D-Link DGS-6600-48TS Configuration Guide - Page 293

Volume 4-Layer 3 Configurations / Chapter 30-Border Gateway Protocol (BGP) Chapter Overview Chapter 30 Border Gateway Protocol (BGP) Chapter Overview The following topics are included in this chapter, please go to the topic for more detailed information: • Chapter Overview • An Introduction to BGP • BGP Configuration commands • Configuration Examples • BGP Configuration Example An Introduction to BGP The purpose of the BGP routing protocol is to provide loop-free routing between autonomous systems. An autonomous system is a routing domain that contains independent routing policies. In order to make the operation of BGP more reliable, BGP uses the TCP protocol as it is connection-oriented. BGP uses port 179 as the destination port and a random port number for the source port. The Switch supports BGP version 4, which is the same version that Internet Service Providers used when building the Internet. Several new BGP features were introduced in RFC 1771, which allowed BGP to meet the demands for Internet connectivity. One of the main uses for BGP is when a user needs to connect their local network to an external network in order to access the Internet or the networks of other external organizations. Whenever a connection is made to an external organization, an external BGP (eBGP) peering session is made. Despite the fact that BGP was originally designed to work as an exterior gateway protocol (EGP), the increased complexity of internal networks has led to many organizations deploying BGP internally as it actually simplifies their internal network. In order for routing information to be exchanged between peers in the same organization, internal BGP (iBGP) peering sessions need to be established. BGP exchanges network reachability information with other devices that are running BGP by using a path-vector routing algorithm. Routing updates are used to exchange information about network reachability between BGP peers. The information contained within the network reachability consists of the network number, attributes that are specific to a path, and a list that details the autonomous system numbers that must be traversed by the route in order to reach the destination network. The AS-path attribute of BGP contains the list of autonomous system numbers that are appended to an update as it traverses across networks. In order to prevent routing loops, BGP rejects any routing updates that contain a local autonomous system number. If a BGP routing update has a local autonomous system then a loop will be created since the route update has previously travelled through the local autonomous system. The distance-vector algorithm and the AS-path loop detection mechanism combined create the BGP path-vector algorithm. By default, only a single-path to a destination host or network is selected as the best path by BGP. The algorithm that BGP uses to select the best path analyzes all the attributes of each path to identify the route that has been designated as the best path in the routing table of BGP. Several attributes are used to analyze the best BGP path. The attributes that are carried by each BGP path are the well-known mandatory, well-known discretionary, and optional transitive types. The user can use the Switch's CLI to alter some of the attributes to influence the way BGP chooses a path. Although it is also possible to configure the standard BGP algorithm that controls the path selection DGS-6600 Configuration Guide 293