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WITCH
3-248
However, if the MAC address is not yet known to the switch, an Address
Resolution Protocol (ARP) packet with the destination IP address is
broadcast to get the destination MAC address from the destination node.
The IP packet can then be sent directly with the destination MAC address.
If the destination belongs to a different subnet on this switch, the packet
can be routed directly to the destination node. However, if the packet
belongs to a subnet not included on this switch, then the packet should be
sent to a router (with the MAC address of the router itself used as the
destination MAC address, and the destination IP address of the destination
node). The router will then forward the packet to the destination node via
the correct path. The router can also use the ARP protocol to find out the
MAC address of the destination node of the next router as necessary.
Note:In order to perform IP switching, the switch should be recognized
by other network nodes as an IP router, either by setting it as the default
gateway or by redirection from another router via the ICMP process.
When the switch receives an IP packet addressed to its own MAC address,
the packet follows the Layer 3 routing process. The destination IP address
is checked against the Layer 3 address table. If the address is not already
there, the switch broadcasts an ARP packet to all the ports on the
destination VLAN to find out the destination MAC address. After the
MAC address is discovered, the packet is reformatted and sent out to the
destination. The reformat process includes decreasing the Time-To-Live
(TTL) field of the IP header, recalculating the IP header checksum, and
replacing the destination MAC address with either the MAC address of the
destination node or that of the next hop router.
When another packet destined to the same node arrives, the destination
MAC can be retrieved directly from the Layer 3 address table; the packet is
then reformatted and sent out the destination port. IP switching can be
done at wire-speed when the destination address entry is already in the
Layer 3 address table.
If the switch determines that a frame must be routed, the route is