Unfortunately, the independent nature of
routers can also be a source of vulnerability whenever a change
occurs in the network's topology. Such changes, by their very
nature, change a network's topology. Figure
illustrates
how a change in the network is, in fact, a change in its topology.
Figure
features another fairly simple, four-node
internetwork with some route redundancy. Figure
summarizes
the routing tables of the four routers. For the sake of this
example, consider this table to be preconvergence routing table
information.
Preconvergence Routing Table Contents
If packets sent by Router C to Server 192.168.253.2 suddenly
become undeliverable, it is likely that an error occurred somewhere
in the network. This could have been caused by a seemingly infinite
number of different, specific failures. Some of the more common
suspects include the following:
- The server has failed completely (due to either a hardware,
software, or electrical failure).
- The LAN connection to the server has failed.
- Router D has experienced a total failure.
- Router D's serial interface port to router C has failed.
- The transmission facility between Gateway Routers C and D has
failed.
- Router C's serial interface port to Router D has failed.
Obviously, the new network topology can't be determined until the
exact location of the failure has been identified. Similarly, the
routers cannot attempt to route around the problem until the failure
location has been isolated. If either of the first two scenarios
occurred, server 192.168.253.2 would be completely unavailable to
all the users of the internetwork, regardless of any route
redundancy that may have been built into the network.
Similarly, if router D had failed completely, all the
LAN-attached resources at that location would be isolated from the
rest of the network. If the failure was either a partial failure of
that router, or elsewhere in the network, however, there might still
be a way to reach Server 192.168.253.2. Finding a new route to
192.168.253.2 requires the network's routers to recognize and agree
on which piece of the network failed. In effect, subtracting this
component from the network changes the network's topology.
To continue with the example, assume that Router D's serial
interface port to router C has failed. This renders the link between
C and D unusable. Figure
illustrates the new network
topology.
Routers using a dynamic routing protocol would quickly determine
that Server 192.168.253.2 was unreachable through their current,
preferred route. Individually, none of the routers could determine
where the actual failure occurred, nor could they determine whether
any viable alternative routes still existed. By sharing information
with each other, however, a new composite picture of the network can
be developed.
Note For the purposes of this chapter, this example uses
an intentionally generic method of convergence. More specific
details about each routing protocol's convergence characteristics
are presented in Part III.
The routing protocol used in this internetwork is relatively
simple. It limits each router to exchanging routing information with
its immediate neighbors, although it supports the recording of
multiple routes per destination. Figure
summarizes the
pairs of immediately adjacent routers illustrated in Figure .
The entries in Figure
that contain the word Yes
indicate a physically adjacent pair of routers that would exchange
routing information. The entries that contain a dash (gray) 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
shows this sharing of routing information between
immediate neighbors.
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. Figure
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.
In Figure , 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.
Figure 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.
As evident in Figure , 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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