3Com LANPLEX 2500, Руководство пользователя, Страница: 49 / 186

Elements of IP Routing 4-7
An active router sends a RIP message every 30 seconds. This message
contains both the IP address and a metric (the distance to the
destination from that router) for each destination. In RIP, each router
that a packet must travel through to reach a destination equals one
hop .
Default Route
In addition to the routes to specific destinations, the routing table may
contain an entry called the
default route . The router uses the default
route to forward packets that do not match any other routing table
entry. A default route is often used in place of routes to numerous
destinations all having the same gateway IP address and interface
number. The default route can be configured statically, or it can be
learned dynamically using RIP.
Address Resolution
Protocol (ARP)
ARP is a low-level protocol used to locate the MAC address correspond-
ing to a given IP address. This protocol allows a host or router to make
its routing decisions using IP addresses while it uses MAC addresses to
forward packets from one hop to the next.
Once the host or router knows the IP address of the next hop to the
destination, the host or router must translate that IP address into a MAC
address before the packet can be sent. To do this translation, the host or
router first looks in its ARP cache, a table of IP addresses with their cor-
responding MAC addresses. Each device participating in IP routing
maintains an ARP cache. See Figure 4-6.
Figure 4-6 Example of an ARP Cache
If the IP address does not have a corresponding MAC address listed, the
host or router broadcasts an
ARP request packet to all the devices on the
network. The ARP request contains information about the hardware and
ARP Cache
158.101.1.1
158.101.2.1
IP Address MAC Address
00308e3d0042
0080232b00ab