Types Of Ospf Areas - Avaya 8600 Engineering

Level
1
2
3
4
This results in a routing preference from most preferred to least preferred of:
1) routing within an OSPF area
2) routing within the OSPF domain
3) routing within the OSPF domain and external routes with External Type 1 metrics
4) routing within the OSPF domain and external routes with External Type 2 metrics
For example, an ASBR can be configured to import RIP routes into OSPF with external Type 1 metrics.
Another ASBR can be configured to import Internet routes and advertise a default route with an external
Type 2 metric. This will result in RIP imported routes having a higher preference than the Internet
imported default routes. In reality, External Type 2 metrics should always be used for BGP Internet routes
whereas External Type 1 should always be used for RIP imported routes. The reason for this is routes
imported into OSPF as External Type 1 are supposed to be from IGP (internal gateway protocols) whose
external metric is comparable to OSPF metrics. With External Type 1 metrics, OSPF will add the internal
cost to the ASBR to the external metric. External Type 2 metrics are used for EGP (exterior gateway
protocols) whose metric is not comparable to OSPF metrics. For External Type 2 metrics, only the internal
OSPF cost to the ASBR router is used in the routing decision.
8.2

Types of OSPF Areas

In reference to the area types below, the following chart displays the various LSA types exchanged
between area‟s. LSA‟s are used to share link state information among routers; there are 7 different types.
They typically contain information about the router and its neighbors and are generated periodically to
ensure connectivity or generated upon the change in state of a router/link (up/down).
LSA Type Description
1 LSA Type 1 are called router-LSAs and is
originated by a router to describe its set of
active interfaces and neighbors.
LSA Type 2 are called network-LSAs and
2
describes a network segment such as
broadcast or NBMA. In a broadcast network,
network-LSAs are originated by the Designated
Router (DR).
3 LSA Type 3 are called network-summary-LSAs.
Describes the networks within an area.
Originated by the Area Border Router (ABR).
November 2010
ERS 8600 / 8800 OSPF Technical Configuration Guide
Description
Intra-area routing
Inter-area routing
External Type 1 metrics
External Type 2 metrics
Area of Distribution
Only within the same area.
Only within the same area.
Passed between areas.
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