Subinterfaces

On a Cisco router, it is possible to configure a physical interface to be many logical interfaces. This is extremely useful in a WAN environment and truly means that the logical topology is independent of the physical configuration. These subinterfaces can be configured to be point-to-point or point-to-multipoint. One of the main determining factors is the number of subnets to be used. A point-to-point interface requires its own subnet to identify it.

Life is very simple in OSPF if the point-to-point option is selected because the routers at each end create adjacencies. This does require more network overhead and restricts some communication—in particular, the capability to indirectly connect through a hub router.

In a point-to-point network, the concept of a broadcast is not relevant because the communication is directly to another router. In a point-to-multipoint network, although OSPF simulates a broadcast, multicast environment, the network traffic is replicated and sent to each neighbor.

Table 5-4 indicates the characteristics and options for each case. Table 5-4 OSPF over NBMA

Point-point Nonbroadcast

Point-to-point

Broadcast

NBMA

Point-to Multipoint

Addressing

Unicast

Multicast

Multicast

Unicast

Multicast

DR/BDR

No

No

Yes

Manual Yes

No

Manual Configuration of Neighbors

Yes

No

No

Yes

No

Hello

30 seconds

10 seconds

10 seconds

30seconds

30seconds

Dead=120

Dead=40

Dead=40

Dead=120

Dead=120

RFC/Cisco

Cisco

Cisco

Cisco

RFC 2328

RFC 2328

Partial mesh, using subinterfaces

Full mesh

Partial mesh

(seen as point-to-point)

Replicates Packets

Yes

Yes

Yes

Yes

Yes

Number of Subnets

1

Many (1 per circuit)

1

1

1

This can be very confusing because it is not clear which type of network corresponds to a particular physical configuration. The following list clarifies this:

• For serial interfaces with HDLC encapsulation, the default network type is point-to-point. Timers: hello 10, dead 40.

• For serial interfaces with Frame Relay encapsulation, the default network type is nonbroadcast. Timers: hello 30, dead 120.

• For serial interfaces with Frame Relay encapsulation and using point-to-point subinterfaces, the default network type is point-to point. Timers: hello 10, dead 40.

• For serial interfaces with Frame Relay encapsulation and using point-to-multipoint subinterfaces, the default network type is nonbroadcast. Timers: hello 30, dead 120.

Now that you understand the mechanism of the OSPF routing protocol, it is useful to understand how to configure the protocol on a Cisco router.

WARNING

If OSPF is used in an environment across different vendor equipment, it should be researched and tested to ensure interoperability.

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