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RIP 2 packets also can contain one of two types of authentication methods that can be used to verify the validity of the supplied routing data.
The first method is a simple password in which an authentication key of up to 16 characters is included in the packet. If this password does not match what is expected, the packet is discarded. This method provides very little security, as it is possible to learn the authentication key by watching RIP packets.
The second method uses the MD5 algorithm to create a crypto checksum of a RIP packet and an authentication key of up to 16 characters. The transmitted packet does not contain the authentication key itself; instead, it contains a crypto-checksum called the digest. The receiving router performs a calculation using the correct authentication key and discards the packet if the digest does not match. In addition, a sequence number is maintained to prevent the replay of older packets. This method provides stronger assurance that routing data originated from a router with a valid authentication key.
