Information Technology Reference
In-Depth Information
layers, defining that layer's role and BGP architecture. It also provides a BGP configuration
template appropriate to that layer.
The Network Core Layer
The primary responsibility of the network core is switching packets at line rate. The net-
work core consists of a small number of routers, usually fewer than 20, that are all connect-
ed in a dense partial mesh or full mesh. The network core is at the very top of the hierarchy,
providing connectivity for the aggregation layer. Figure 9-1 shows a core network.
Figure 9-1
Sample Network Core
Aggregation Routers
Aggregation Routers
The core routers terminate two types of links: core links and aggregation uplinks. The core
links form the actual core by interconnecting the core routers. These links are typically the
highest-capacity links in the network. The aggregation uplinks provide connectivity for the
aggregation layer into the network core. The core routers are not centrally located, because
this would reduce the network's fault tolerance. A single location for all core routers would
provide a single point of failure for the entire network.
It is uncommon to see policy application or packet filtering at the network core. The traffic
levels on core devices present a scaling challenge for processing-intensive operations in the
forwarding path.
