Bgp Overview - D-Link DI-1750 Reference Manual

6.12 Configure BGP Task List

6.12.1 BGP Overview

This chapter describes how to configure Border Gateway Protocol (BGP). For a complete description of
the BGP commands in this chapter, refer to the "BGP Commands" chapter.
The Border Gateway Protocol, as defined in RFCs 1163, 1267 and 1771, is an Exterior Gateway
Protocol (EGP). It allows you to set up an interdomain routing system that provides the loop-free
exchange of routing information between autonomous systems. This section will describe following
contents:
6.12.2 D-Link BGP Implementation
In BGP, each route consists of a reachable destination (network or prefix), a list of autonomous systems
that information has passed through (called the autonomous system path), and a list of other path
attributes. D-Link supports RFC1771 defined BGP Versions 4.
The primary function of a BGP system is to exchange network reachability information with other BGP
systems, including information about the list of autonomous system paths. This information can be
used to construct a graph of autonomous system connectivity from which routing loops can be pruned
and with which autonomous system-level policy decisions can be enforced. BGP Version 4 supports
classless interdomain routing (CIDR), which lets you reduce the size of your routing tables by creating
aggregate routes, resulting in supernets. CIDR eliminates the concept of network classes within BGP
and supports the advertising of IP prefixes. CIDR routes can be carried by Open Shortest Path First
(OSPF), Enhanced IGRP (EIGRP), and Intermediate System-to-Intermediate System (ISIS)-IP, and
Routing Information Protocol Version 2(RIP2).
The external gateway route provides better control capability than internal gateway route. It is also the
main distinguish of external and internal gateway route. The BGP implemetation offers multiple optional
methods for the route control:
To filter route, you can use access-list, aspath-list and prefix-list based on neighbor, you can also use
access-list and prefix-list based on port.
To change the route attribute, you can use route-map to change the BGP route attributes, such as MED,
Local Preference, weight, etc.
To communicate with interior dynamic routing protocol (OSPF, RIP, etc), you can use redistribute to
automatically generate the BGP routing information. You can also manually configure network and
aggregate to generate the BGP route. At the time of generating BGP route, you can configure the route
attribute with route-map.
To control the priority of a BGP route in the system, you can configure the administrate distance of the
BGP route with the command distance.
6.12.3 How BGP Select s Paths
BGP bases its decision process on the attribute values. When faced with multiple routes to the same
destination, BGP chooses the best route for routing traffic toward the destination. The following process
summarizes how BGP chooses the best route:
(1) If the next hop is inaccessible, do not consider it.
(2) If the path is internal, synchronization is enabled, and the route is not in the IGP, do not consider
the route.
(3) Prefer the path with the largest weight (weight is a D-Link proprietary parameter).
(4) If the routes have the same weight, prefer the route with the largest local preference.
(5) If the routes have the same local preference, prefer the route that was originated by the local
router. For example, a route might be originated by the local router using the network bgp router
configuration command, or through redistribution from an IGP.
(6) If the local preference is the same, or if no route was originated by the local router, prefer the
route with the shortest autonomous system path.
Model Name
- 245 -