Incredibly Bad Network Designs That Still Work
By Andres Villalva
Kudos to IT giants such as Cisco, Juniper and Microsoft. Despite some horrible network
designs out in the field their equipment continues to work. In many cases, so well that the
designer isn't aware of the abomination that is the network architecture. Here are our top
4 picks for network designs that can make your eyes water.
1. Dodgy Net - This design consists of many IP subnets all residing on one single
VLAN. For the uninitiated the general rule is 1 IP subnet per VLAN. This helps to segment layer
2 and layer 3 traffic consistently across the network.
Technically, however, it is possible to run all IP subnets on a single VLAN. Of course,
you get the worst of both worlds with this approach. IP broadcasts are encapsulated by layer
2 frames that have no boundaries and are in turn seen by every IP device on the network. Those
devices outside the IP subnet of the originating host promptly discard the packet but by that
stage both performance and security have been compromised.
Correcting dodgy net designs does require a lot of planning and management because every
access port VLAN and trunk port has to be identified, labelled and configured.
Configuring Dodgy Net is akin to slipping on a warm sweater in winter then jumping into
a cold pool. It just doesn't make sense.
2. Static City - Most network engineers first learn about routing using static
routes. Learning to propagate routes via routing protocols comes later but for some lost souls
the penny never drops and their network designs inevitably become static cities.
Consider that modern networks can host thousands of subnets and hundred/thousands of
routing devices. Imagine now having to write down each subnet from the perspective of each
device and manually tell it which direction to send the packet. That is a lot of work and it
becomes an administrative nightmare in large networks where changes occur on a daily basis.
Here is a simple example of how the workload involved in adding manual routes can grow
exponentially. A network with 800 subnets hosted on 50 devices requires 40,000 static entries.
Of course, this doesn't take into account summarization but even if you can reduce that number
to just 10 percent of original entries that is still 4,000 routes to manage and update.
Every time a new network is added, removed or modified 50 devices must be reconfigured to
reflect the new changes. Even on a relatively stable network 1 change per month will add up
to 600 devices changes per year. 1 Change per week and that number grows to a staggering
2600 changes per year.
The good news is that this problem is relatively easy to fix because the administrative
distance feature on routers means that you can configure and implement network protocols together
with static routes allowing you to configure the entire dynamic routing solution and test it
without having to remove a single static.
But many organizations that have experienced this type of growth are reluctant to change
for fear of breaking something unforeseen and many of them have utilized default gateways as
a way to reduce an otherwise unmanageable problem.
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