PPP Different Subnet but Still Works!

Latest reply: Oct 15, 2019 13:32:42 159 2 1 2

Hi,


I just want to share an interesting topic that I learned during the training that I attended.


PPP Connection

As you can see in the picture above, the s0/0/0 of R1 and R2 are 1.1.1.1/24 and 2.2.2.2/24. No routing protocols or L3 protocols are configured on each router. The only configuration is ip addressing on s0/0/0 and link-protocol PPP on the interfaces.


If you try to ping R1 and R2 the ping will succed as shown in the image below:


Ping


So how is this possible? there is no routing protocol, the ip addresses are not on the same subnet.


PPP has a feature called PPP neighbor route. This feature allows for connected interface that are not on the same IP subnet to communicate with one another by installing a /32 route for the neighbors IP address which can be seen as connected. Its activated on Huawei devices by default and if you want to disable it you can run the command ppp peer hostroute-suppress on the interface and this disables the PPP neighbor route feature. 

  • x
  • convention:

Popeye_Wang
Admin Created Oct 15, 2019 09:52:17 Helpful(0) Helpful(0)

The Internet Protocol Control Protocol (IPCP) of PPP can obtain the IP address of the peer device. Even if the IP addresses of the two ends are on different network segments, they can ping each other.
IPCP is a Network Control Protocol (NCP) for establishing and configuring Internet Protocol over a Point-to-Point Protocol link. Through IPCP, a PPP endpoint can request a specific IP address from its peer(Static negotiation). It can also ask the peer to suggest an IP address by requesting the address 0.0.0.0 (Dynamic negotiation).
  • x
  • convention:

wissal
MVE Created Oct 15, 2019 13:32:42 Helpful(0) Helpful(0)

Hello,

The differece comes from the standard behaviour of ppp protocol. Apart from LCP (Link Control Protocol) and Authenticaton there is also a collection of NCP (Network Control Protocol) which come to play after link is estabilished and auth confirmed.
Especially, if we are talking IP, there is IPCP protocol (in NCP phase) which negotiates IP addresses of both link's ends.
As a result it puts remote ppp-link end's IP into local routing table as a host address. What's more,
that way 2 ends of the link can even have IP addresses from different IP networks and still connectivity is assured (because remote address is in local RIB and router knows, through which int it is reachable).
  • x
  • convention:

Telecommunications%20engineer%2C%20currently%20senior%20project%20manager%20at%20an%20operator%2C%20partner%20of%20Huawei%2C%20in%20the%20radio%20access%20network%20department%2C%20for%2020%20years%20I%20managed%20several%20types%20of%20projects%2C%20for%20the%20different%20nodes%20of%20the%20network.

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