tree mode pvst

spanning-tree extend system-id

spanning-tree vlan 1-10 hello-time 1

spanning-tree vlan 1-10 forward-time 4

spanning-tree vlan 1-10 max-age 6

hope this works

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Re: Spanning Tree Confusion

Hi Anand,

Thanks for the reply.

I'm still surprized that why the communication gets blocked the moment I plug in the new switch to both the 2980s.

Regards,

Wilson Samuel

Re: Spanning Tree Confusion

yeah that behaviour is normal & maximum of 30 seconds of delay you will get, but if you configure the given spantree command, it will might take less than 10 sec. or max. of 15 sec. reason of the delay is because it will find the root swtich. the command we are specify is to minimize that delay. it cannot be less than 2-3 sec.

hope this helps.

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Re: Spanning Tree Confusion

Honestly, I could not guess the design of your network from your description;-) Do you have 3 switches and 2 routers, or are you considering two of your switches as routers (as you could indeed). Could you explain how they are connected together.

If I understand correctly, you put all the switches in the same vtp domain in transparent mode, and you are only using vlan 1 right?

At last, is your loss of connectivity temporary or permanent when you are connecting your new switch (and also, what is the new switch btw).

I would not blindly reduce STP timers to their minimum value without understanding the reason for your problem.

Regards,

Francois

Re: Spanning Tree Confusion

Hi,

Herewith I'm posting the required config.

At this moment the 2980B and 3560A are NOT connected, and everything is ok. To achieve the STP redundancy the moment I connect 3560A with 2980B (Root Bridge) both the switches ie 2980A and 3560A become unaccessible.

Also in the config the HSRP is configured and the MPLS Router is the Active Router.

VTP is being used and the Root Bridge is VTP Server and rest of the switches are Clients.

The Gigports on all the Switches are in Trunk Mode.

Now, whenever the connection is made between 2980B and 3560A using the Gigports, I'm not able to ping from the MPLS Router to either the 2980A or 3560A.

Any help would be appreciated.

Kind Regards,

Wilson Samuel

Re: Spanning Tree Confusion

Hi,

Sorry for the late answer. I cannot find an obvious cause for your problem (there are too many possibilities in fact;-)).

STP has to block a port in your topology. It's very likely to be gi0/1 on the 3560. For some time, both uplinks on the 3560 could block (this depends also on your stp mode).

Is the connection permanently lost, or just for about 30 seconds or for about 300 seconds (one is 2xforward_delay, the other is the cam aging time)? Are you running MST? Do you have any feature configured (rootguard, bpduguard in particular)?

The simplest way to troubleshoot that is to inspect on the path between the two L3 devices that are trying to communicate the stp state of every switch involved. You can also check the consistency of the cam table for the mac address of the L3 devices along this path.

Regards,

Francois

Re: Spanning Tree Confusion

Hi,

Thanks for the reply anyway. Moreover, I have tried the most simplest troubleshooting technique and now doubt that one of the GBIC / Controller is having problems.

So its not that something I'm missing but its rather seems to be a hardware issue.

Regards,

Wilson Samuel

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