Sweet you da man ..... From: Bob McCouch [mailto:[email protected]] Sent: Wednesday, May 16, 2012 4:31 PM To: Bodnar, Edward Cc: [email protected] Subject: Re: [OSL | CCIE_RS] ?
Ed, Check this out for how a server can use it: http://www.miquels.cistron.nl/isc-dhcpd/ A real-world use in an SP environment would be static DHCP reservations for broadband subscribers. For example, a local broadband ISP up this way doesn't *route* static IP blocks to business customers, they just give you a static reservation so your equipment always picks up the same IP. You configure your firewall with DHCP, but you'll always get the same IP, guaranteed. They implement it using the MAC address (I know this because I always have to furnish the MAC of the equipment I'm installing) but they *could* do it based on option 82, and instead just determine that the specific port on the specific switch that services customer X will always get a reserved IP regardless of what equipment the customer puts on. So it allows you to tie a DHCP reservation (or a pool) to either the aggregation device the endpoints are coming through (that's the remote-id) and/or the physical port/VLAN they're coming in on (circuit-id) *rather* than tying that info to something like the device-specific MAC address. And again, this is implemented on IOS as DHCP classes: http://www.cisco.com/en/US/docs/ios/12_3t/12_3t4/feature/guide/gdhcpopt.html On Wed, May 16, 2012 at 4:20 PM, Bodnar, Edward <[email protected]<mailto:[email protected]>> wrote: Ya I think I need to read about this some more. I think I see how this works on the router now. But how does the DHCP server take that string and use it to assign IP's base off that. I can't quite get my head around that. Thanks for the info. This is not a very well documented feature. From: Bob McCouch [mailto:[email protected]<mailto:[email protected]>] Sent: Wednesday, May 16, 2012 4:05 PM To: Bodnar, Edward Cc: [email protected]<mailto:[email protected]> Subject: Re: [OSL | CCIE_RS] ? Hi Ed, Take a look at this: http://www.cisco.com/en/US/docs/switches/lan/catalyst3550/software/release/12.1_19_ea1/configuration/guide/swdhcp82.html#wp1069615 That little blurb explains a bit how DHCP Option 82 is used. For more detail, consult RFC 3046 (a short read). Doing a "show ip dhcp snooping" on a switch shows the following as part of the output: Insertion of option 82 is enabled circuit-id default format: vlan-mod-port remote-id: 001e.7a07.c500 (MAC) So you can see the default values used for each of the sub-option fields. These would allow a server to make some sort of decision based on the information provided in those fields, specifically that the relay was the "remote-id" with that MAC address, and the circuit ID was something like 452-0-24 for port 24 of module 0 in VLAN 452. What you referenced as one command is actually two: (config)#ip dhcp snooping info option format remote {hostname|string {blah}} and (config-if)#ip dhcp snooping vlan 500 information option format-type circuit-id string {blah} Note one is global and one is interface specific. Once you grok the purpose of the sub-options, that makes perfect sense. They can be used to override the default formats for those options listed above. So you could change the remote-id from a MAC to either the switch hostname or an arbitrary string like "Columbus-Edge-CPE" and you can modify the circuit-id to be an interface specific string like "CustID_5555555" or something like that. Again, it's so the DHCP server can serve the desired addresses based on the info it can glean from these hints. See the DHCP class function for how it could be done on IOS. Why would you use each? Because the lab tells you to. :-) For the record I had to research that detail... I knew they were related to Opt 82 but had to dig around a little bit to figure all that out. HTH, Bob On Wed, May 16, 2012 at 2:25 PM, Bodnar, Edward <[email protected]<mailto:[email protected]>> wrote: Can anybody provide some clarity around these commands. Ip dhcp snooping information option format-type ( circuit-id | remote-id ) Need info on what they do and why I would use them. _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com<http://www.ipexpert.com> Are you a CCNP or CCIE and looking for a job? Check out www.PlatinumPlacement.com<http://www.PlatinumPlacement.com> http://onlinestudylist.com/mailman/listinfo/ccie_rs _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com Are you a CCNP or CCIE and looking for a job? Check out www.PlatinumPlacement.com http://onlinestudylist.com/mailman/listinfo/ccie_rs
