To Another AS

External routes are routes passed between a router within the OSPF domain and a router in another autonomous system or routing domain. The routes discovered by OSPF in this way can have the cost of the path calculated in one of two ways:

• E1—The cost of the path to the ASBR is added to the external cost to the next-hop router outside the AS.

• E2—The cost of the path to the ASBR is all that is considered in the calculation. This is the default configuration. This is used when there is only one router advertising the route and no selection is required. If both an E1 and an E2 path are offered to the remote network, the E1 path will be used.

The Path

The Path

At the side of the routing table is a column indicating the source of the routing information. Typically, this is the routing protocol. In the instance of OSPF, however, it includes the LSA type that provided the path.

Table 6-2 shows the codes used in the routing table.

Table 6-2 OSPF Routing Table Codes and Associated LSAs

Table 6-2 shows the codes used in the routing table.

Table 6-2 OSPF Routing Table Codes and Associated LSAs

LSA Type

Routing Table Entry

Description

1 Router Link

O

This is generated by the router, listing all the links to which it is connected, their status, and their cost. It is propagated within the area.

2 Network Link

O

This is generated by the designated router on a multiaccess LAN to the area.

3 or 4 Summary Link (between areas)

IA

LSA Type 3 includes the networks or subnets within an area that may have been summarized and that are sent into the backbone and between ABRs. LSA Type 4 is information set to the ASBR from the ABR. These routes are not sent into totally stubby areas.

5 Summary Link/External Link (between autonomous systems)

E1 OR E2

The routes in this LSA are external to the autonomous system. They can be configured to have one of two values. E1 will include the internal cost to the ASBR to the external cost reported by the ASBR. E2 does not compute the internal cost— it just reports the external cost to the remote destination.

TIP The exam focuses on the LSA Types 1 through 5.

Was this article helpful?

0 0

Post a comment