Core Layer

Core Layer
The core layer is literally the core of the network. At the top of the hierarchy, the core layer is
responsible for transporting large amounts of traffic both reliably and quickly. The only purpose
of the core layer of the network is to switch traffic as quickly as possible. The traffic transported
across the core is common to a majority of users. However, remember that user data is processed
at the distribution layer, and the distribution layer forwards the requests to the core, if needed.
If there is a failure in the core, every single user can be affected. Therefore, fault tolerance at
this layer is an issue. The core is likely to see large volumes of traffic, so speed and latency are
driving concerns here. Given the function of the core, we can now look at some design specifics
to consider. Let’s start with some things you know you don’t want to do:
 Don’t do anything to slow down traffic. This includes using access lists, routing between
VLANs, and packet filtering.
 Don’t support workgroup access here.
 Avoid expanding the core when the internetwork grows (that is, adding routers). If performance
becomes an issue in the core, give preference to upgrades over expansion.
There are a few things that you want to make sure to get done as you design the core:
 Design the core for high reliability. Consider Data Link technologies that facilitate both speed
and redundancy, such as FDDI, FastEthernet (with redundant links), Gigabit Ethernet, or
even ATM.
Core
layer
Distribution
layer
Access
layer
FDDI Ring
Users’ machines Users’ machines Users’ machines
Workgroups
Understanding the Cisco Hierarchical Model 397
 Design with speed in mind. The core should have very little latency.
 Select routing protocols with lower convergence times. Fast and redundant Data Link connectivity
is no help if your routing tables are shot!