[c-nsp] Setting relay agent IP on 4500

[Jason Lixfeld]

> On Jul 31, 2017, at 11:57 AM, Phil Mayers <p.mayers at imperial.ac.uk> wrote:
> 
> On 31/07/17 15:38, Jason Lixfeld wrote:
> 
>> Agreed. However, if the relay is configured to use option 82 sub option 5 (RFC3527), the relay can overwrite the giaddr to an IP that is reachable by the DHCP server (typically the egress interface instead of the ingress interface).  The relay would also inserts the link subnet into the option 5 field, and the server uses that value to select the subnet it should allocate from, instead of using giaddr to signal that.
> 
> I am assuming that IOS does not do this on a "global" IP helper, then?

No.  My impression is that the “global" argument just tells the relay that the server is not in a VRF (either the one configured in the helper, or the one on the interface where the command is configured).  It doesn’t have any affect on giaddr or anything else, but I’m only using the global argument on these boxes, I don’t have any that use the vrf argument, or do not include either a vrf or global argument.