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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 calculated only during
setup. Once the route has been determined, all packets in the current flow are simply
switched or forwarded across the chosen path. This takes advantage of the high
throughput and low latency of switching by enabling the traffic to bypass the routing
engine once path calculation has been performed.
4.2.3.Routing Path Management Routing Path Management involves the determination and updating of all the routing
information required for packet forwarding, including:
• Handling routing protocols
• Updating the routing table
• Updating the Layer 3 switching database
4.2.4.ICMP Router Discovery Before a host can send IP datagrams beyond its directly attached subnet, it must find
the address of at least one operational router on that subnet. Typically, this can be
accomplished by reading a list of one or more router addresses from a configuration file
at startup time. On multicast links, some hosts also discover router addresses by
listening to routing protocol traffic.
The ICMP Router Discovery message is an alternative router discovery method that
uses a pair of ICMP messages on multicast links. It eliminates the need to manually
configure router addresses and is independent of any specific routing protocol.
ICMP Router Discovery messages are called “Router Advertisements” and “Router
Solicitations.” Each router periodically multicasts a Router Advertisement from each of
its multicast interfaces, announcing the IP address(es) of that interface. Hosts discover
the addresses of their neighboring routers simply by listening for advertisements. When
a host attached to a multicast link starts up, it may multicast a Router Solicitation to ask
for immediate advertisements, rather than waiting for the subsequent, periodic ones to