DHCP For PPP Link

September 25, 2008 at 6:06 pm | Posted in IP Services | 1 Comment

I came accross an weird issue today when practicing Vol2 Lab 13, Task 2.3: The R4 Serial interface was  not able to get IP address via DHCP, until i changed the R5 Serial0/1 to use unnumbered off Loopbac1, instead of manual IP.

This could be because of routing for the subnet 139.1.45.0/24 is not available, till the interface is fully up, by then IPCP is no longer negotiating IP address for the Serial interface. The loopback interface fix arround this issue. It may also because of other hidden issue I am not aware of. Anyway, this issue is a good candidate for the IOS bug, or weird caveat that need to remember by heart.

[21 Oct 2008] Also see other related issue found at https://enotepad.wordpress.com/2008/10/21/dhcp-for-ppp-link-revisited/

[22 Oct 2008]

This issue has been confirmed in the IEWB Vol1 ver5. We ether need to use ip unnnumbered off a loopback, or static routing for that Serial PPP interface. This is needed because the Serial link isn’t in the UP/UP state until IP is acutally negotiated. This means that when the proxy request is received at the server, the server does not have a route back to the relay in order to send the reply back.

This can be observed by turning on “debug ip packet details” on the server, and we can see that DHCP reply packets (UDP src=67, dst=67) are unroutable.

RSRack1R5#sh run int s0/1
interface Serial0/1
ip address 139.1.45.5 255.255.255.0
encapsulation ppp
peer default ip address dhcp

ip dhcp-server 139.1.15.1

RSRack1R4#sh ip int s0/1
Serial0/1 is up, line protocol is up
Internet address will be negotiated using IPCP
Broadcast address is 255.255.255.255
Peer address is 139.1.45.5
MTU is 1500 bytes

RSRack1R4#sh ip int brief | in Serial0/1
Serial0/1                  unassigned YES IPCP   up                    up

Change to use IP unnumbered on R5

RSRack1R4#sh run int s0/1
interface Serial0/1
ip address negotiated
ip rip advertise 3
encapsulation ppp

RSRack1R5#sh run
!
interface Serial0/1
ip unnumbered Loopback1
encapsulation ppp
peer default ip address dhcp

interface Loopback1
ip address 139.1.45.5 255.255.255.0

ip dhcp-server 139.1.15.1

RSRack1R4#sh ip int s0/1
Serial0/1 is up, line protocol is up
Internet address is 139.1.45.4/32
Broadcast address is 255.255.255.255
Address determined by IPCP
Peer address is 139.1.45.5

RSRack1R4#sh ip int brief | in Serial0/1
Serial0/1                  139.1.45.4 YES IPCP up                    up

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  1. […] DHCP for PPP Link (revisited) October 21, 2008 at 11:08 pm | In IP Services | Another caveat found today, in relation to the topic posted previously. https://enotepad.wordpress.com/2008/09/25/dhcp-for-ppp-link/ […]


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