In the example shown in Figure 19, authentication is configured on router LA and router SanDiego, but not on router SanJose. Router LA and router SanDiego accept packets from each other because they contain message digests generated by an accepted key. Router SanJose accepts packets from router LA and router SanDiego, and simply ignores the message digest included in their packets. Router LA and router SanDiego reject packets from router SanJose because those packets do not include a message digest.
Figure 19: Packet Flow Between Routers With and Without Authentication Set
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