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HSRP virtual router versus dynamic routing of both routers implied in HSRP

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Aug 16th, 2002
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I have a remote office where the default gateway of the LAN is the HSRP's virtual router. Of course, this means i have 2 routers terminating this LAN who shared the HSRP functions.


I know that in case of failure of the active router, the standby router takes the lead. This is ok for me.


Also we have activated the "interface tracking" feature in case of the WAN link failure. With this one, if the monitored interface goes down, the standby router will take the lead also. It's fine too.


Where it's more tricky, it's when the WAN network is composed of multiple technology, like Frame relay access, converted in ATM PVC and so on. Then, if the ATM goes down, the frame relay access, i think, will never be aware of this failure and the "interface tracking" will never be triggered to failover to the standby router.


Conclusion: It's the job of EIGRP to detect this kind of failure and switch the communications path (from all stations in the LAN) to the standby router.


Where i'm not sure, is if the hsrp's active router will redirect the outgoing trafic (LAN to WAN) to the standby router, probably with ICMP redirect. Is someone knows the behaviors in this case?


Benoit Dube

CGI Inc


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bourse Fri, 08/16/2002 - 05:46
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Hi Benoit. If the routers talk EIGRP on the LAN side, then the active router's routing tables will indicate the path via the standby router. It is important that the HSRP routers use EIGRP (or OSPF or whatever) to advertise their routes because interface tracking has its limitations. Consider a faulty circuit. The interface goes down and the HSRP tracking the interface will fall over to the standby router. Life is good. You call the telco company about the failure and the first thing they do is start looping the circuit to determine where the problem is. The loop will put the interface back in the UP/UP state and HSRP will come back to your primary router. If the primary router doesn't have routing information via the backup router then it will drop the packets since it likely doesn't have any routes in its routing table.

bdube Fri, 08/16/2002 - 07:06
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Thanks David,


Unfortunately, we don't advertise route to inside hosts. Hosts always send packet to their default gateway, the IP/MAC address of the virtual router as configured with HSRP.


Thanks a lot about the limitation of HSRP in loop case.


Benoit


bourse Fri, 08/16/2002 - 11:19
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I meant that the routers should advertise their routes to each other, not the hosts, on the LAN segment. If the routers are neighbors on the LAN side, then if a router loses it's WAN connection, it will still know how to get to the central site via the other router. If the routers are not neighbors, then if a router loses its WAN connection, it will have no routes (unless you have static routes) and if it happens to be the HSRP active router it will drop all the packets since it won't know how to route them.

rfroom Fri, 08/16/2002 - 05:46
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??? Conclusion: It's the job of EIGRP to detect this kind of failure and switch the communications path (from all stations in the LAN) to the standby router.


No, but it is routing protocols responsibility


HSRP can be configured for redirects in later software revisions, late 12.1 and 12.2 to work for the case you are describing.

bdube Fri, 08/16/2002 - 07:11
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Hi Richard,


You are right, it's the routing protocols responsibility. It will be EIGRP.


Do you confirm HSRP will redirects to standby router if a path is disable in the routing table of the primary router ?


Thanks


Benoit


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