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196 Chapter 6: IP Routing
Exterior Gateway Protocols (EGPs)
EGPs are used to connect autonomous systems (ASs). Here's an example of an EGP: Border
Gateway Protocol (BGP) is an Internet protocol that allows ASs to share routing information so
that efficient, loop-free routes can be established. An AS is a set of routers, typically under a
single technical administration, that uses an Interior Gateway Protocol and common metrics to
route packets within the AS. It uses an Exterior Gateway Protocol to route packets to other ASs.
BGP is commonly used within and between Internet service providers (ISPs). The BGP
protocol is defined in RFC 1771.
A Cisco IP routing table is based on several factors:
·
Static routing entries
·
Local interface configuration
·
Local interface status
·
Dynamic routing protocols and routing metrics
·
Redistribution between routing protocols
·
Policy decisions implemented with access lists
The following paragraphs provide some analysis of how these factors interact.
Three processes are involved in building and maintaining the routing table in a Cisco router:
·
Routing processes, which actually run a network (or routing) protocol, such as EIGRP,
BGP, IS-IS, and OSPF
·
The routing table itself, which accepts information from the routing processes and also
replies to requests for information from the forwarding process
·
The forwarding process, which requests information from the routing table to make a
packet-forwarding decision
Figure 6-8 depicts the router's build-and-maintain process. This process is described in the
following paragraphs.
Table 6-3
Categories of IGPs
Protocol
Update
Hello
Type
RIP
Routes
No
Distance-vector
IGRP
Routes
No
Distance-Vector
OSPF
Links
Yes
Link-state
EIGRP
Routes
Yes
Hybrid
87200333.book Page 196 Wednesday, August 22, 2001 2:37 PM