Internet Protocol Control Protocol in PPP links.


 this blog we will look the PPP link's ip route / address negotiation .

In the above diagram, there is not static routing / protocol implemented just serial interface with ppp encapsulation link brought up .
  if you try to ping the R1 interface ip 192.168.100.1  from R2 interface ip 192.168.1.1 will it succeed ?


R2#ping 192.168.100.1
Lets check the router interface:
R1#show run int s1/0
Building configuration...
Current configuration : 160 bytes
!
interface Serial1/0
ip address 192.168.200.1 255.255.255.0 secondary
ip address 192.168.100.1 255.255.255.0
encapsulation ppp
serial restart-delay 0

R2#show run int s1/0
Building configuration...

Current configuration : 110 bytes
!
interface Serial1/0
ip address 192.168.1.1 255.255.255.254
encapsulation ppp
serial restart-delay 0
end

Answer is Yes you can ping.
R2#ping 192.168.100.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.100.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/36/84 ms
R2#

Check the routing table: " 192.168.100.1" directly connected !!

R2#show 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
192.168.1.0/30 is subnetted, 1 subnets
C 192.168.1.0 is directly connected, Serial1/0
192.168.100.0/32 is subnetted, 1 subnets
C 192.168.100.1 is directly connected, Serial1/0

How it works :
ip address negotiated through IPCP as defined in RFC 1332
“The IP Control Protocol (IPCP) is responsible for configuring, enabling, and disabling the IP protocol modules on both ends of the point-to-point link.“

enabled the debugging to what is going on :
*Mar 1 00:31:48.207: Se1/0 PPP: I pkt type 0x8021, datagramsize 14 link[ip]
*Mar 1 00:31:48.211: Se1/0 IPCP: I CONFREQ [Open] id 3 len 10
*Mar 1 00:31:48.211: Se1/0 IPCP: Address 192.168.100.2 (0x0306C0A86402)
*Mar 1 00:31:48.215: Se1/0 IPCP: O CONFREQ [Open] id 3 len 10
*Mar 1 00:31:48.215: Se1/0 IPCP: Address 192.168.1.1 (0x0306C0A80101)
*Mar 1 00:31:48.215: Se1/0 IPCP: O CONFACK [Open] id 3 len 10
*Mar 1 00:31:48.219: Se1/0 IPCP: Address 192.168.100.2 (0x0306C0A86402)
*Mar 1 00:31:48.367: Se1/0 PPP: I pkt type 0x8021, datagramsize 14 link[ip]
*Mar 1 00:31:48.367: Se1/0 IPCP: I CONFACK [ACKsent] id 3 len 10
*Mar 1 00:31:48.371: Se1/0 IPCP: Address 192.168.1.1 (0x0306C0A80101)
Further Reading:
Internet Protocol Control Protocol in PPP links - http://tools.ietf.org/html/rfc1332


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