System information

The Network Layer
Book Title
7-96
IP routing protocols are dynamic. Dynamic routing calls for routes to be calculated at regular
intervals by software in the routing devices. This contrasts with static routing, where routes are
established by the network administrator and do not change until the network administrator changes
them. An IP routing table consists of destination address/next hop pairs. A sample entry, shown in
Figure 7-7, is interpreted as meaning “to get to network 34.1.0.0 (subnet 1 on network 34), the next
stop is the node at address 54.34.23.12.”
Figure 7-7 An Example of an IP Routing Table
IP routing specifies that IP datagrams travel through internetworks one hop at a time. The entire route
is not known at the outset of the journey. Instead, at each stop, the next destination is calculated by
matching the destination address within the datagram with an entry in the current node’s routing
table. Each node’s involvement in the routing process consists only of forwarding packets based on
internal information, regardless of whether the packets get to their final destination. In other words,
IP does not provide for error reporting back to the source when routing anomalies occur. This task
is left to another Internet protocol, the Internet Control Message Protocol (ICMP).
ICMP
ICMP performs a number of tasks within an IP internetwork. The principal reason it was created was
for reporting routing failures back to the source. In addition, ICMP provides helpful messages such
as the following:
• Echo and reply messages to test node reachability across an internetwork
• Redirect messages to stimulate more efficient routing
• Time exceeded messages to inform sources that a datagram has exceeded its allocated time to
exist within the internetwork
• Router advertisement and router solicitation messages to determine the addresses of routers on
directly attached subnetworks
A more recent addition to ICMP provides a way for new nodes to discover the subnet mask currently
used in an internetwork. All in all, ICMP is an integral part of all IP implementations, particularly
those that run in routers.
IRDP
The ICMP Router Discovery Protocol (IRDP) uses router advertisement and router solicitation
messages to discover addresses of routers on directly attached subnets.
The way IRDP works is that each router periodically multicasts router advertisement messages from
each of its interfaces. Hosts discover the addresses of routers on the directly attached subnet by
listening for these messages. Hosts can use router solicitation messages to request immediate
advertisements, rather than wait for unsolicited messages.
Destination
address
34.1.0.0
78.2.0.0
147.9.5.0
17.12.0.0
Next
hop
54.34.23.12
54.34.23.12
54.32.12.10
54.32.12.10