PPP Peer Neighbor Route Feature

Have you ever come across an issue where you have got two Cisco routers, connected through back to back serial cable and are configured with two different IP subnets over the WAN link.

Lets take an example:

R1 is connected to R2 through back to back DCE/DTE cable. On R1's Serial0/0 we have ip address configured as 1.1.1.1 255.255.255.252; on R2's Serial0/0 we have configured ip address as 2.2.2.2 255.255.255.252. Now what you think ....will R1 be able to ping R2 address ?

Weird Situation :-O

Lets fix it using PPP peer neighbor route feature...trust me...not many people know this cool ppp feature :-)

PPP peer neighbor route feature discover the peer router's ip address and install a /32 host route for that address. Below is the sample config.
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R1(config)#do sh ip int brief
Interface IP-Address OK? Method Status Protocol

FastEthernet0/0 unassigned YES unset administratively down down

Serial0/0 1.1.1.1 YES manual up up

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R2(config)#do sh ip int brief
Interface IP-Address OK? Method Status Protocol

FastEthernet0/0 unassigned YES unset administratively down down

Serial0/0 2.2.2.2 YES manual up up

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R1(config)#do ping 2.2.2.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
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R1(config)#int s0/0
R1(config-if)#encapsulation ppp
R1(config-if)#exit
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R2(config)#int s0/0
R2(config-if)#encapsulation ppp
R2(config-if)#exit
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R1(config)#do ping 2.2.2.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 12/47/112 ms
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R1(config)#do sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

1.0.0.0/30 is subnetted, 1 subnets
C 1.1.1.0 is directly connected, Serial0/0
2.0.0.0/32 is subnetted, 1 subnets
C 2.2.2.2 is directly connected, Serial0/0
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Enjoy...

Best Regards,
Deepak Arora