Border Gateway Protocol (BGP) is a critical routing protocol that enables the exchange of routing information between different autonomous systems (ASes) on the internet. When multiple paths are available to reach a destination, BGP relies on a set of well-defined path selection rules to determine the optimal path. This article aims to provide an overview of the BGP path selection rules, shedding light on the factors that influence route selection.
Highest Local Preference:
BGP first considers the local preference attribute (LOCAL_PREF) to determine the preferred route. Local preference is an attribute assigned within an autonomous system (AS) and reflects the preference for a specific route. A higher local preference value indicates a more preferred path.
Locally Originated Routes:
If multiple routes have the same local preference, BGP gives preference to locally originated routes. Locally originated routes are those that have been originated within the local AS and advertised to neighboring ASes.
Shortest AS Path:
If multiple routes have the same local preference and originate locally, BGP selects the route with the shortest AS_PATH attribute. The AS_PATH attribute represents the sequence of autonomous systems that the route has traversed. Shorter AS paths are generally preferred as they indicate fewer network hops.
Lowest Origin Type:
If multiple routes have the same local preference, originate locally, and have the same AS path length, BGP compares the origin type attribute (ORIGIN). The origin type can be one of three values: IGP (Interior Gateway Protocol), EGP (Exterior Gateway Protocol), or incomplete. BGP prefers routes with a lower origin type value.
Lowest Multi-Exit Discriminator (MED):
In cases where multiple paths to the destination have the same local preference, origin type, and AS path length, BGP considers the Multi-Exit Discriminator (MED) attribute. MED is an optional attribute that reflects the preferred path from an adjacent AS. A lower MED value indicates a more preferred path.
eBGP over iBGP:
If all the aforementioned attributes are the same, BGP favors paths learned via external BGP (eBGP) over paths learned via internal BGP (iBGP). This rule ensures that routing decisions are based on paths learned from neighboring ASes rather than within the same AS.
Lowest IGP Metric to Next-Hop:
If none of the above rules result in a definitive decision, BGP compares the Interior Gateway Protocol (IGP) metric to the next-hop IP address. The IGP metric represents the cost associated with reaching the next-hop router. BGP selects the route with the lowest IGP metric.
Border Gateway Protocol (BGP) path selection rules play a crucial role in determining the optimal route for internet traffic. By considering factors such as local preference, AS path length, origin type, MED, and the type of BGP session, BGP ensures efficient and reliable routing decisions across autonomous systems. Understanding these rules is essential for network engineers and administrators to effectively manage BGP routing and maintain robust internet connectivity.
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