Qtech QSW-3900-48-SFP-DC [101/245] Classification of ospf networks

QTECH Software Configuration Manual

8-100

The intra-area and inter-area routes describe the network topology of the AS, while external routes describe

routes to destinations outside the AS.

OSPF classifies external routes into two types : Type-1 and Type-2. A Type-1 external route is an IGP

route, such as a RIP or static route, which has high credibility and whose cost is comparable with the cost of an OSPF

internal route. The cost from a router to the destination of the Type-1 external route= the cost from the router to the

corresponding ASBR+ the cost from the ASBR to the destination of the external route.

A Type-2 external route is an EGP route, which has low credibility, so OSPF considers the cost from the

ASBR to the destination of the Type-2 external route is much bigger than the cost from the ASBR to an OSPF

internal router. Therefore, the cost from the internal router to the destination of the Type-2 external route= the cost

from the ASBR to the destination of the Type-2 external route. If two routes to the same destination have the same

cost, then take the cost from the router to the ASBR into consideration.

8.1.3 Classification of OSPF Networks

8.1.3.1 OSPF network types

OSPF classifies networks into four types upon the link layer protocol :

· Broadcast : When the link layer protocol is Ethernet or FDDI, OSPF considers the network type broadcast

by default. On Broadcast networks, packets are sent to multicast addresses (such as 224.0.0.5 and 224.0.0.6).

· NBMA (Non-Broadcast Multi-Access) : When the link layer protocol is Frame Relay, ATM or X.25, OSPF

considers the network type as NBMA by default. Packets on these networks are sent to unicast addresses.

· P2MP (point-to-multipoint) : By default, OSPF considers no link layer protocol as P2MP, which is a

conversion from other network types such as NBMA in general. On P2MP networks, packets are sent to

multicast addresses (224.0.0.5).

· P2P (point-to-point) : When the link layer protocol is PPP or HDLC, OSPF considers the network type as

P2P. On P2P networks, packets are sent to multicast addresses (224.0.0.5).

8.1.3.2 II. NBMA network configuration principle

Typical NBMA networks are ATM and Frame Relay networks.

You need to perform some special configuration on NBMA interfaces. Since these interfaces cannot

broadcast hello packets for neighbor location, you need to specify neighbors manually and configure whether the

neighbors have the DR election right.

An NBMA network is fully meshed, which means any two routers in the NBMA network have a direct

virtual link for communication. If direct connections are not available between some routers, the type of interfaces

associated should be configured as P2MP, or as P2P for interfaces with only one neighbor.

Differences between NBMA and P2MP networks :

· NBMA networks are fully meshed, non-broadcast and multi access. P2MP networks are not required to be

fully meshed.

· It is required to elect the DR and BDR on NBMA networks, while DR and BDR are not available on P2MP

networks.

· NBMA is the default network type, while P2MP is a conversion from other network types, such as NBMA in

general.

· On NBMA networks, packets are unicast, and neighbors are configured manually on routers. On P2MP

networks, packets are multicast.