IOS Features & Management

Troubleshooting: IP Addressing Tricks & Tips Using PPP

Let’s say you have this scenario:

R4 and R5 are connected via a serial cable. Configure IP Addressing so that R4 and R5 are in the 192.168.45.0/24. R4 should have an IP of 192.168.45.4/24. R5 should have an IP address off 192.168.45.5/24.

The catch: Do not configure an IP Address with the ip address 192.168.45.4 255.255.255.0 command directly on R4’s s1/1 interface.

Let’s have a look at the different ways we can solve this!

IP Unnumbered

R4:

interface lo0
 ip add 192.168.1.45.4 255.255.255.0
!
interface Serial1/1
 ip unnumbered Loopback0
 encapsulation ppp
 clock rate 64000

R5:

interface Serial1/1
 ip address 192.168.45.5 255.255.255.0
 encapsulation ppp
 no peer neighbor-route

R4#sh ip route | b Gateway
Gateway of last resort is not set

     192.168.45.0/24 is variably subnetted, 2 subnets, 2 masks
C       192.168.45.5/32 is directly connected, Serial1/1
C       192.168.45.0/24 is directly connected, Loopback0

R5#sh ip route | b Gateway
Gateway of last resort is not set

C    192.168.45.0/24 is directly connected, Serial1/1

The no peer neighbor-route is not needed on R5, but I put it there to illustrate a point. R5 doesn’t need the peer neighbor-route to R4 because it will see the 192.168.45.0/24 subnet as directly connected on its s1/1 interface. R4 does need the peer route to 192.168.45.5/32 otherwise it will forward any packets destined to that address out Lo0.

PPP IPCP

R4:

interface Serial1/1
 ip address negotiated
 encapsulation ppp
 clock rate 64000

R5:

interface Serial1/1
 ip address 192.168.45.5 255.255.255.0
 encapsulation ppp
 no peer neighbor-route
 peer default ip address 192.168.45.4

R4#sh ip route | b Gateway
Gateway of last resort is not set

     192.168.45.0/32 is subnetted, 2 subnets
C       192.168.45.5 is directly connected, Serial1/1
C       192.168.45.4 is directly connected, Serial1/1

R5#sh ip route | b Gateway
Gateway of last resort is not set

C    192.168.45.0/24 is directly connected, Serial1/1

We have used IPCP to give R4 and IP address. R4 needs the peer route, otherwise its routing table will only have a route to the 192.168.45.4/32 network. R5 doesn’t need the peer neighbor-route because that subnet is directly connected on the interface.

PPP Multilink

R4:

interface Multilink1
 ip address 192.168.45.4 255.255.255.0
 no peer neighbor-route
!
interface Serial1/1
 no ip address
 encapsulation ppp
 clock rate 64000
 ppp multilink
 ppp multilink group 1

R5:

interface Multilink1
 ip address 192.168.45.5 255.255.255.0
 no peer neighbor-route
!
interface Serial1/1
 no ip address
 encapsulation ppp
 ppp multilink
 ppp multilink group 1

R4#sh ip route | b Gateway
Gateway of last resort is not set

C    192.168.45.0/24 is directly connected, Multilink1

The IP address isn’t on R4’s s1/1 interface! :)

Once again the no peer neighbor-route command is not needed, but is used to show that we don’t require a /32 to the remote end of the PPP connection.

PPP over Frame Relay

R4:

interface Serial1/1
 no ip address
 encapsulation frame-relay
 no keepalive
 clockrate 64000
 no frame-relay inverse-arp
!
interface Serial1/1.45 point-to-point
 frame-relay interface-dlci 45 ppp Virtual-Template1
!
interface Virtual-Template1
 ip address 192.168.45.4 255.255.255.0
 no peer neighbor-route

R5:

interface Serial1/1
 no ip address
 encapsulation frame-relay
 no keepalive
 no frame-relay inverse-arp
!
interface Serial1/1.45 point-to-point
 frame-relay interface-dlci 45 ppp Virtual-Template1
!
interface Virtual-Template1
 ip address 192.168.45.5 255.255.255.0
 no peer neighbor-route

R4#sh ip route | b Gateway
Gateway of last resort is not set

C    192.168.45.0/24 is directly connected, Virtual-Access1

R5#sh ip route | b Gateway
Gateway of last resort is not set

C    192.168.45.0/24 is directly connected, Virtual-Access1

We have disabled LMI with the no keepalive command so we can do back-to-back frame-relay. Once again we have used no peer neighbor-route it illustrate that we don’t need a specific /32 to the other end.

Cool Huh? Why the hell would you need this stuff outside the CCIE? You wouldn’t! Just configure the damn interface with 192.168.45.4/24! Since when was the CCIE about best practices and how you would do it in real life?! :)

Anybody know of any other methods?

© 2008 Arden Packeer. All rights reserved. The opinions expressed herein are my own personal opinions and do not represent my employer's view in anyway. This material is not sponsored or endorsed by Cisco Systems, Inc. Cisco, Cisco Systems, CCIE and the CCIE Logo are trademarks of Cisco Systems, Inc. and its affiliates