Figure 12-4 shows
several steps that occur over time, as follows:
|
1. |
Each router knows only its respective, directly connected
routes, as shown by the routing tables at the top of the
figure.
|
|
2. |
R1 sends a routing update to R2. The term routing update refers to a
routing protocol message that contains information about IP networks and
subnets. Notice that R1's routing update simply lists the subnets that R1 has in
its routing table. |
|
3. |
Each router still has a routing table, with R2's now larger
because it learned some routes. R2 decided to add a route to subnet 150.1.1.0the
subnet where Hannah resides. R2 didn't have a route to subnet 150.1.1.0 before,
but after R2 learned about that subnet from the routing update that R1 sent, it
now knows about this additional subnet.
|
Also notice the outgoing interface and next-hop router of this
new route. R2 uses its own Ethernet1 interface as the outgoing interface. From
Figure 12-4, it seems that R2 would use
its interface E1 to forward packets to subnet 150.1.1.0, but R2 chooses to put
that interface into the route because that's the interface in which it received
the routing update. R2 also puts R1's IP address of 150.1.2.1 as the next-hop
router because that's the IP address of the router that sent the routing update.
From Figure 12-4, you can see that this
route's next-hop router IP address should be R1's IP address 150.1.2.1.
Notice that although R2 learned a route based on getting the
routing update from R1, R1 has not learned any routes yet. Routers learn routes
with routing protocols when they receive routing updates from other routers;
routers send routing updates so that other routers can learn from them. For R1
to learn routes, another router needs to tell it about other routes with a
routing update. Figure 12-5 shows R2
advertising its routes, with R1 learning a route to subnet 150.1.3.0.
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