Lab 3 - OSPF in NBMA Networks

Prerequisites: CCNP level skills.

Cisco IOS OSPF modes of Operation:

1. Point-to-point (Cisco)
2. Broadcast (Cisco)
3. Non-broadcast (RFC 2328)
4. Point-to-multipoint (RFC 2328)
5. Point-to-multipoint non-broadcast (Cisco)

Topology

Pic. 1 - NBMA Topology Diagram.

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

Workflow

1. Get familiar with the topology.
2. Read the 'Task List'.
3. Read the 'Questions' and provide the answers BEFORE configuring the routers.
4. Configure the lab according to the 'Task List' .
5. Compare the answers (step 3) with the lab results.

Task List

• Configure IP addresses as per Pic. 1.
• Configure Frame-Relay using DLCIs listed in the topology (Pic. 1). Make sure that S0/0 interfaces do NOT learn any other DLCIs.
• Enable OSPF in all routers. All interfaces should be in area 0. Use default OSPF mode.
• Check if OSPF adjacency has been built and if the loopback interfaces are reachable.

Questions

1. What is the default OSPF mode of operation on NBMA networks (here: Frame-Relay)?
2. Is DR/BDR elected by default in NBMA networks?
3. If DR/BDR are elected, which router in the topology can be DR and which can be BDR?
4. Do OSPF packets use multicast or unicast transmissions by default?
5. What kind of well-known issue with regards to IP-to-DLCI mapping can a router run into? How can I check it? How can I fix it?
6. If Frame-Relay hub-and-spoke topology is used, what are the differences between OSPF non-broadcast mode and OSPF point-to-multipoint mode?
7. R1's interface S0/0 (Frame-Relay) uses the default OSPF mode, and R2's interface S0/0 (Frame-Relay) is configured as multipoint interface (S0/0.1 multipoint). What OSPF mode is going to be used on S0/0.1?
8. With hub-and-spoke topology (the spokes do not have circuit provisioned) what is the behavior of the next-hop address between the spokes?

Lab Solution

Note!
Since Frame-Relay is not a full meshed, the spoke routers must NOT become DR or BDR! 

Note!
When static mapping IP-to-DLCI is uses the broadcast keyword allows broadcast and multicast to be sent out as unicast (pseudo broadcast/multicast).

R1 Configuration:

!

interface Loopback0

 ip address 172.16.101.1 255.255.255.0
 ip ospf network point-to-point
!

interface Serial0/0
 ip address 10.1.1.1 255.255.255.0
 encapsulation frame-relay

 frame-relay map ip 10.1.1.2 102 broadcast

 frame-relay map ip 10.1.1.3 103 broadcast

 no frame-relay inverse-arp

!

router ospf 1
 router-id 1.1.1.1
 log-adjacency-changes
 network 10.1.1.1 0.0.0.0 area 0
 network 172.16.101.1 0.0.0.0 area 0
 neighbor 10.1.1.2
 neighbor 10.1.1.3
!

R2 Configuration:

!

interface Loopback0

 ip address 172.16.102.2 255.255.255.0
 ip ospf network point-to-point
!

interface Serial0/0
 ip address 10.1.1.2 255.255.255.0
 encapsulation frame-relay

 ip ospf priority 0
 frame-relay map ip 10.1.1.1 201 broadcast
 frame-relay map ip 10.1.1.3 201
 no frame-relay inverse-arp

!

router ospf 1
 router-id 2.2.2.2
 log-adjacency-changes
 network 10.1.1.2 0.0.0.0 area 0
 network 172.16.102.2 0.0.0.0 area 0
!

R3 Configuration:

!

interface Loopback0

 ip address 172.16.103.3 255.255.255.0
 ip ospf network point-to-point
!

interface Serial0/0
 ip address 10.1.1.3 255.255.255.0
 encapsulation frame-relay

 ip ospf priority 0
 frame-relay map ip 10.1.1.1 301 broadcast
 frame-relay map ip 10.1.1.2 301
 no frame-relay inverse-arp
!

router ospf 1
 router-id 3.3.3.3
 log-adjacency-changes
 network 10.1.1.3 0.0.0.0 area 0
 network 172.16.103.3 0.0.0.0 area 0
!

Verification

Note!
R1 (the hub router) uses the 'neighbor' statement to send hello packet using unicast. Without this command, the adjacency will not get established.

Note!
R2 and R3 are DROTERs as they must NOT become DR/BDR since they have no full reachability to other routers in the topology.

Pic.  2 - OSPF Neighbors.

Note!
Frame-Relay static mapping on R2 also shows how to reach R3 via R1 (no broadcast keyword is necessary since no broadcast/multicast are gonna be sent between R2 and R3 directly).

Pic. 3 - Frame-Relay Mapping on R2.

Pic. 4 - OSPF Routing Table on R2.

Pic. 5 - Traceroute from R2 to R3's Loopback.