Redirects and Relays – Ciscco Certifed Support Technici

What if you have a network like the one illustrated in Figure 3-15? If host A has its default gateway set to router B, it does not seem as though it can ever send any packets to E.Figure 3-15 Redirects Router B is configured as host A’s default gateway. When A sends a packet toward F, it will send the packet to B, which will either route the packet back out the same interface, toward C, or it will drop the packet (if B has been configured not to forward traffic back out the interface on which it arrived).

To resolve this point, router B can send a  redirect to A, telling the host it should send traffic destined to 2001:db8:3e8:e030::/60 toward router C. Redirects are transmitted through the Internet Control Message Protocol (ICMP) in both IPv4 and IPv6.

Host A will insert this information into its local routing table.

Hosts also have routing tables but do not normally run a routing protocol to populate them. Host routing tables will be considered in the next chapter.

We can use the same illustration to consider one final host configuration problem and its solution. If A is the only configuration server attached to this network, then E is going to run into a problem trying to lease an address and retrieve other configuration information. DHCP relies on local segment broadcast packets. If E sends a DHCP request to the physical address broadcast address, A will never receive this request because router B breaks up the broadcast domain; B will not forward broadcast packets from one segment to another.

To resolve this problem, router B can be configured as a DHCP relay. When  B receives a broadcast DHCP interface on any interface, it will forward the DHCP request—as a unicast—to host A’s IP address.

Chapter Review

Addresses are complex; they must be defined, formatted, and assigned. This chapter discussed address assignment and mapping in detail. Addresses are not enough for moving packets through a network, however. Network devices like routers and switches need to know where and how hosts are attached to the network and where to send packets so they can be delivered.

This chapter considered calculating, assigning, and mapping IP addresses. Routing and switching are also explained here at a high level, and we pulled the lid off a router to get a better idea of how it really works.

Now that you have a general idea of how addresses are used to move packets from a source host to a destination host and how addresses are assigned to a host, it is time to look at how to configure a few different kinds of hosts. The next chapter considers how to configure Windows, Apple Macintosh, and Linux hosts for network connectivity.

One key to doing well on the exams is to perform repetitive spaced review sessions. Review this chapter’s material using either the tools in the book or interactive tools for the same material found on the book’s companion website. Refer to the online Appendix D, “Study Planner,” element for more details.

Table 3-2 outlines the key review elements and where you can find them. To better track your study progress, record when you completed these activities in the second column.

Table 3-2 Chapter Review Tracking