PPP IPCP Negotiation IP Address Assignment


PPP provides a way to assign ip address to a remote client on a serial link using IPCP protocol. Remember IPCP is part of PPP protocol suite.

Assuming you have a very simply 2 routers topology shown below;

ppp ipcpQuestion:

Configure the serial link between R1 & R2 with PPP encapsulation

R2 should assign ip address to R1 interface

Answer:

To configure this, we need to configure an ip address to R2  interface with appropriate mask.

Enable R2 to supply ip address to R1 with “peer default ….” command

And finally disable peer neighbor route on both end of the link

R3:

interface Serial0/0
ip address negotiated
encapsulation ppp
clock rate 2000000
end

R2:

interface Serial0/0
ip address 160.1.4.2 255.255.255.0
encapsulation ppp
peer default ip address 160.1.4.3
clock rate 2000000
end

Output on R3:

R3(config-if)#do sh ip route conn
3.0.0.0/24 is subnetted, 1 subnets
C       3.3.3.0 is directly connected, Loopback0
160.1.0.0/16 is variably subnetted, 3 subnets, 2 masks
C       160.1.1.0/24 is directly connected, Serial0/1
C       160.1.3.0/24 is directly connected, FastEthernet0/0
C       160.1.4.3/32 is directly connected, Serial0/0

 

——–

Serial0/1                  160.1.1.3       YES NVRAM  up                    up
Serial0/0                  160.1.4.3       YES IPCP   up                    up
Loopback0                  3.3.3.3         YES NVRAM  up                    up

Output on R2:

R2(config-if)#do sh ip route conn
2.0.0.0/24 is subnetted, 1 subnets
C       2.2.2.0 is directly connected, Loopback0
160.1.0.0/16 is variably subnetted, 3 subnets, 2 masks
C       160.1.2.0/24 is directly connected, FastEthernet0/0
C       160.1.4.0/24 is directly connected, Serial0/0
C       160.1.4.3/32 is directly connected, Serial0/0

You can see from these outputs that ip address assigned to s1/2 interface on R3 is from IPCP. You also noticed that output from “sh ip route” displayed 2 routes with /32 pointing in the direction of the link.

By default, when PPP link is enabled on a link, PPP creates a host routes for it’s peer on that link in it’s RIB and this is why we have one of the /32 route.  Basically, the router adds it remote peer’s ip address to it’s routing table with a /32 mask (And infact on both end of the links). This is applicable to static ip address assignment or negotiated on a PPP enabled link.

Ok, so why is the assigned ip address for R3 displayed with a mask of /32 in R3 RIB ?

When a PPP client peers assigns ip address to it’s remote peer on the link, it by default assigns the route as a /32 route and this is why we have “160.1.4.3/32″ in R3 RIB.

There are two issues here:

1) How doe we remove this peer route from the router’s routing table since we really do not need it

2) How do we make IPCP assign the remote peer address with the correct mask (instead of /32).

To fix the first issue, we need to add an extra command to the serial links on both end of the link and then shut/no shut. This is shown below

R3:

interface Serial0/0
ip address negotiated
encapsulation ppp
no peer neighbor-route
clock rate 2000000
end

R2:

interface Serial0/0
ip address 160.1.4.2 255.255.255.0
encapsulation ppp
 no peer neighbor-route
peer default ip address 160.1.4.3
clock rate 2000000
end

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About ccie4all
Hello, and welcome to the first post of my CCIE blog This blog has got one simple goal and that is to improve our skills in Cisco Networking field so we can become best engineers on a job market. Wordpress Blog https://ccie4all.wordpress.com/ information about the changes made to Gns3 BGP , MPLS and R&S CCIE labs. In order to access and download all provided materials and receive important updates from Gns3 BGP , MPLS and R&S CCIE labs under GNS3 tab in the main header please go ahead and subscribe to https://ccie4all.wordpress.com/ ! All other posts have not been affected and can be accessed at any given time. Enjoy ! Tom

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