PPP Peer neighbor route - для чего?

PPP has something called peer neighbor route. It installs a /32 route for the neighbors IP address which can be seen as connected. That is why it succeeds. You can disable the peer neighbor route per interface with no peer neighbor-route command.

Возьмем к примеру такую схему.

The ping was succeeded between these two routers with ppp encapsulation . what is the reason for that ?

with HDLC

Gateway of last resort is not set

C    10.0.0.0/8 is directly connected, Serial0/0/0

with PPP

Gateway of last resort is not set

C    10.0.0.0/8 is directly connected, Serial0/0/0

     20.0.0.0/32 is subnetted, 1 subnets

C       20.1.1.1 is directly connected, Serial0/0/0

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.

Default behavior on Cisco routers using PPP encapsulation.

Example: R1----10.1.1.1/24------------------10.2.2.2/24-------R2

R1#sh ip route connected

     10.0.0.0/24 is subnetted, 1 subnets

C       10.1.1.0 is directly connected, Serial1/0

Note its only showing R1's  connected network Because HDLC encapsulation

R1#sh int s1/0 | in HDLC

  Encapsulation HDLC, crc 16, loopback not set

Change it to PPP now.

RX(config)#int s1/0

RX(config-if)#encapsulation ppp

R1#sh int s1/0 | in PPP

  Encapsulation PPP, LCP Open

R1#sh ip route connected

     10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

C       10.2.2.2/32 is directly connected, Serial1/0

C       10.1.1.0/24 is directly connected, Serial1/0

This happen because of the IPCP (Internet Protocol Control Protocol), the router installed the route 10.2.2.2 into the IP routing table.Lets debug and verify it.

R1#debug ppp negotiation

PPP protocol negotiation debugging is on

*Apr 28 23:37:15.479: Se1/0 IPCP:   Address 10.1.1.1 (0x03060A010101)

*Apr 28 23:37:15.479: Se1/0 CDPCP: O CONFREQ [Closed] id 1 len 4

*Apr 28 23:37:15.483: Se1/0 PPP: Process pending ncp packets

*Apr 28 23:37:15.483: Se1/0 IPCP: Redirect packet to Se1/0

*Apr 28 23:37:15.483: Se1/0 IPCP: I CONFREQ [REQsent] id 1 len 10

*Apr 28 23:37:15.483: Se1/0 IPCP:   Address 10.2.2.2 (0x03060A020202)

*Apr 28 23:37:15.487: Se1/0 IPCP: O CONFACK [REQsent] id 1 len 10

*Apr 28 23:37:15.487: Se1/0 IPCP:   Address 10.2.2.2 (0x03060A020202)

*Apr 28 23:37:15.491: Se1/0 IPCP: I CONFACK [ACKsent] id 1 len 10

*Apr 28 23:37:15.491: Se1/0 IPCP:    Address 10.1.1.1 (0x03060A010101)

*Apr 28 23:37:15.495: Se1/0 IPCP: State is Open

*Apr 28 23:37:15.495: Se1/0 CDPCP: I CONFACK [REQsent] id 1 len 4

*Apr 28 23:37:15.503: Se1/0 IPCP: Install route to 10.2.2.2

*Apr 28 23:37:17.455: Se1/0 CDPCP: Timeout: State ACKrcvd

*Apr 28 23:37:17.539: Se1/0 CDPCP: State is Open

Вывод:

the PPP peer neighbour route is useful when each end of the PPP link is on a different subnet like when using ip unnumbered interfaces( borrowing the IP address from another interface for the PPP interface).

if both ends are on same subnet then you can safely disable the peer neighbour route.