Small OSPF issues

kpjungle

Hi,

I have a few issues, regarding some inconsistent information from the BSCI Exam Certification Guide.

One issue, is that on page 177, it is mentioned:

What is the default OSPF network type for a physical serial interface on a cisco router?

and the answer is according to the book:

the default ospf network type for serial interface is nonbroadcast multiaccess.

I cant really figure out how they get to that. If i assign an ip address to a standard serial interface, and include this interface's ip address in the router ospf process (by the network command), so it becomes part of the OSPF, and do a show ip ospf interface, it is listed as a point-to-point ospf network type. Only if the encapsulation is frame relay, it defaults to non-broadcast.

The book's errata doesnt list any errors regarding this. Am I wrong in this?

Second question is about the "ip ospf retransmit-interval <seconds>" command and the "ip ospf transmit-delay <seconds>" command.

I understand that the retransmit one, specifies how long to wait for an LSA ack, before retransmitting the ack. The second one is how long you believe it will take to send an LSA to a neighbor. However, the book says that the retransmit interval should not be smaller than the transmit interval. I dont understand how these directly relate to each other. ofcourse you should wait longer than it takes to transfer before retransmitting, but does the transmit interval actually do something to the LSA's that affect anything, i cant where it really fits in. If i have a standard point to point link and use this command, i can change the timers all I want without anything bad happens. Can someone provide an example to put it in context for me?

Thanks!

GT-Rob

Ya it would be point-to-point on serial interfaces.

That said, if it was a serial interface that was running frame relay, then it would be nonbroadcast. They might have forgot to put that in :P

Plazma

yes this only applies when you are doing a network over an NBMA cloud.

ccnpninja

It depends on the network the serial interface is attached to. If it's a point-to-point serial connection (Leased Line for example) the OSPF network type would be point-to-point. If it's an NBMA network (like Frame Relay), it would be the NBMA network type.
An OSPF network type should be viewed as an OSPF operational mode, or an OSPF interpretation of the underlying network topology.
HTH.

dtlokee

It really depends on the encapsulation, PPP or HDLC = point to point by default, Frame Relay = Nonbroadcast multiaccess by default

ccnpninja

Thanks Dtlokee for the clarification. We can not get a better answer than from a CCIE.