Packet Forwarding Review

When a host must communicate with a device on its local subnet, it can generate an Address Resolution Protocol (ARP) request, wait for the ARP reply, and exchange packets directly. However, if the far end is located on a different subnet, the host must rely on an intermediate system (a router, for example) to relay packets to and from that subnet.

A host identifies its nearest router, also known as the default gateway or next hop, by its IP address. If the host understands something about routing, it recognizes that all packets destined off-net must be sent to the gateway's MAC address rather than the far end's MAC address. Therefore, the host first sends an ARP request to find the gateway's MAC address. Then packets can be relayed to the gateway directly without having to look for ARP entries for individual destinations.

If the host is not so savvy about routing, it might still generate ARP requests for every off-net destination, hoping that someone will answer. Obviously, the off-net destinations cannot answer because they never receive the ARP request broadcasts; these requests are not forwarded across subnets. Instead, you can configure the gateway to provide a proxy ARP function so that it will reply to ARP requests with its own MAC address, as if the destination itself had responded.

Now the issue of gateway availability becomes important. If the gateway router for a subnet or VLAN goes down, packets have no way of being forwarded off the local subnet. Several protocols are available that allow multiple routing devices to share a common gateway address so that if one goes down, another automatically can pick up the active gateway role. The sections that follow describe these protocols.

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