BGP全系列.ppt

BGP Basics Reasons for Using BGP You need to scale your IGP You’re multihomed and need to implement routing policy You need to transit full Internet routes Interior vs. Exterior Routing Interior Automatic discovery Generally trust your IGP routers Routes go to all IGP routers Exterior Specifically configured peers Connecting with outside networks Set administrative boundaries Why Do We Need an EGP? Scaling a large network—“Divide and Conquer” Hierarchy Periodic IGPs/flooding Isolate network instability Complex policies Control reachability to prefixes Merge separate organizations Connect multiple IGPs Concept of Autonomous System A network(s) sharing the same routing policy Possibly multiple IGPs Usually under single administrative control Contiguous internal connectivity Numbering range form 1 to 65,535—Globally unique—“AS Number” Private range: 64512–65535 IGP of Each AS Is Hidden You Need to Transit Internet Routes (i.e., You Are an ISP) Basic to Basics Runs over TCP—Port 179 Path vector protocol Incremental updates “Internal” and “External” BGP General Operation Learns multiple paths via internal and external BGP speakers Picks the most preferred path, installs itin the IP forwarding table, forwards to BGP neighbors Policies applied by influencing the bestpath selection Summary of Operation TCP connection established (port 179) Both peers attempt to connect—There is an algorithm to resolve “connection collisions” Exchange messages to open and confirm the connection parameters Initial exchange of entire table Incremental updates after initial exchange Keepalive messages exchanged when there are no updates What Are Incremental Updates? IGPs typically rebroadcast routes BGP runs over TCP = reliable date delivery Once BGP sends a route to a peer, it assumes the peer will keep it unless: A replacement route is sent—Implicit withdraw of old route The route is withdrawn—Explicit withdraw The BGP session goes down (keepalive failure) BGP Sessions—TCP Port 179,