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150 Chapter 5: RIP, IGRP, and Static Route Concepts and Configuration

Distance Vector Concepts

Distance vector logic is pretty simple on the surface. However, the distance vector features that help prevent routing loops can actually be pretty difficult to grasp at first.

Distance vector protocols work by having each router advertise all the routes they know out all their interfaces. Other routers that share the same physical network receive the routing updates and learn the routes. The routers that share a common physical network are generally called neighbors. For instance, all routers attached to the same Ethernet are neighbors; the two routers on either end of a point-to-point serial link are also neighbors. If all routers advertise all their routes out all their interfaces, and all their neighbors receive the routing updates, eventually every router will know the routes to all the subnets in the network. It’s that simple!

The following list spells out the basic distance vector logic and introduces a few important concepts (which are explained over the next several pages):

Routers add directly connected subnets to their routing tables, even without a routing protocol.

Routers send routing updates out their interfaces to advertise the routes that this router already knows. These routes include directly connected routes, as well as routes learned from other routers.

Routers listen for routing updates from their neighbors so that they can learn new routes.

The routing information includes the subnet number and a metric. The metric defines how good the route is; lower metric routes are considered better routes.

When possible, routers use broadcasts or multicasts to send routing updates. By using a broadcast or multicast packet, all neighbors on a LAN can receive the same routing information in a single update.

If a router learns multiple routes to the same subnet, it chooses the best route based on the metric.

Routers send periodic updates and expect to receive periodic updates from neighboring routers.

Failure to receive updates from a neighbor in a timely manner results in the removal of the routes previously learned from that neighbor.

A router assumes that, for a route advertised by Router X, the next-hop router in that route is Router X.

The following examples explain the concepts in the list in a little more depth. Figure 5-2 demonstrates how Router A’s directly connected subnets are advertised to Router B. In this case, Router A advertises two directly connected routes.

Distance Vector Concepts 151

Figure 5-2 Router A Advertising Directly Connected Routes

162.11.8.1

162.11.5.0Router A

 

 

 

 

 

 

 

 

 

Routing Update

 

 

 

 

 

 

 

 

 

1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

s0

s1

162.11.5.0

 

1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

162.11.9.0

 

1

 

 

162.11.9.0

 

 

 

162.11.8.0

 

 

 

 

 

 

 

 

s0

 

 

 

 

s0

 

 

 

Router C

 

 

 

 

 

 

 

Router B

 

 

 

 

 

 

 

 

 

 

 

 

E0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

162.11.10.0

 

 

 

 

 

 

 

162.11.7.0

 

 

 

Table 5-2 shows the resulting routing table on Router B.

Table 5-2 Router B

Group (Mask Is

Outgoing

Next-Hop

 

255.255.255.0)

Interface

Router

Comments

 

 

 

 

162.11.5.0

S0

162.11.8.1

This is one of two routes learned via

 

 

 

the update in the figure.

 

 

 

 

162.11.7.0

E0

This is a directly connected route.

 

 

 

 

162.11.8.0

S0

This is a directly connected route.

 

 

 

 

162.11.9.0

S0

162.11.8.1

This is one of two routes learned via

 

 

 

the update in the figure.

 

 

 

 

Two interesting facts about what a Cisco IOS software-based Cisco router puts in the routing table become obvious in this example. First, just like for any other directly connected route, the two directly connected routes on Router B do not have an entry in the table’s Next-Hop Router field, because packets to those subnets can be sent directly to hosts in those subnets. In other words, there is no need for Router B to forward packets destined for those subnets to another router, because Router B is attached to those subnets. The second interesting fact is that the Next-Hop Router entries for the routes learned from Router A show Router A’s IP address as the next router. In other words, a route learned from a neighboring router goes through that router. Router B typically learns Router A’s IP address for these routes simply by looking at the routing update’s source IP address.

152 Chapter 5: RIP, IGRP, and Static Route Concepts and Configuration

If for some reason Router A stops sending updates to Router B, Router B removes the routes it learned from Router A from the routing table.

The next example gives you some insight into the metric’s cumulative effect. A subnet learned via an update from a neighbor is advertised, but with a higher metric. Just like a road sign in Decatur, Ga. might say “Turn here to get to Snellville, 14 miles,” another road sign farther away in Atlanta might read “Turn here to get to Snellville, 22 miles.” By taking the turn in Atlanta, 8 miles later, you end up in Decatur, looking at the road sign that tells you the next turn to get to Snellville, which is then 14 miles away. This example shows you exactly what happens with RIP, which uses hop count as the metric. Figure 5-3 and Table 5-3 illustrate this concept.

Figure 5-3 Router A Advertising Routes Learned from Router C

162.11.5.0

 

 

 

 

Router A

162.11.8.1

 

 

 

 

 

 

 

 

Routing Update

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2

162.11.10.0

2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

s0

s1

162.11.5.0

1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

162.11.9.0

1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

162.11.9.0

 

 

 

 

 

162.11.8.0

 

 

Routing Update

1

 

 

 

 

 

 

 

 

 

 

 

 

162.11.10.0

1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

s0

 

 

 

 

 

s0

 

 

Router C

 

 

 

 

 

 

 

 

 

 

 

 

 

Router B

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

162.11.10.0

 

 

 

 

 

 

 

162.11.7.0

 

 

Table 5-3 Router B Routing Table After Receiving the Update Shown in Figure 5-3

 

Outgoing

 

 

 

Group

Interface

Next-Hop Router

Metric

Comments

 

 

 

 

 

162.11.5.0

S0

162.11.8.1

1

This is the same route that

 

 

 

 

was learned earlier.

 

 

 

 

 

162.11.7.0

E0

0

This is a directly connected

 

 

 

 

route.

 

 

 

 

 

162.11.8.0

S0

0

This is a directly connected

 

 

 

 

route.

 

 

 

 

 

162.11.9.0

S0

162.11.8.1

1

This one was also learned

 

 

 

 

earlier.

 

 

 

 

 

162.11.10.0

S0

162.11.8.1

2

This one was learned from

 

 

 

 

Router A, which learned it

 

 

 

 

from Router C.