PPP - Negotiated address via DHCP

This kind of task may seem more difficult than it really is. I, in fact, spent way too long one morning/afternoon/evening trying to get this scenario to work. Turns out my server did not have a route back to the requester's subnet. So here it is without all the crap (ok, some of it) I went through:

Topology:

R5---R2---R1

R5 to R2 is PPP.
R5 needs to negotiate its address.
R1 is to supply this address.

R2-R5: 150.100.25.x/24
R1-R2: 150.100.12.x/24

R5 config is EASY:

interface Serial0/1
ip address negotiated

R2 is also easy, we configure it's interface to supply the address via DHCP and then specify a DHCP server:

R2(config)#int s1/1
R2(config-if)#peer default ip address dhcp
R2(config-if)#exit
R2(config)# ip dhcp-server 150.100.12.1

On R1 we configure the pool and everything is cool, right?

R1(config)#ip dhcp pool R5
R1(dhcp-config)#network 150.100.25.0 /24
R1(dhcp-config)#exit
R1(config)#ip dhcp excluded-address 150.100.25.1 150.100.25.4
R1(config)#ip dhcp excluded-address 150.100.25.6 150.100.25.255

Let's check R5, to see if it got an address:

R5#show ip int brief | inc l1/1
Serial1/1 unassigned YES IPCP up up

Nothing! Let's do some debugging on R1 with an ACL to match DHCP packets:

R1(config)#access-list 150 pe udp any any eq bootpc
R1(config)#access-list 150 pe udp any any eq bootps
R1(config)#access-list 150 pe udp any eq bootpc any
R1(config)#access-list 150 pe udp any eq bootps any

R1#debug ip packet 150 detail
IP packet debugging is on (detailed) for access list 150

*Mar 1 00:15:27.995: IP: s=150.100.12.1 (local), d=150.100.25.2, len 328, unroutable
*Mar 1 00:15:27.999: UDP src=67, dst=67

R1 has no route to 150.100.25.0/24 yet! Let's configure one and then manually shut/no shut the interface on R5:

R1(config)#ip route 150.100.25.0 255.255.255.0 150.100.12.2

R1#debug ip dhcp server events

*Mar 1 00:19:27.263: DHCPD: Sending notification of DISCOVER:
*Mar 1 00:19:27.263: DHCPD: htype 1 chaddr 0000.0c07.79e1
*Mar 1 00:19:27.267: DHCPD: circuit id 00000000
*Mar 1 00:19:27.267: DHCPD: Seeing if there is an internally specified pool class:
*Mar 1 00:19:27.271: DHCPD: htype 1 chaddr 0000.0c07.79e1
*Mar 1 00:19:27.271: DHCPD: circuit id 00000000
*Mar 1 00:19:28.411: DHCPD: Adding binding to radix tree (150.100.25.5)
*Mar 1 00:19:28.415: DHCPD: Adding binding to hash tree
*Mar 1 00:19:28.419: DHCPD: assigned IP address 150.100.25.5 to client 0063.6973.636f.2d31.3530.2e31.3030.2e32.352e.322d.5365.7269.616c.312f.31.
*Mar 1 00:19:28.495: DHCPD: Sending notification of ASSIGNMENT:
*Mar 1 00:19:28.499: DHCPD: address 150.100.25.5 mask 255.255.255.0
*Mar 1 00:19:28.499: DHCPD: htype 1 chaddr 0000.0c07.79e1
*Mar 1 00:19:28.503: DHCPD: lease time remaining (secs) = 86400
*Mar 1 00:20:17.647: DHCPD: checking for expired leases.
*Mar 1 00:22:17.647: DHCPD: checking for expired leases.
*Mar 1 00:24:17.647: DHCPD: checking for expired leases.

Now check R5:

R5#show ip int bri s1/1
Interface IP-Address OK? Method Status Protocol
Serial1/1 150.100.25.5 YES IPCP up up
R5#

*** IMPORTANT ***

R1 needs a route back to the 150.100.25.0/24 subnet. In this case I have a default route from R1 toward R2. This is EXTREMELY important. I wasted many minutes of my life trying to get this thing to come up. My DHCP configuration was correct but the DHCP server did not have a route back to the requester!