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dhcp on hsrp

If I configure dhcp on hsrp and the first router fails over to the second router, does the second router know which ip addresses the first router has leased to clients?

Kelvin

4 REPLIES

Re: dhcp on hsrp

I doubt if that will happen. But you can write the lease information ( automatic bindings) to a tftp/ftp server and make the second router also read and write the database information from that server.

Again i m not 100 percent sure about this feature working the way i said. Command used is.

ip dhcp database url [timeout seconds | write-delay seconds]

is of the form, tftp:// or ftp://user:password@

Hope that helps as well as works out!

Re: dhcp on hsrp

Also, DHCP will be forwarding (or serving) on both routers if configured for it. DHCP requests are broadcasts and HSRP only listens to / forwards unicast traffic sent to the active HSRP address directly.

For example, if you have 3 routers on a subnet and all of them have ip-helper configured to forward to a DHCP server all 3 routers will forward DHCP requests.

New Member

Re: dhcp on hsrp

I gather the original post was curious as to how to handle it if the DHCP was specifically handled locally on the multiple router interfaces, not forwarded on to the DHCP servers elsewhere. Depending on the size of the network, there would likely be great advantage to doing the latter.

Here's a link on the Cisco IOS DHCP features:

http://www.cisco.com/en/US/products/sw/iosswrel/ps1831/products_command_summary_chapter09186a0080087e7b.html#xtocid1046213 (wraps)

If the DHCP server was only set up on the HSRP router interface with the highest priority (ie - the one that is active under normal conditions when both interfaces are up) then the worst case exposure in the event of an outage to the primary is that clients would temporarily not get new DHCP leases or renewals. Unless you have very mobile clients, this exposure is rather minimal given DHCP clients will simply used cached prior leased information.

The ideal network DHCP solution is to have multiple DHCP servers (services) located centrally in the network, ones for which the IP address assignment ranges do not overlap for a give segment (scope in MS terms). For example, DHCP-SVR1 doles out 10.10.10.10-119, DHCP-SVR2 doles out 10.10.10.120-239 (given a net mast of 255.255.255.0, reserves 1-9 for HSRP & admin & 240+ for misc like printers). You then set up all your HSRP router interfaces, both primary and secondary, to use IP HELPER pointers to primary and secondary DHCP servers. Set half your segments to use DHCP-SVR1 as their primary and DHCP-SVR2 as their secondary. Obviously, do the opposite for the others.

On a medium to large network, having the DHCP servers certrally managed simplifies the overall management of these DHCP address leases.

Good luck...

New Member

Re: dhcp on hsrp

Thanks for the help. It was hypothetical and we came to the same conclusion to have multiple servers with partial dhcp scopes to prevent overlaps.

Much appreciated

Kelvin

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