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New Member

IP Redirects and multiple active HSRP groups

I have a LAN with three routers. We have ip redirects turned on, and in accordance with the document on HSRP and IP redirects, we have three HSRP groups. Each router is active on one of the HSRP groups. It works fine.

We are going to be removing one of the routers, yet we want to keep that third HSRP IP address (because we have a variety of clients who have different timeout characteristics for ip redirects). So we are planning to simply let the HSRP flop over to one of the other two routers.

This would leave one router as Active in two HSRP groups, and the other as Active in just one.

So here's the question: if a client gets redirected to a router that is ACTIVE in more than one HSRP group, what IP address is sent as the redirect? Does it send the IP address of the lowest numbered HSRP group (in our case Group 1)?

That would be preferable if it did. We'd want to eventually shut off that third IP address completely.

Thank you

  • Other Network Infrastructure Subjects
3 REPLIES
Bronze

Re: IP Redirects and multiple active HSRP groups

Um, I am a little confused as to how exactly you have your HSRP set up. You have 3 HSRP groups in the same IP network? I am also a bit confused as to why you are relying on ICMP redirects so much.

From what I understand you have a network with clients that are using 1 of 3 HSRP addresses as their GW. If a packet is sent to one of the HSRP IP's and the router decides one of the other GW's is a better route it sends a redirect to the client and the client starts using the (better?) HSRP addresses as the route for the destination. Is this roughly correct? Also, can you please post a link to the document of which you speak.

--Warren.

New Member

Re: IP Redirects and multiple active HSRP groups

First, here is that document:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121newft/121t/121t3/dt_hsrpi.htm

Second, here is a more detailed description of our situation:

We have a very large, flat LAN. On that LAN are three routers with the various WAN links (ATM, etc.). One router is obviously the default gateway, we enable ip redirects so the various client devices can learn of the better paths to limit redirected traffic as much as possible.

So, per the attached document, we have the following:

Router A, hardware address of .36, HSRP group 1 active router for .13 (the gateway)

Router B, hardware address of .37, HSRP group 2 active router for .14.

Router C, hardware address of .24, HSRP group 3 active router for .12.

Each router is a standby router in the other two groups. So each router has three HSRP statements on the LAN interface with various priority metrics comprising the active-standby relationships

Per the attached document, if Router A has to redirect a client to a remote site via Router C, the client learns of the HSRP group 3 address, not the hardware address. Reason: if Router C fails, all of those clients can still have a path out (via whomever takes that HSRP address for HSRP group 3). If they learned of the hardware address, they'd be dead.

Now, my question is this: we want to get Router C off this LAN. When we do this, HSRP Group 3 will flop over to Router B. So what will happen to redirects now? If Router A has to redirect the client to Router B, and Router B is active in two HSRP groups, which HSRP group address is included in the redirect? Group 2 or Group 3?

It's an esoteric problem, I grant you.

New Member

Re: IP Redirects and multiple active HSRP groups

I'm quite sure that the interface address (.36 or .37 in your case) is used when redirecting traffic. At L2 the mac is bia.

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