3Com 2500 manual IP Routing, Transmission, Errors

IP Routing Transmission Errors 4-9

IP Routing	Because each router only knows about the next hop, it is not aware of
Transmission	problems that might be further “down the road” toward the destination.
Errors	Destinations can be unreachable if:
	■ Hardware is temporarily out of service
	■ You inadvertently specified a nonexistent destination address
	■ The router does not have a route to the destination network
	To help routers and hosts know of problems in packet transmission, an
	error-reporting mechanism called Internet Control Message Protocol
	(ICMP) provides error reporting back to the source when routing
	problems arise. ICMP allows you to determine whether a delivery
	failure resulted from a local or a remote malfunction.
	ICMP does the following:
	■ Tests the reachability of nodes (ICMP Echo Request and ICMP Echo Reply)
	A host or gateway sends an ICMP echo request to a specified
	destination. If the destination receives the echo request, it sends an
	ICMP echo reply back to the original sender. This process tests whether
	the destination is reachable and responding and verifies that the major
	pieces of the transport system work. The ping command is one
	frequently used way to invoke this process.
	■ Creates more efficient routing (ICMP Redirect)
	Often the host route configuration specifies the minimal possible
	routing information needed to communicate (for example, the address
	of a single router). The host relies on routers to update its routing table.
	In the process of routing packets, a router may detect a host not using
	the best route. The router then sends the host an ICMP redirect,
	requesting that the host use a different gateway when sending packets
	to that destination. The next time the host sends a packet to that same
	destination, it uses the new route.
	■ Informs sources that a packet has exceeded its allocated time to exist
	within the network (ICMP Time Exceeded)