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  • Redundant Routing using HSRP

    I'm trying to provide redundant default gateways for 2950 L2 switches that are daisy-chained between L3 3550 switches as shown in the attached drawing. All connections are fiber, and there's no opportunity to home run all the L2 switches to the L3 gateways with new fiber connections. I'd like to provide fail-over if the fiber link between Switch A and Switch B is cut (this would also cut the gig link between Node 1 and 2). So lets say that both Switch A and Switch B are using Node 1 as a default gateway, the cable gets cut, HSRP determines that Node 2 is the new active gateway, but now Switch A can't reach it because it's on the wrong side of the cut. I thought about using GLBP to load balance between the two nodes, but if the cable gets cut would half the packets be lost? Is it possible to do this with spanning tree? Other ideas?
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  • #2
    Re: Redundant Routing using HSRP

    Hi rbautch,

    I have done a similar setup about 3 weeks ago. What you will need to do is use HSRP in conjunction with a dynamic routing protocol like eigrp (if you are a cisco shop). HSRP alone won't work because it doesn't determine the path that a packet should take. HSRP is enough only if the link between switch A and switch B gets cut. But what about the link between Node 3 and Node 1 or Node 3 and Node 2? That is where the dynamic routing helps and that will give you a more robust failover. Below is the link for HSRP configuration:

    http://www.cisco.com/en/US/docs/swit...de/swhsrp.html


    The most important HSRP commands (other than creating the VIP) ip standby preempt, ip standby priority, ip standby track interface.

    Then for the routing enable an eigrp instance on each L3 switch and tell it what networks to advertise.

    I am more than willing to continue this offline just for the fact that this could be a faily lengthy discussion and we can just post the results. Either way just my $.02

    Ryan

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    • #3
      Re: Redundant Routing using HSRP

      Thanks for the response. I'm running OSPF between the nodes, so that part is working fine. Any cut between the nodes means that all nodes can still talk to each other. I'm still not seeing how HSRP could help a cut between A and B. I have HSRP set up to track the gig interface that connects the nodes together, so if the entire cable gets cut between switch A and B, Node 2 becomes the active router. The problem is that Switch A (which was fine before Node 2 became active) can no longer reach it's default gateway which used to be Node 1 but now is Node 2. It's on the wrong side of the cut! So, I'm trying to figure out how to send it clockwise around the other side of the ring to reach Node 2.

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      • #4
        Re: Redundant Routing using HSRP

        Would it be possible to get a sample of what your HSRP config looks like? I don't think STP will work because based on your diagram there are no other switches along the path. The only way STP would work is if there was a switch plugged in to both A and B (say switch C). That way if the link between swich A and B is severed swich C would be the alternate switch path.

        I would love to continue working with you troubleshooting this but I would be interested to see if anyone else chimes in with their input.

        Ryan

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