Information Technology Reference
In-Depth Information
An IGP has some or all of the following characteristics:
•
It performs topology discovery
•
It strives to achieve fast convergence
•
It requires periodic updates to ensure routing information accuracy
•
It is under the same administrative control
•
It assumes a common routing policy
•
It provides limited policy control capability
Because of these characteristics, an IGP is not suitable to provide interdomain routing. For
example, an interdomain routing protocol should be able to provide extensive policy
control, because different domains often require different routing and administrative
policies. As another example, periodic refresh of IGP routes is not scalable when the
number of prefixes is at the Internet level.
From the start, BGP was designed to be an interdomain protocol. Two of the most important
design goals were policy control capability and scalability. However, BGP typically is not
suitable to replace an IGP because of its slower response to topology changes. When BGP
is used to provide intradomain reachability, such as in an MPLS VPN, BGP tunings are
often needed to reduce the convergence time.
Both IGP and BGP have their place. When designing networks, it is important to use both
types of protocols appropriately. A more detailed comparison of BGP and IGP is provided
in Chapter 2.
