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Hold the phone!...In Packet Tracer I connected 2 routers together and connected them over a PPP link. Purposefully put them on different subnets. They can ping each other?!

My CCNA just expired today after never really using it. I'm trying to refresh my knowledge (because I just happened to get a job where I will use it).

I said it in the title. It's a 2 router topology connect by a serial port. I encapsulated both interfaces in PPP, and purposefully set my hostname:West router's interface to 204.65.13.1/28 and hostname:East router to 1.1.1.1/24. Then did a show interface on each router to see the difference in output. But everything was the same; up and up.

I can ping both ways! Did I completely forget how this should work, or is this a packet tracer error? Or, is this a PPP thing that I forgot about

btw, show ip route shows routes to each network, directly connected (I don't have any routing protocols running either)

Really, the topology looks like this 0<------>0 That's it. No other configurations.

edit: Just changed encapsulation back to hdlc. I'm no longer able to ping. What gives?

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PPP on Cisco installs host routes to the other side by default.

Perform a "show ip route" both before and after you bring the interfaces up, and you'll see.

Original Poster2 points · 3 months ago

So this is the intended behavior? I don't think I even knew this when I passed my CCNA.

This is intended behavior, but I don't believe it is taught at the CCNA level.

You can disable this behavior on the interface with "no peer neighbor-route"

Likewise, you can create an automatic default route with "ppp ipcp route default"

Original Poster1 point · 3 months ago

Huh. Ok. I thought they had to be on the same network. I guess I'll have to read up on PPP.

And you're probably right about not being taught at CCNA level. Those commands are not supported in Packet Tracer.

Thank you. I thought I lost my mind/skillset for a minute. But that was just something I never knew about or noticed before.

Host routes are /32, which means a single IP on a subnet. Another way to think about it is with loopback interfaces. If you assign an IP address 10.10.10.1/32 to a loopback address, and then advertise that interface (or subnet) into a routing protocol (or use static routing), that IP address becomes reachable to all devices that can reach the attached router via either the advertising routing protocol or static route.

The same goes for PPP with the default host-route installed. When the PPP connection is made, the /32 to the IP address of the other end of the link is installed in the local routing table. Because it's a /32, it does not matter that the IP address is not on the same subnet as the local interface, because the local router knows that it can reach that other IP address through that interface because of the /32 host route.

I know it sounds confusing and contrary to what is traditionally taught in networking. The key is to remember that a /32 host route is a subnet of one.

5 points · 3 months ago

I don't think that's completely accurate.

If you joined two routers together by an Ethernet interface, on different subnets, and pointed the host route out the interface, I'm fairly confident it won't work.

The reason this works in PPP isn't because of the host routes, but because it's a point-to-point interface. In other words, the routers know that the only device on the other side of the link is the other router.

You're right. I forgot to take the point-to-point nature into account. There I go again confusing people! LOL

Thank you

2 points · 3 months ago

This is good knowledge to know going forward. Thank you everyone, I feel smarter now.

CCNP|CCDP|CCNA-V|CMNA
5 points · 3 months ago

On a point-to-point circuit, there's only one place for the traffic to go (the other side), so you don't necessarily need routes just to ping your neighbor.

This is different than an ethernet connection where there could be multiple devices in the segment, which is why we have ARP to match an IP to a MAC address. With ethernet you can't just put traffic on the wire and know it'll get to the right device.

Just 'cause it ain't in my flair doesn't mean I don't have certs
3 points · 3 months ago

Technically speaking, you very much DO need routes, but you don't need to have devices on the same subnet. PPP will install host routes by default, which will be seen in the routing table whenever the PPP session is up.

Original Poster1 point · 3 months ago

Ok. Thank you. I have another question. I put OSPF on the "misconfigured" topology that I originally described + a switch and a pc pulling DHCP from the DHCP pool I put on the router. The routers didn't form an adjacency, and I could not ping from the PC.. But once I changed the "East" interface to the same subnet as the "West" interface, and added the network under ospf config, they formed an adjacency and I was able to ping from the PC.

I almost see what's going on here, but not exactly. If my pings work from the routers, why doesn't configuring ospf fix the ping from PC "problem"?

ninja edit: I'm confident that I didn't mess up configuring OSPF when the 2 routers were on different subnets.

PPP connections are different from "point to point" in the sense that if you connected g0/0 on r1 to the same port on r2, they need to be in the same subnet to communicate.

PPP is different from an Ethernet link.

Meow 🐈🐈Meow 🐱🐱 Meow Meow🍺🐈🐱Meow A+!
1 point · 3 months ago

PPP installs a /32 "peer neighbor-route" of its neighbour to ensure connectivity. It's a dial-up networking thing where ISPs would give their dial-up customers IPs from many different ranges.

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