3Com 2500 manual Default Route

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 ARP is a low-level protocol used to locate the MAC address correspond- Protocol (ARP) 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.

ARP Cache

IP Address MAC Address

158.101.1.1 00308e3d0042

158.101.2.1 0080232b00ab

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