Troubleshooting guide

BGP Overview

BGP Neighbors and Messages

Unlike other routing protocols, BGP does not automatically discover its neighbors. The transport medium

for BGP is Transmission Control Protocol (TCP), port 179. TCP is a connection-oriented protocol; thus,

providing an extra layer of reliability for BGP communication.

BGP neighbors must be manually configured. After the three-way TCP handshake has been established

between two BGP neighbors, a peering session is established through an open message that contains a hold

time and BGP router ID. During the exchange of the open message, a router will decide if their neighbor is

in the same AS (iBGP) or a different AS (eBGP). Once a session is established, keepalive messages are

periodically exchanged between the peers to maintain connections and verify paths held by the router

sending the keepalive.

BGP routers send update messages to their neighbors whenever a path changes. There is only one path per

update. Each update message contains information on the path to a destination network and the path

attributes, such as origin, AS path, and neighbor. Routes that are no longer available or withdrawn routes

are included in updates.

Notification messages are sent when an error has been received. The BGP connection is closed after the

notification message has been sent.

BGP Attributes

BGP attributes are properties that are used to determine the best route to a destination when multiple paths

exist to a single destination. An understanding of how BGP attributes influence route selection is important

when designing networks.

The following BGP attributes are supported by AOS:

• LOCAL_PREF

• MULTI_EXIT_DISC or MED

•Origin

• AS_PATH

• NEXT_HOP

• Community

Local Preference

The local preference (LOCAL_PREF) attribute is used to choose a path when there are multiple exit points

from the local AS. Adjusting the local preference value can affect the local router’s decision, as well as the

decision of other iBGP routers in the same AS when determining the best route to a destination.

The higher the local preference, the more desirable a route becomes.