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New Member

STP details needed with my scenario

i have 2 6509 switches with 20.1.1.x and 20.1.1.x and their router module with 131.8.2.x and 131.8.2.x,both the switches are connected with fiber uplink gibic's each having some 5 vlans of 20.x.x.x and one vlans of 131.8.x.x ranges with HSRP.

one of the 6509 switch is connected to 131.8.2.x 3500 switch with fiber uplink which connects to our building with giga speed fiber.if unfortunately the link from 6509 to 3500 fails then all our servers on both switches will be not reachable.

i want a solution from which we can connect both the 6509 to 3500 and lt work.i know thru STP it will work but i need to know the configuration steps.


Re: STP details needed with my scenario

Not sure if I understand your network design 100%, but it sounds like you are trying to protect against a fiber link failure between service provider's 3500 switch and one of your 6509s. These two switches connect over a VLAN that supports the subnet. Your second 6509 is connected to the first 6509, and also has an interface on that VLAN with a IP address. But the second 6509 is not connected directly to the service provider's 3500. I assume the 3500 is either a 3512, 3524, or 3548 switch.

If the 3500 switch has only two Gig Ethernet ports (3512, 3524, 3548 types) and both are already used for fiber, then you can get a redundant connection from the 3500 to the second 6509 switch only if you use a 10/100 link over Category 5 UTP cable. Plug it directly into the second 6509 if it has 10/100 ports in the chassis; or else plug it into an access switch that plugs into the second 6509. Either way, Spanning Tree will give the Gig fiber link between the 3500 and the first 6509 a lower cost (=4) than the cost (=19) of the 10/100 connection between the 3500 and the second 6509, so everything will prefer to flow in and out the Gig fiber under normal circumstances.

When you have a fiber link failure between 3500 switch and the first 6509, the 100-meg full duplex connection between the 3500 and the second 6509 (or an access switch plugged into the second 6509) should change from blocking to forwarding. It may be slower than the Gig fiber, but slow connectivity to your servers is better than NO connectivity.

If you want all Gig fiber connections, then the least expensive way to get three is to use a 3508G switch. (Had a customer once who did this, instead of using their ISP's 3524.) Cost: about US$5,000.

If cost is an issue, then since the connection is only for backup just do it with the 10/100 Cat5 cable. You probably already have the ports available; simply configure them as you have the fiber link configured. Let Spanning Tree take care of the rest. Just remember to use an Ethernet crossover cable between switches. Cost: about US$5.

Hope this helps.

New Member

Re: STP details needed with my scenario

yes i have C3508G-XL which has free eth gib ports for second link to 6509 but i need to know how to configure STp for both the links for redundancy.

Re: STP details needed with my scenario


I can give you quickly a short answer that may not be adequate: just configure your second uplink on the 3500 exactly as you did for the first one, connect it to the other 6509 and you are done.

Spanning tree is supposed to be plug-n-play so the resulting will work but may not be optimal. Tuning it is correctly requires a good knowledge of the network and this forum is not adapted to discuss your particular case.

At least, enable uplinkfast on your 3500, so that it is not becoming root of the STP domain. The command is: spanning-tree uplinkfast.