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oundary routers. They describe <strong>network</strong><br />

destinations that are external <strong>to</strong> <strong>the</strong> OSPF <strong>network</strong>.<br />

� Link state acknowledgment—These are sent <strong>to</strong> <strong>the</strong><br />

LSA sender <strong>to</strong> acknowledge that <strong>the</strong> LSA has been<br />

received.<br />

• Packet length—The <strong>to</strong>tal size of <strong>the</strong> message in bytes.<br />

• Router ID—The ID number of <strong>the</strong> router sending <strong>the</strong> message.<br />

Usually, <strong>the</strong> IP address of <strong>the</strong> interface that is sending <strong>the</strong><br />

message.<br />

• Area ID—The ID number of <strong>the</strong> destination area.<br />

• Checksum—Cyclical redundancy check (CRC) of <strong>the</strong> message.<br />

Excludes <strong>the</strong> au<strong>the</strong>ntication fields.<br />

• Au<strong>the</strong>ntication type and value—OSPF supports process<br />

au<strong>the</strong>ntication. The type field specifies what type is being used,<br />

and <strong>the</strong> value field is <strong>the</strong> au<strong>the</strong>ntication key.<br />

OSPF messages are sent ei<strong>the</strong>r by using a technique known as<br />

flooding or directly <strong>to</strong> a node address. Flooding is accomplished by<br />

having each router forward an OSPF message out of each of its<br />

connected interfaces. This is <strong>the</strong> most common means of propagating<br />

<strong>network</strong> changes throughout <strong>the</strong> OSPF routing domain. If a message<br />

is intended for all <strong>the</strong> routers in <strong>the</strong> OSPF domain, <strong>the</strong> message is sent<br />

<strong>to</strong> <strong>the</strong> multicast address 224.0.0.5. In cases where router adjacencies<br />

exist, OSPF messages intended for <strong>the</strong> designated router or backup<br />

designated router are sent using <strong>the</strong> multicast address 224.0.0.6.<br />

After an OSPF message is received, it must be verified before it can be<br />

processed. There are eight steps <strong>to</strong> OSPF message verification:<br />

1. The IP packet CRC checksum is verified.<br />

2. The destination IP address must be for <strong>the</strong> router interface that<br />

received <strong>the</strong> message or one of <strong>the</strong> designated multicast<br />

addresses.<br />

3. The IP pro<strong>to</strong>col ID must identify that <strong>the</strong> message is an OSPF<br />

message (89 is OSPF's IP pro<strong>to</strong>col ID).<br />

4. The source IP address must be checked <strong>to</strong> make sure <strong>the</strong><br />

message did not originate from <strong>the</strong> receiving station.<br />

5. The OSPF version number is verified.<br />

6. The OSPF header CRC checksum is checked.<br />

7. The area ID must be ei<strong>the</strong>r that of <strong>the</strong> interface or <strong>the</strong> backbone<br />

area.<br />

8. The OSPF au<strong>the</strong>ntication type and value must correspond and<br />

be correct.

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