DHCP relay question in small network

Question

Baby network person here.

I'm trying to set up a small network such as the one in attached picture. 3 routers, 3 switches, 3 endpoints. Color boxes denote subnets.

If I want to have only 1 DHCP server, which router would be the best/how would it be set up? I believe I should be able to set any one router to work as DHCP and hand out IPs from different pools for any endpoints(except for routers which have static IPs).

So, questions: 1. If R1 becomes DHCP server, how does it determine which pool to use for PC2 & PC3? Even with DHCP relay setup, they'd be coming from the same interface(same line). Or does R2 & R3 replace the source address when relaying the DHCP broadcast packets so R1 can look at that and use it to determine which pool?

2. Extension of Q1, how does DHCP server decide between multiple pools? If R2 was DHCP server, does it look at which interface it came from(which would cause problem for multiple VLANs coming via multiple hops to same interface) or does it look at source router IP address that started DHCP relay?

Kinda confused here, any help would be awesome!

Answer 1

It's based on the sending interface where you have configured the ip-helper option(or dhcp relay in juniper). Look up giaddr.

DHCP Configuration Guide, Cisco IOS Release 15SY: IP Addressing § Information About DHCP Relay Server ID Override and Link Selection Option 82 Suboptions

If you have 2 ip adresses on the svi/interface it will send from the primary then the secondary.