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New Member

OSPF neighbor on PPP and HDLC


We have very simple configuration, two routers connected on a serial link having ip address as and second is with encapsulation as HDLC default. We have enabled OSPF on both routers by

router ospf 100

network area 0

and on second router

router ospf 100

network area 0

This works ok

OSPF neighbor relationship is formed and data is transfered on the link.

But when we changed the encapsulation of serial link to PPP of both the links it is not forming neighbor relationship. I get the ping to other side of the link that means link is ok but it is not forming relationship.

Do we need any more configuration like authentication ( ppp auth chap ) on both routers ? Is it mandatory ?

Any configuration example on is highly appreciable.

Thanx in advance



Re: OSPF neighbor on PPP and HDLC

Hall of Fame Super Silver

Re: OSPF neighbor on PPP and HDLC


Andrea is correct that ppp authentication is not mandatory. It should work if you do not do authentication on both ends or it should work if you do authentication on both ends.

If OSPF worked correctly when configured with HDLC then just changing the encapsulation to ppp should not prevent OSPF from working. I suspect that something changed more than just specifying encapsulation ppp on both ends. Perhaps you could post the config of both routers and we might be able to find the problem.



Cisco Employee

Re: OSPF neighbor on PPP and HDLC

There shouldn't be anything special about ppp encapsulation and ospf. Try running "debug ip ospf adj" on either side of the link and see if you get any helping messages.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
México móvil: +52 1 55 8312 4915
Cisco México 
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New Member

Re: OSPF neighbor on PPP and HDLC

The default OSPF network type on either HDLC or PPP is point-to-point, so you shouldn't have an issue.

Have you tried 'clear ip ospf process 100' on both routers? You shouldn't even need this, but it's worth a shot.

You should also try 'debug ip ospf adj', and 'debug ip ospf hello' to see where a problem may lie.

The other thing that you can do, which you shouldn't have to do on a point-to-point network link is to define the neighbor's. The neighbor command is useful in a NBMA environment where the hello's cannot be multicasted.

Good luck.

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