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New Member

DHCP Snooping

Does anyone have detail documentation of how DHCP snooping works? I have read the "Cat4500 features best practices for sups". Would like to get more deeper. Part of the problem I have is clients must manually release their previous DHCP addresses before obtaining new addresses. This is nuisance for laptop users. Not sure whether this is how DHCP snooping suppose to work?

2 REPLIES
New Member

Re: DHCP Snooping

DHCP snooping is more or less to prevent untrusted machines on the outside of your protected network from requesting leases from your server. This problem sounds more like a DHCP lease timeout issue. When the DHCP server receives a request for an address from a client, it assigns an unused IP address from its pool to the client's MAC address and then maintains that mapping for the duration of the lease. If the client goes away and then comes back, the server will always try to assign the same IP address to the client. The only way that the server will NOT assign the same IP address to the client is if the lease has expired, or if the client requests a release. Just a suggestion: check out your scope settings and decrease the lease duration. Just outta curiousity.. Why do you not want the clients to maintain the same IP addresses?

Rich

New Member

Re: DHCP Snooping

My DHCP scope is cofigured for 3 day lease. I do want my clients to maintain the same IP addresses, and that is not the problem. For example, if I have a DCHP server configured at home for my broadband connection. I get an address of 192.168.10.50. When I come in to work, my laptop is getting a NAK from the DHCP server for an new IP address. I have to manually release my IP first, then renew. Don't know why the laptop kept the address from home network to begin with. If I turn off DHCP snooping on the switch, my laptop would get an IP address immediately. Never have to release and renew.

Part2: If my laptop is connected to a corporate network switch with DHCP snooping enabled, and if I don't gracefully shut it down but unplug it from the network. When I try to connect my laptop to another DHCP snooping enabled switch, I won't be able to obtain a new address until manually release it from laptop.

I don't know the details of how DHCP snooping work and was hoping for someone to provide that info.

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