Table 76 Routers that Share Routing Information with Immediate Neighbors

Router

A

B

C

D

A

---

Yes

Yes

No

B

Yes

---

No

Yes

C

Yes

No

---

Yes

D

No

Yes

Yes

---

The entries in Table 7-6 that contain the word Yes indicate a physically adjacent pair of routers that would exchange routing information. The entries that contain a dash denote the same router: A router cannot be adjacent to itself. Finally, those entries that contain the word No indicate nonadjacent routers that cannot directly exchange routing information. Such routers must rely on their adjacent neighbors for updates about destinations on nonadjacent routers.

From this table, it is apparent that because they are not directly connected to each other, Routers A and D must rely on Routers B and C for information about each other's destinations. Similarly, Routers B and C must rely on Routers A and D for information about each other's destinations.

Figure 7-6 shows this sharing of routing information between immediate neighbors. Figure 7-6: Immediate neighbors sharing routing data.

Figure 7-6 shows this sharing of routing information between immediate neighbors. Figure 7-6: Immediate neighbors sharing routing data.

The important implication in this scenario is that, because not every router is immediately adjacent to every other router, more than one routing update may be required to fully propagate new routing information that accommodates the failed link. Therefore, accommodating topological change is an iterative and communal process.

For the sake of simplicity, assume that convergence occurs within two routing table updates in this example. During the first iteration, the routers are starting to converge on a new understanding of their topology. Routers C and D, because of the unusable link between them, cannot exchange routing information. Consequently, they invalidate this route and all destinations that use it. Table 7-7 summarizes the contents of the four routers' routing tables during the convergence process. Note that the contents of some routing tables may reflect the mistaken belief that the link between Routers C and D is still valid

Table 7-7: Midconvergence Routing Table Contents

Gateway Router

Destination

Next Hop

Number of Hops to Destination

A

172.16.0.0

B

1

A

192.168.125.0

C

1

A

192.168.253.0

B or C

2

B

10.0.0.0

A

1

B

192.168.125.0

A or D

2

B

192.168.253.0

D

1

C

10.0.0.0

A

1

C

172.16.0.0

A only (D failed)

2

C

192.168.253.0

D - Invalid route

Not reachable

D

10.0.0.0

B or C

2

D

172.16.0.0

B

1

D

192.168.125.0

C - Invalid route

Not reachable

In Table 7-7, Routers C and D have invalidated the route between them. Routers A and B, however, still believe that their routes through this link are viable. They must await a routing update from either Router C and/or D before they can recognize the change in the internetwork's topology.

Table 7-8 contains the contents of the four routers' routing tables after they have converged on a new topology. Remember that this is an intentionally generic depiction of the convergence process; it is not indicative of any particular routing protocol's mechanics.

Table 7-8: Postconvergence Routing Table Contents

Router

Destination Name

Next Hop

Number of Hops to Destination

A

172.16.0.0

B

1

A

192.168.125.0

C

1

A

192.168.253.0

B only

2

B

10.0.0.0

A

1

B

192.168.125.0

A only

2

B

192.168.253.0

D

1

C

10.0.0.0

A

1

C

172.16.0.0

A only

2

C

192.168.253.0

A

3

D

10.0.0.0

B only

2

D

172.16.0.0

B

1

D

192.168.125.0

B only

3

As evident in Table 7-8, all the routers in the internetwork eventually agree that the link between C and D is unusable, but that destinations in each autonomous system are still reachable via an alternative route.

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