Lab 1 - OSPF Point-to-Point Mode - Solution

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Prerequisites: CCNP level skills.

This post contains the answers and configuration for the LAB 1.

Answers
Q1:

What is the default OSPF mode of operation on F1/0 interface?

Answer:
The default OSPF mode of operation is 'broadcast'.

Q2:
Are hello packets sent using unicast or multicast? 
Answer:
Hello packets will use multicast 224.0.0.5.

Q3:

Does F1/0 interface participate in DR/BDR election? Why?

Answer:
Yes, since broadcast and NBMA interfaces do participate in DR/BDR election unless have been configured with the priority=0.
The reason for electing DR/BDR on broadcast and NBMA networks is to limit the propagation of LSAs. DRother routers establish full adjacency with DR and BDR only. DRothers establish 2-way adjacency with other routers in DRother role.

Q4:

What is the default OSPF mode of operation on S0/1 interface?

Answer:
The default encapsulation on S0/1 is HDLC which is point-to-point protocol.
The default OSPF mode is point-to-point. This is the default OSPF mode on the following:

• Interfaces running point-to-point protocols (HDLC, PPP)
• Subinterfaces configured as point-to-point (int s0/1.point-to-point) 

Q5:
Are hello packets sent using unicast or multicast?
Answer:

Since point-to-point connections support multicast (like broadcast networks), multicast 224.0.0.5 is going to be used when sending hello packets. 

Q6:
Does it participate in DR/BDR election? Why?
Answer:
OSPF point-to-point mode does not participate in the election of DR/BDR since there are only two routers echanging LSAs. There is no excess of LSA exchanges typical for multi-access networks.

Q7:
What are the default hello and dead interval timers used on F1/0 and S0/1?
Answer:
On both interfaces the timers are:

• hello=10 seconds
• dead=40 seconds. 

Q8:
Is there any other OSPF mode of operation that could work with the default OSPF mode on F1/0? What would you have to do to make it work?
Answer:
The default OSPF mode on broadcast networks is broadcast.
You can configure OSPF-enabled interface with NBMA mode which also elect DR/BDR. In order to make them work together you have to adjust the timers as they are not same between these two modes. NBMA mode uses:

• hello=30 seconds
• dead=120 seconds

Q9:

Is there any other OSPF mode of operation that could work with the default OSPF mode on S1/0? What would you have to do to make it work?

Answer:
The default OSPF mode on S0/1 (HDLC encapsulation) is point-to-point.
The "somewhat" compatible modes of operation with the 'point-to-point' are:

• point-to-multipoint
• point-to-multipoint nonbrodcast

Also, their hello and dead interval timers must be tweaked. They use 30/120 whereas point-to-point use 10/40.

Q10:

Without using the 'network' statement, how can you advertise the subnet 10.1.1.0/24?

Answers:
Quick solutions could be:

1. The interface command: 'ip ospf 1 area 0' (here 1 is the process ID).
2. Redistribution of connected networks into OSPF.

As for the solution to the tasks, here goes...

Pic. 1 - OSPF Point-to-Point Topology.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

R1 Configuration:

!

interface Loopback0
 ip address 172.16.101.1 255.255.255.0
!
interface Serial0/1
 ip address 10.1.13.1 255.255.255.0
!
router ospf 1
 log-adjacency-changes
 network 10.1.1.1 0.0.0.0 area 0
 network 10.1.13.0 0.0.0.255 area 0
!

R3 Configuration:

!

interface Loopback0
 ip address 172.16.103.3 255.255.255.0
!

interface Serial0/1

 ip address 10.1.13.3 255.255.255.0

!

router ospf 1

log-adjacency-changes

network 10.1.3.3 0.0.0.0 area 0

network 10.1.13.0 0.0.0.255 area 0

!