PPP Encapsulation Changes Routing Table

typesh

Hey everyone...

Had another config question I wanted to throw out there...

I have (2) 2501s connected to each other's Serial1 interface.

R1 has been given the ip address 150.1.1.1 255.255.0.0 on its Serial 1 interface(with a clock rate as well)

R2 has been given the ip address 150.1.1.2 255.255.0.0 on its Serial 1 interface

I initially used HDLC between the routers and each router had this in its routing table:

C 150.1.0.0/16 is directly connected, Serial1

Then I decided to change the encapsulation on both routers to use PPP instead of HDLC. Immediately (on both routers) the routing table has changed to this:

R2#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

150.1.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 150.1.1.1/32 is directly connected, Serial1
C 150.1.0.0/16 is directly connected, Serial1


AND


R1#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route

Gateway of last resort is not set


C 150.1.1.2/32 is directly connected, Serial1
C 150.1.0.0/16 is directly connected, Serial1



It seems that using PPP has caused a /32 mask identifying the specific IP of the other router on that serial link. I searched through my running-config to make sure I didn't incorrectly configure an interface, but everything seems fine.

Does anyone know why this happens?

Thank you.

Bl8ckr0uter

This is a feature of ppp. When routers advertise to each other using ppp it uses the /32 (host) mask prefix. Others can confirm.

typesh

knwminus wrote: »

This is a feature of ppp. When routers advertise to each other using ppp it uses the /32 (host) mask prefix. Others can confirm.

Nice! Thanks knwminus, I never knew that.

EdTheLad

I'm not sure why it does it but you can disable this feature using the command “no peer neighbor-route“.PPP by default will send the locally configured address as a /32 to the remote router, this will allow two directly connected interfaces on different subnets to communicate.

Bl8ckr0uter

No problem.

EdTheLad wrote: »

I'm not sure why it does it but you can disable this feature using the command “no peer neighbor-route“.PPP by default will send the locally configured address as a /32 to the remote router, this will allow two directly connected interfaces on different subnets to communicate.

Nice followup. I knew there was a way to do it but i could not remember it.