Does PPP "advertise" the /32 host route?

CodeBlox

Couldn't find a better word than advertise so forgive me if it's inaccurate but while playing with PPP(authentication and enabling) I noticed some odd behavior. No routing protocol was enabled, yet the router at the other end of the point to point link had an entry in its routing table for the distant interface, a /32 route. Whats even wierder is that the IP addresses can be in totally different subnets, sharing the same link and still be pingable! Seems really buggy to me, how can this be and is there ever any use for to devices to share a point to point link using totally different subnet information and still be able to communicate?

Forsaken_GA

IOS creates neighbor routes as part of the PPP negotiation by default. The fact that they are on different subnets has no bearing, it's a /32 route, a host route. It has no subnet, as it's the only possible address on that link. Since the routing table has an interface associated with the IP, it always knows how to get to the other side (ie, if it's destined for that IP, shove it out that interface, and then it's the other ends problem)

CodeBlox

Ohhh okay. But is there anything useful about this behavior? Or is the feature enable for historical reasons?

Forsaken_GA

well it keeps you from having to define a route down to that host manually if you want them to actually talk to each other!

JJBladester

CodeBlox wrote: »

Couldn't find a better word than advertise so forgive me if it's inaccurate but while playing with PPP(authentication and enabling) I noticed some odd behavior. No routing protocol was enabled, yet the router at the other end of the point to point link had an entry in its routing table for the distant interface, a /32 route. Whats even wierder is that the IP addresses can be in totally different subnets, sharing the same link and still be pingable! Seems really buggy to me, how can this be and is there ever any use for to devices to share a point to point link using totally different subnet information and still be able to communicate?

First off, I know this is an old post.

I just ran across this post because I was working on a PacketTracer Standard ACL lab and found some strange /32 routes. The lab uses three routers connected via serial point-to-point links. EIGRP was enabled and so was PPP. Doing a

show running-config

did not reveal that a

network

command was explicitly given for the /32 host route.

So, it makes sense that PPP is the "culprit" in this case. PPP creates a /32 peer route by default which allows routers that may be (but don't have to be) on different subnets to talk with each other. The option can be turned off with

no peer neighbor-route

.

This post does a great job explaining this further.
Remove unwanted PPP peer route « ipSpace.net by @ioshints