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IPv4 Routing Protocols

The routing (forwarding) process depends heavily on having an accurate and up-to-date IP
routing table on each router. This section takes another look at routing protocols, considering
the goals of a routing protocol, the methods routing protocols use to teach and learn
routes, and an example.

First, consider the goals of a routing protocol, regardless of how the routing protocol works:
■ To dynamically learn and fill the routing table with a route to each subnet in the
internetwork.
■ If more than one route to a subnet is available, to place the best route in the routing
table.
■ To notice when routes in the table are no longer valid, and to remove them from the
routing table.
■ If a route is removed from the routing table and another route through another neighboring
router is available, to add the route to the routing table. (Many people view this
goal and the preceding one as a single goal.)
■ To work quickly when adding new routes or replacing lost routes. (The time between
losing the route and finding a working replacement route is called convergence time.)
■ To prevent routing loops.

Routing protocols all use some similar ideas to allow routers to learn routing information
from each other. Of course, each routing protocol works differently; otherwise, you would
not need more than one routing protocol. However, many routing protocols use the same
general steps for learning routes:

Step 1. Each router, independent of the routing protocol, adds a route to its routing
table for each subnet directly connected to the router.

Step 2. Each router’s routing protocol tells its neighbors about the routes in its routing
table, including the directly connected routes and routes learned from other
routers.

Step 3. After learning a new route from a neighbor, the router’s routing protocol adds
a route to its IP routing table, with the next-hop router of that route typically
being the neighbor from which the route was learned.

For example, Figure 4-12 shows the same sample network as in Figure 4-11, but now with
a focus on how the three routers each learned about subnet 150.150.4.0. Note that routing
protocols do more work than is implied in the figure; this figure just focuses on how the
routers learn about subnet 150.150.4.0.
Free CISCO CCNA Routing and Switching ICND1 Study Guide
Figure 4-12 Router R1 Learning About Subnet 150.150.4.0

Follow items A through F shown in the figure to see how each router learns its route to
150.150.4.0. All references to Steps 1, 2, and 3 refer to the list just before Figure 4-12.
Step A. Subnet 150.150.4.0 exists as a subnet at the bottom of the figure, connected to
Router R3.
Step B. R3 adds a connected route for 150.150.4.0 to its IP routing table (Step 1); this
happens without help from the routing protocol.
Step C. R3 sends a routing protocol message, called a routing update, to R2, causing
R2 to learn about subnet 150.150.4.0 (Step 2).
Step D. R2 adds a route for subnet 150.150.4.0 to its routing table (Step 3).
Step E. R2 sends a similar routing update to R1, causing R1 to learn about subnet
150.150.4.0 (Step 2).
Step F. R1 adds a route for subnet 150.150.4.0 to its routing table (Step 3). The route
lists R1’s own Serial0 as the outgoing interface and R2 as the next-hop router
IP address (150.150.2.7).

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