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To form OSPF neighbor relationships on WAN connections, OSPF still must meet the
same requirements as on LANs. The area number must match with each neighbor; the IP
subnet number and mask of each router must match; authentication must pass; and so on.
In short, the items in Table 5-5 earlier in this chapter must be true.
However, the operation of OSPF on WAN links of various types requires some additional
thought, particularly when developing an implementation and verification plan. In particular,
depending on the WAN technology and configuration, the following additional questions
may matter for proper OSPF operation over WAN connections:
■ Will the routers discover each other using multicast OSPF Hello messages, or do the
neighbors require predefinition?
■ Will the routers try to elect a DR, and if so, which router should be allowed to be the
DR?
■ With which other routers should each router become an OSPF neighbor?
The first two of these items depend in part on the setting of the OSPF network type, and
the third question depends on the WAN service. This section first examines the concept
of OSPF network types and then examines the use of OSPF over common WAN technologies
included in the CCNP ROUTE exam.
OSPF Network Types
The OSPF network type–a per-interface setting–directs OSPF in regard to three important
facts:
■ Whether the router can expect to discover neighbors using multicast Hello messages
■ Whether only two or more than two OSPF routers can exist in the subnet attached to
the interface
■ Whether the router should attempt to elect an OSPF DR on that interface
For instance, LAN interfaces require a DR because of the default OSPF network type of
broadcast. OSPF defines this interface network type to use multicast Hellos to dynamically
discover neighbors, allows more than two routers to be in the same subnet, and to attempt
to elect a DR. Conversely, point-to-point links and point-to-point WAN
subinterfaces default to use a network type of point-to-point, meaning that only two
OSPF routers can exist in the subnet, neighbors can be dynamically discovered through
Hellos, and that the routers do not elect a DR.
Note: The discussion of the motivation for using OSPF DRs occurs in Chapter 6 section
“Background of Designated Routers.” For now, note that DRs become useful but are not
always required, when a subnet supports more than two OSPF routers (for example, on a
LAN subnet).
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Chapter 5: OSPF Overview and Neighbor Relationships 163
In production networks, the network type is often ignored, because there is no motivation
to change this setting–you pick a combination that works, and most everyone ignores it.
For the sake of CCNP ROUTE, you need to be aware of the setting and know a few of the
caveats found in some Frame Relay configurations. To keep the discussion focused on the
core topics, Chapters 6 and 7 avoid the Frame Relay configurations that require more consideration
of the OSPF network type setting. Chapter 8’s section “OSPF over Multipoint
Frame Relay” discusses the OSPF network types in more depth.
Table 5-8 summarizes the OSPF network types and their meanings. Note that this per-interface
or per-subinterface setting is configured with the ip ospf network type interface
subcommand; the first column in the table lists the exact keyword according to this command.
Note that the gray highlighted rows will be discussed in Chapter 8.
OSPF Neighborship over Point-to-Point Links
Point-to-point serial links can be a bit boring. You configure IP addresses on either end,
configure the clock rate if using a back-to-back serial cable in a lab, and no shutdown the
interfaces. When enabling OSPF on the interfaces, no extra effort is required compared to
LANs–just enable OSPF on the interface, and rely on the default OSPF network type of
point-to-point.
However, serial links can provide a convenient and uncluttered place to experiment with
OSPF network types. As such, Figure 5-5 shows a small network with two routers, with
Key
Topic
Table 5-8 OSPF Network Types
Interface Type Uses DR/BDR?
Default Hello
Interval
Dynamic
Discovery of
Neighbors?
More Than
Two Routers
Allowed in the
Subnet?
Broadcast Yes 10 Yes Yes
Point-to-point1 No 10 Yes No
Loopback No — — No
Nonbroadcast2
(NBMA)
Yes 30 No Yes
Point-to-multipoint
No 30 Yes Yes
Point-to-multipoint
nonbroadcast
No 30 No Yes
1Default on Frame Relay point-to-point subinterfaces.
2Default on Frame Relay physical and multipoint subinterfaces.
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Example 5-8 that follows showing several examples of the OSPF network type. (This small
network matches a portion of the network shown in Figure 5-1 earlier in this chapter.)
Example 5-8 demonstrates OSPF network types with all defaults on the HDLC link between
R1 and R2.
Example 5-8 OSPF Network Types, Default, on an HDLC Link
R1#show run int s0/0/0
Building configuration...
Current configuration : 102 bytes
!
interface Serial0/0/0
ip address 10.1.12.1 255.255.255.252
no fair-queue
clock rate 1536000
!
router ospf 1
network 10.0.0.0 0.255.255.255 area 0
!
end
R1#show ip ospf interface s0/0/0
Serial0/0/0 is up, line protocol is up
Internet Address 10.1.12.1/30, Area 1
Note: All IP addresses begin with 10.1 unless otherwise noted.
RID 1.1.1.1 RID 2.2.2.2
R1 R2
Area 1
Area 0
S0/0/0
12.1/30
Fa0/0
1.1/24
S0/0/1
12.2/30
Fa0/0
2.2/25
Figure 5-5 Simple Two Router Internetwork
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Chapter 5: OSPF Overview and Neighbor Relationships 165
Process ID 1, Router ID 1.1.1.1, Network Type POINT_TO_POINT, Cost: 64
! lines omitted for brevity
R1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 0 FULL/ - 00:00:31 10.1.12.2 Serial0/0/0
Example 5-6 begins listing R1’s configuration on the serial link, mainly to make the point
that the OSPF network type has not been explicitly configured. The show ip ospf interface
command then lists the network type (point-to-point). Based on Table 5-7, this type
should dynamically discover neighbors, and it does, with neighbor 2.2.2.2 (R2) being
listed at the end of the example. In particular, note that under the state heading in the
show ip ospf neighbor command output, after the /, only a dash is listed. This notation
means that no attempt was made to elect a DR. If the network type had implied a DR
should be elected, then some text would be listed after the /, for example, “/DR” meaning
that the neighbor was the DR. (Refer back to the end of Example 5-4 for an example of the
output of show ip ospf neighbor in which a DR has been elected.)
Example 5-9 shows an alternative where both routers change their OSPF network type on
the serial link to nonbroadcast. This change is nonsensical in real designs and is only done
for the purposes of showing the results: that the neighbors are not discovered dynamically,
but once defined, a DR is elected.
Note: R2 has been preconfigured to match the configuration on R1 in Example 5-9,
namely, the OSPF network type has been changed (ip ospf network non-broadcast), and
R2 has been configured with a neighbor 10.1.12.1 OSPF router subcommand.
Example 5-9 Configuring OSPF Network Type Nonbroadcast on an HDLC Link
R1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#interface s0/0/0
R1(config-if)#ip ospf network ?
broadcast Specify OSPF broadcast multi-access network
non-broadcast Specify OSPF NBMA network
point-to-multipoint Specify OSPF point-to-multipoint network
point-to-point Specify OSPF point-to-point network
R1(config-if)#ip ospf network non-broadcast
R1(config-if)#^Z
R1#show ip ospf neighbor
R1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router ospf 1
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R1(config-router)#neighbor 10.1.12.2
R1(config-router)#^Z
R1#
*Apr 28 20:10:15.755: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on Serial0/0/0 from
LOADING to FULL, Loading Done
R1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 1 FULL/DR 00:01:58 10.1.12.2 Serial0/0/0
The example begins with R2 already configured, so the neighbor relationship has already
failed. When the OSPF network type changes on R1’s S0/0/0, the routers do not dynamically
discover each other, based on the network type (nonbroadcast). However, by completing
the configuration in the example by adding R1’s neighbor 10.1.12.2 command, the
neighbor relationship is formed. Also, note that the final show ip ospf neighbor command
lists a state of FULL, then a /, and then DR, meaning that a DR was indeed elected, as required
by this OSPF network type.
Neighborship over Frame Relay Point-to-Point Subinterfaces
Frame Relay design allows several options for IP addressing and subnetting. One option
treats each pair of routers on the ends of each PVC as a point-to-point topology, with one
subnet assigned to each pair of routers. Another option treats more than two routers as a
group, whether connected with a full mesh or partial mesh of PVCs, with a single subnet
assigned to that group.
Many Frame Relay designs use the first option, treating each pair of routers on the ends
of a PVC as a single subnet, as shown in Figure 5-6. In such cases, it makes sense to treat
each PVC as a separate point-to-point connection, assigning a single subnet (at Layer 3) to
each Layer 2 PVC.
With this design, if all the routers use point-to-point subinterfaces as shown in R1’s configuration
in the figure, you can ignore the OSPF network (interface) type, and OSPF
works fine. IOS point-to-point subinterfaces unsurprisingly default to use OSPF network
type point-to-point. The two routers discover each other using multicast OSPF Hellos,
they do not bother to elect a DR, and everything works well.
Chapter 8 discusses alternative Frame Relay configurations.
Neighborship on MPLS VPN
Multiprotocol Label Switching (MPLS) virtual private networks (VPN) create a WAN
service that has some similarities but many differences when compared to Frame Relay.
The customer routers connect to the service, often times with serial links, but other times
with Frame Relay PVCs or with Ethernet. The service itself is a Layer 3 service, forwarding
IP packets through the cloud. As a result, no predefined PVCs need exist between the customer
routers. Additionally, the service uses routers at the edge of the service provider
cloud–generically called provider edge (PE) routers–and these routers are Layer 3-aware.
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