PPP and host routes

Maybe some of you did configure a ppp connection and wondered about the new host route in the routing table after the PPP connection went alive. This is nothing to worry about since its a normal behavior of PPP and IPCP and also allows the routers to not even share the same subnet to get the line up and running.
Given the following diagram with a simple back to back connection between R4 and R5. Both interfaces have a different subnet.


The configuration is plain and simple, while R5 is the DCE:

R4#sh run int s0/1
interface Serial0/1
ip address 10.1.1.4 255.255.255.0
encapsulation ppp

R5#sh run int s0/1/0
interface Serial0/1/0
ip address 10.1.2.5 255.255.255.0
encapsulation ppp
clock rate 128000

To see whats happening, issue a debug ppp negotiation and do a shut/no shut on one of the interfaces:

R4:
May 27 21:54:16.303: Se0/1 IPCP: Address 10.1.1.4 (0x03060A010104)
*May 27 21:54:16.303: Se0/1 IPCP: State is Open
*May 27 21:54:16.307: Se0/1 IPCP: Install route to 10.1.2.5

R5:
*Jun 16 13:31:09.797: Se0/1/0 IPCP: Address 10.1.2.5 (0x03060A010205)
*Jun 16 13:31:09.797: Se0/1/0 IPCP: State is Open
*Jun 16 13:31:09.797: Se0/1/0 IPCP: Install route to 10.1.1.4

IPCP is aware of its own interface address and the one of its peer, based on that IPCP is going to install this host route into the router routing table:

R4#sh ip route
C 10.1.1.0/24 is directly connected, Serial0/1
C 10.1.2.5/32 is directly connected, Serial0/1

R5#sh ip route
C 10.1.2.0/24 is directly connected, Serial0/1/0
C 10.1.1.4/32 is directly connected, Serial0/1/0

And the nice part is, they are able to ping each other now, even though its a point-to-point link and they have different IP subnets:

R4#ping 10.1.2.5
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.2.5, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 12/15/16 ms

R5#ping 10.1.1.4
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.1.4, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 12/15/16 ms

If the link loses the connection, IPCP will also automatically remove the IP from the routing table:

R5(config)#int s0/1/0
R5(config-if)#shutdown

*Jun 16 16:16:49.848: %LINK-5-CHANGED: Interface Serial0/1/0, changed state to administratively down
*Jun 16 16:16:49.848: Se0/1/0 PPP: Sending Acct Event[Down] id[9]
*Jun 16 16:16:49.848: Se0/1/0 CDPCP: State is Closed
*Jun 16 16:16:49.848: Se0/1/0 IPCP: State is Closed
*Jun 16 16:16:49.848: Se0/1/0 PPP: Phase is TERMINATING
*Jun 16 16:16:49.848: Se0/1/0 LCP: State is Closed
*Jun 16 16:16:49.848: Se0/1/0 PPP: Phase is DOWN
*Jun 16 16:16:49.848: Se0/1/0 IPCP: Remove route to 10.1.1.4
*Jun 16 16:16:50.848: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/1/0, changed state to down

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2 comments

  1. Joyce

    Hello,

    I saw your post about the remote host route being installed by default and the command to remove the default behavior.

    I have a question regarding this, does this remote host /32 route being installed into local routing table, say R4 is installing 10.1.2.5/32 after the PPP negotiation, will R5 periodically advertise this /32 to R4 every so often? Or is this R4 install of this /32 just “one time” after the initial negotiation and there is no periodic refreshment of this route? I know we have default keepalive every 10 seconds but I am interested to know if this remote host route is being advertised periodically or not as it has a big impact of my needs.

    thanks,
    Joyce

    • pashtuk

      Hello Joyce
      Sorry for the late answer and sadly I wasnt able to test it yet but Im pretty sure this Route installation will only happen when the link comes up. To be sure use the debug ip routing on one of the routers and check if you see periodic updates for the specific IP while the link is up and running.
      May I ask you on what kind of implementation that matters for you? Might be interessting 🙂 If you do not want to discuss this on a public blog, please send me a mail to pashtuk at gmail.com 🙂

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