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.

6.2.3 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

6.2.4 ICMP Router Discovery

Before a host can send IP datagrams beyond its directly attached subnet, it must discover 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 start-up 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 arrive.

Router Discovery messages do not constitute a routing protocol: they enable hosts to discover the existence of neighboring routers, but not which router provides a route to a particular destination. If a host chooses a poor first-hop router for a particular destination, it should receive an ICMP Redirect from that router, identifying a better one.

6.2.5 Proxy ARP

When a node in the attached subnetwork does not have routing or a default gateway configured, ARP Proxy can be used to forward an ARP request to a remote subnetwork. When the switch receives an ARP request for a remote network and ARP Proxy is enabled, it determines if it has the best route to the remote network, and then answers the ARP request by sending its own MAC address to the requesting node. That node then sends traffic to the switch, which in turn uses its own routing table to forward the traffic to the remote destination. End stations that require Proxy ARP must view the entire network as a

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