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Identifying devices using specific default gateways

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Feb 17th, 2003
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I have a router which is used by serveral devices as a default gateway.

I need to remove this router but want to ensure there is no service disruption.

Is there a way to locate which devices are using this router apart from the ARP cache more accurately ?

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vcjones Mon, 02/17/2003 - 06:04
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Presumably there is another router still on the LAN (otherwise, you wouldn't need a default gateway). The easiest way to deal with the problem is to not even try! Just define a secondary address on one of the remaining routers to support the address of the router being removed. Better yet use HSRP (VRRP if not Cisco routers) and let the migration from router to router be handled automatically.

Good luck and have fun!

Vincent C Jones


mistryj Tue, 02/18/2003 - 02:34
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The LAN switches (6509) are now taking over the role. I already have IP account configured on the ethernet interface of the router to be migrated which has been running for a week but nothing in the tables.

Can you do HSRP between a router and a switch ?

Secondary addresses is an option but it only resolves the problem in the short term.

lauren Mon, 02/17/2003 - 08:27
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You could use IP accounting or netflow to show which source/destination pairs are communicating through your router. Netflow is probably overkill for just a quick check, so Id opt for IP accounting. Turn it on on the interface in question, give it a large enough cache so it doesnt drop entries, and leave it for a while.

The other option, which may or may not work depending on how things go, is to send a broadcast ping. Ping the broadcast address of the network in question from a non local address and see what replies you get. It used to work a treat but in these days of denial of service attacks, Ive not seen it work in a while.


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