PPP Peer Neighbor Route

Erick N. Borgard CCNP

Peer neighbor route is a PPP feature that allows for connected interfaces that are not on the same IP subnet to communicate with one another.  This is the default behavior on Cisco routers using PPP encapsulation.

Setup:

R1——–R2

R1 has an IP address of 10.1.100.1/24 on the S0/0 interface.

R2 has an IP address of 10.1.200.2/24 on the S0/0 interface.

Let’s take a look at the IP routing table on R1

R1#show ip route

Gateway of last resort is not set

10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C       10.1.100.0/24 is directly connected, Serial0/0
C       10.1.200.2/32 is directly connected, Serial0/0

Notice that there is a connected host(/32) route to 10.1.200.2 even though R2 has an IP address that is not in the 10.1.100.0/24 subnet like R1.  Let’s try and ping R2 S0/0 interface.

R1#ping 10.1.200.2
 
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.200.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/8/20 ms
 
 

Here we can see that a ping is successful.  We can disable this feature by issuing the no peer neighbor-route command at the interface level.  Let’s disable this feature on R1 S0/0 interface, then examine the routing table again at R1.  We will also shut and no shut the interface on R1 to complete the changes.

R1(config)#interface serial 0/0
R1(config-if)#no peer neighbor-route 
R1(config-if)#shut
R1(config-if)#
%LINK-5-CHANGED: Interface Serial0/0, changed state to administratively down
%LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0, changed state to down
R1(config-if)#no shut
R1(config-if)#
%LINK-3-UPDOWN: Interface Serial0/0, changed state to up
R1(config-if)#
%LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0, changed state to up
R1(config-if)#do sh ip route
Gateway of last resort is not set

10.0.0.0/24 is subnetted, 1 subnets
C       10.1.100.0 is directly connected, Serial0/0

Notice that the /32 route to R2 serial interface is now gone and we have lost reachability to that interface without the use of the PPP Peer Neighbor Route feature.  If we re-enable the peer neighbor route feature on the interface, we will see that the /32 route will be injected into the routing table once again.  Let’s re-enable the S0/0 interface on R1 and enable debug ppp negotiations.

R1(config-if)#IPCP:    Address 10.1.100.1 (0x03060A016401)
Se0/0 CDPCP: O CONFREQ [Closed] id 1 len 4
Se0/0 PPP: Process pending ncp packets
Se0/0 IPCP: Redirect packet to Se0/0
Se0/0 IPCP: I CONFREQ [REQsent] id 1 len 10
Se0/0 IPCP:    Address 10.1.200.2 (0x03060A01C802)
Se0/0 IPCP: O CONFACK [REQsent] id 1 len 10
Se0/0 IPCP:    Address 10.1.200.2 (0x03060A01C802)
Se0/0 IPCP: I CONFACK [ACKsent] id 1 len 10
Se0/0 IPCP:    Address 10.1.100.1 (0x03060A016401)
Se0/0 IPCP: State is Open
Se0/0 IPCP: Install route to 10.1.200.2
Se0/0 CDPCP: I CONFACK [REQsent] id 1 len 4
R1(config-if)#

We can see that by the use of IPCP, the router installed the route 10.1.200.2 into the IP routing table.