Chapter 5. First Hop Redundancy Protocols
Layer 2 devices, such as end hosts and switches, need to go through a Layer 3 device to communicate with hosts on different VLANs or subnets. This Layer 3 device is called a default gateway. In Figure 5-1, the end hosts are connected to a Layer 2 switch. If one of them needs to reach a server in another part of the network, it must go through either Router A or Router B.
Figure 5-1 Layer 2 and Layer 3 Network
You can manually configure hosts to use a specific default gateway, but that leads to a single point of failure. Suppose the end hosts had RTR A’s IP address hard-coded as their default gateway, ...
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