Ip Routing Transmission Errors - 3Com CoreBuilder 2500 User Manual

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HAPTER OUTING WITH
IP Routing
Transmission Errors
IP T
ECHNOLOGY
When devices on the network receive this packet, they examine it. If their
address is not the target protocol address, they discard the packet. When
a device receives the packet and confirms that its IP address matches the
target protocol address, the receiving device places its MAC address in
the target hardware address field and sends the packet back to the
source hardware address. When the originating host or router receives
this ARP reply, it places the new MAC address in its ARP cache next to the
corresponding IP address. See Figure 4-8.
ARP cache
IP address MAC address
158.101.1.1 00308e3d0042
158.101.2.1 0080232b00ab
158.101.3.1 0134650f3000
Figure 4-8 Example of ARP Cache Updated with ARP Reply
After the MAC address is known, the host or router can send the packet
directly to the next hop.
Because a router knows only about the next network hop, it is not aware
of problems that may be closer to the destination. Destinations may be
unreachable if:
Hardware is temporarily out of service.
You specified a nonexistent destination address.
The routers do not have a route to the destination network.
To help routers and hosts discover problems in packet transmission, a
mechanism called Internet Control Message Protocol (ICMP) reports
errors back to the source when routing problems occur. With ICMP, you
can determine whether a delivery failure resulted from a local or a remote
problem.