IP Link-State Routing Protocols - CCIE Routing and Switching Exam Certification Guide - page 382

OSPF Network with a Virtual Link Connecting an Area to the Backbone Figure 8-5

Area 3

Area 0

Virtual Link

Area 1

Area 2

Router A

Area 4

Router B

In this command, ip-address is the RID of the remote router of the virtual link. The virtual link
can be verified with the show ip ospf virtual-link command.

OSPF Router Authentication
OSPF supports the authentication of routes using 64-bit simple password or cryptographic
Message Digest 5 (MD5) authentication. MD5 authentication provides higher security than
plain text authentication. The same type of authentication must be configured on all routers
within an area. Authentication passwords do not need to be the same for the routers throughout
the area, but they must be the same between neighbors.

To enable authentication with MD5, use the following command under router ospf :

area area-id authentication message-digest
Then configure the interfaces with the MD5 keys by using the following interface command:

ip ospf message-digest-key 1 md5 key

OSPF Summarization
OSPF ABRs can be configured to summarize networks using the area range command. For
example, if a router has two Ethernet interfaces with the networks 200.100.100.0/24 and
200.100.101.0/24, and these networks are in the same area, ABR can be configured to
summarize as follows:

area 10 range 200.100.100.0 255.255.254.0
Routes can also be summarized by an ASBR when routes are redistributed into OSPF. Rather
than advertising routes individually in an external LSA, a summarized route can be advertised.