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In this picture, both Distribution switches also have redundant connections to the network Core, but the Core itself is not shown in detail. It is not shown because, up to this point, the network design is fairly
generic. Later in this chapter, I will discuss the different options for locations of routers in a large-scale LAN. They can be in the Core, in the Distribution Level, or in both. The appropriate types of connections
for these different design types are slightly different.
The key to working with backup links, switches, and routers is in the automated fault recovery system used. Since a Distribution Area is essentially a set of parallel broadcast domains, the best way to
implement redundancy is to use Spanning Tree.
I mentioned earlier in this chapter that to use Spanning Tree effectively, the two Distribution switches must have a trunk connection between them. Another way of looking at this is by our simplicity requirement.
The Spanning Tree Protocol needs to have a Root Bridge, which is the main switch for a Distribution Area through which all traffic must pass. Simplicity tells you that you should have every other switch in the
Distribution Area connected as directly as possible to this Root Bridge. If possible, the Root Bridge should have a single trunk connection to every other switch in the area. Then, the backup Root Bridge switch also
must have a direct trunk connection to the primary. Similarly, every other switch in the area needs a direct trunk to the backup Root Bridge in case the primary fails.
There are inherent scaling problems with directly connecting every switch to both of the two Distribution switches, which will always limit the number of switches in a Distribution Area, as I have already
discussed. Keeping your Distribution Areas relatively small and modular will be good for overall network performance anyway.
Router-to-router redundancy has different requirements for multiple connections than the switch-to-switch case I was just discussing. The dynamic routing protocols for IP operate completely differently from the
Spanning Tree Protocol. Instead of shutting down redundant links, IP routing protocols seek only to rate the different path options and select the most appropriate at the time. If one path goes away, another is
selected from the list of possibilities.
Again, simplicity is the watchword for IP dynamic routing protocols. Every router in an OSPF area must maintain information about all of its neighboring routers the ones with which it shares a direct link, and
routing table information about every other device in the area. It is important to keep the topology of an area as simple as possible. The simplest schemes connect everything redundantly, but with as few
connections as possible.
3.7 Large-Scale LAN Topologies
There are three main options for large-scale topology. If you want to use VLANs, and their benefits should be clear by now, then you need to have routers to interconnect them. Your options basically come down to
where to put these routers. You can put them in the Core or in the Distribution Level, or you put them in both. It is usually best to avoid putting routers at the Access Level of a LAN, but for very large networks it
is easy to see that you get much better scaling properties if you include routers in the Distribution Level.
3.7.1 Routers in the Core Level
Perhaps the simplest and most obvious way to build a large-scale hierarchical network is to use a model like that shown in
Figure 3-24 . In this diagram, several different Distribution Areas are connected via a
central Routing Core consisting of two routers. All Distribution Areas are redundantly connected to both Core Routers from both Distribution switches.
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Figure 3-24. A hierarchical network with central routing
The result of all these redundant connections is that any device in the Core or Distribution Levels can fail without affecting network operation. Each Distribution Area has redundant Distribution switches, either of
which can act as a Root Bridge for this area. Both Distribution switches have connections to both of the two Core routers. If either Core Router fails, you have complete redundancy.
The best part of the redundancy in this network is its simplicity. There are only two central routers there may be additional routers connecting to remote sites, as I will discuss shortly, and either can quickly take
over all central routing functions in case the other fails. Because of the way that these routers are connected to one another and to all Distribution switches, they can both be used simultaneously. However,
the extent to which these routers share the load depends on how the dynamic routing protocols are configured.
The limitation to this design is the capacity of one of the Core Routers. You must configure these two routers so that either is able to support the entire network load in case the other fails. So
Figure 3-25 shows
a simple way of overcoming this limitation. It still has a central routing Core, but now there are four routers in the Core. Each pair of routers is responsible only for a small part of the network..
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Figure 3-25. Central routing model for increased capacity
There are many different ways to connect such a Core. Figure 3-25
shows a Core with four routers that are interconnected with a full mesh. I have already indicated that a full mesh does not scale well, so if the
network will need further expansion, full mesh would not be a good option. Figure 3-26
shows a similar network but with six central routers connected to one another by a pair of central switches.
Note that there need be no VLANs defined on these two switches. Both switches S1 and S2 have connections to all six routers. A natural way to define the IP segments on these switches is to have one
switch carry one subnet and the other carry a different subnet. Then if either switch fails, the dynamic routing protocol takes care of moving all traffic to the second switch.
In this sort of configuration, it is generally useful to make the routers act in tandem. Assuming that Distribution Areas consist of two Distribution switches and several Access switches, you would connect
both switches to both routers in this pair, and you can connect several Distribution Areas to each pair of routers. The actual numbers depend on the capacity of the routers. All connections will be fully redundant.
Then only the Distribution switches that are part of this group of Distribution Areas will connect to this pair of routers. The next group of Distribution Areas will connect to the next pair of Core Routers.
3.7.2 Routers in the Distribution Level
