[c-nsp] Setting relay agent IP on 4500

[Phil Mayers]

On 03/08/17 21:27, Jason Lixfeld wrote:
>
>> On Jul 31, 2017, at 12:17 PM, Phil Mayers <p.mayers at imperial.ac.uk> wrote:
>>
>> I guess you could use a dummy relay that added a fake RFC3527 option but yuck...
>
> What do you mean by this, exactly?  Relay to a relay?  Didn’t think that was possible because a relay only accepts broadcasts… or am I lacking clue?


You can chain relays e.g. client to router, router to relay in PoP, PoP 
to DHCP server. It's pretty rare AIUI and not especially useful in many 
cases, but it does work.

e.g. from the ISC dhcrelay man page:

"""
The DHCP Relay Agent listens for DHCPv4 or DHCPv6 queries
from clients or other relay agents
"""             ^^^^^^^^^^^^^^^^^^

One problem with all of this of course, is that even if you get the 
relay stuff working, it'll only help you in the INIT, SELECTING and 
REQUESTING / REBINDING states. RENEWING (unicast DHCPREQUEST at T1) 
usually don't get touched by the relay in a router-as-relay setup.

(Various option82 setups need or force the relay to intervene even on 
unicast renews, but AFAIK IOS doesn't do this for L3 interface helpers)