R3303-HP HSR6800 Routers Layer 3 - IP Routing Configuration Guide

Router types

OSPF classifies routers into the following types based on their positions in the AS:

• Internal router—All interfaces on an internal router belong to one OSPF area.

• Area Border Router (ABR)—Belongs to more than two areas, one of which must be the backbone

area. An ABR connects the backbone area to a non-backbone area. An ABR and the backbone

area can be connected through a physical or logical link.

• Backbone router—At least one interface of a backbone router must reside in the backbone area.

All ABRs and internal routers in area 0 are backbone routers.

• Autonomous System Boundary Router (ASBR)—Exchanges routing information with another AS. An

ASBR might not reside on the border of the AS. It can be an internal router or an ABR.

Figure 19 OSPF router types

Route types

OSPF prioritizes routes into the following levels:

• Intra-area route

• Inter-area route

• Type-1 external route

• Type-2 external route

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

routes to external ASs.

A Type-1 external route has high credibility. The cost from a router to the destination of a 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.

Area 1

Area 2

Area 3

Area 4

Backbone router

ASBR

IS-IS

RIP

Internal router

ABR

Area 0