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PLANET CS-2001 User Manual

Page 41

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Note:

1.

Dynamic Routing Protocols can be categoried into the following two categories:

Distance-Vector Routing Protocol: Uses the Bellman-Ford algorithm to

calculate paths. Examples of distance-vector routing protocols include

RIPv1/2 and IGRP (Cisco's proprietary protocol). Using RIP, the maximum

hop count from the first router to the destination is 15. Any destination greater

than 15 hops away is considered unreachable. A distance-vector routing

protocol requires that a router informs its neighbors of topology changes

periodically and, in some cases, when a change is detected in the topology of a

network. Its suitable for networks containing fewer than 100 hops (routers), or

small networks that are not changed frequently.

Link state routing protocols use the Dijkstra algorthim to calculate the Shortest

Path First (SPF) address some of the limitations of distance vector routing

protocols. For example, link state routing protocols provide faster convergence

than do distance vector routing protocols and have no limit to the amount of

hops. VLSM and CIDR are also supported. Although link state routing

protocols are more reliable and require less bandwidth than do distance vector

routing protocols, they are also more complex, more memory-intensive, and

place a greater load on the CPU.

2.

Dynamic routing protocols depend on the location of the router in the Autonomous

Sytstem (AS). There are two classifications:

IGP - Interior Gateway Protocol: Examples include: RIP, IGRP, EIGRP, OSPF

and IS-IS are interior gateway protocols.

EGP - Exterior Gateway Protocol: Uses a simple tree topology, but posses

some shortcomings that the Border Gateway Protocol (BGP) has fixed.

3.

Within the Internet, an autonomous system (AS) is a collection of connected

Internet Protocol (IP) routing prefixes under the control of one or more network

operators that presents a common, clearly defined routing policy to the Internet.

Multiple organizations can run BGP using private AS numbers to an ISP that

connects all those organizations to the Internet. Even though there are multiple

Autonomous Systems supported by the ISP, the Internet only sees the routing

policy of the ISP. That ISP must have an officially registered Autonomous System

Number (ASN). A unique ASN is allocated to each AS for use in BGP routing. AS

numbers are important because the ASN uniquely identifies each network on the

Internet.

Until 2007, AS numbers were defined as 16-bit integers, which allowed for a

maximum of 65536 assignments. The Internet Assigned Numbers Authority

(IANA) has designated ASN numbers 64512 through 65534 to be used for