Information Technology Reference
In-Depth Information
smaller networks, this layer is often omitted. The access layer can be aggregated directly
on the core routers. As the access layer grows, the aggregation layer becomes more
important. The aggregation routers form the middle level of the network hierarchy.
Figure 9-2
Aggregation Layer
Network Core
Core Routers
Aggregation Routers
Edge Routers
An aggregation router has two types of links: uplinks and downlinks. Uplinks are to the net-
work core routers and the circuits to the edge routers. Typically, there are two uplinks for
each aggregation router to two separate core routers. This provides uplink and core router
redundancy. In addition to the physical redundancy, BGP redundancy is provided by having
the aggregation router configured as a route reflector client to both directly connected core
routers. Figure 9-2 shows only two aggregation routers connected to the two core routers,
but a real network would have many more aggregation routers homed to the two core
routers.
The aggregation routers have downlinks to the edge routers. The aggregation layer gets
its name from providing aggregation to the edge routers. The use of an aggregation layer
reduces the core routers' port density requirements and increases the network's extensibil-
ity. Aggregation routers are not connected directly to other aggregation routers, because this
reduces the predictability of traffic flow and can reduce the effectiveness of capacity
planning.
The aggregation routers provide the second level of hierarchy in the BGP route reflection
architecture. The aggregation routers are route reflector clients of the core routers and are
route reflectors for the edge routers. The only BGP sessions on the aggregation routers are
iBGP sessions, because external connections are terminated on the edge routers.
Two peer groups are required on the aggregation routers. The first peer group is the iBGP
sessions to the upstream core routers. The second peer group is the iBGP sessions to the
edge routers. The modularity provided allows the network to be expanded with minimal
