Information Technology Reference
In-Depth Information
Interior Gateway Protocol Layout
The most common IGPs used in ISP networks are OSPF and IS-IS. The choice of which
protocol to use is outside the scope of this topic; however, both protocols can be deployed
in a multiarea or single-area environment.
The purpose of the IGP in an ISP network is to support the BGP infrastructure. This
includes providing reachability for the BGP peering sessions and next-hop resolution for
BGP learned prefixes. The IGP should encompass only routers in the ISP network itself,
not customer edge (CE) devices, even if those devices are managed by the ISP.
The number of devices in the average ISP network is typically small enough that a
single area is used. Additional factors lead to ISPs using a single area, such as the need
for MPLS traffic engineering and end-to-end IGP metric visibility.
Network Layout
The network design developed in this chapter employs several principles for building a
stable and scalable network:
•
Hierarchy
—The most common method of enhancing a network's scalability is to
introduce a hierarchy into the network. This distributes the network's complexity and
reduces the concentration-of-scaling issue. Hierarchy is used in both the physical
topology and the BGP peering layout.
•
Modularity
—Modularity in network design increases the network's extensibility. A
modular design increases the network's predictability, providing a more deterministic
traffic flow. It also increases the efficiency of troubleshooting network events.
•
Redundancy
—Redundancy provides the foundation for a fault-tolerant network. The
use of redundancy reduces the impact of link or device failure. It is important to keep
in mind that excess redundancy can create scaling issues by reducing the level of
hierarchy in the network.
•
Simplicity
—Simplicity in network design results in fewer human mistakes and a
reduced set of code issues. In ISP networks, the quantity of routing information puts
additional stress on the routers, increasing the probability of encountering problems.
The overall design is developed in a hierarchical manner. The hierarchy is broken into three
major components:
•
The network core layer
•
The aggregation layer
•
The network edge layer
Each of these layers has a clearly defined role. A device's configuration is optimized for
the layer in which it resides. The core is at the top of the hierarchy, followed by the
aggregation layer, and then the edge layer at the bottom. This section covers each of these
