Bgp path selection criteria

BGP Path Selection

Weight. Prefer the route with the highest Weight. ...
Local Preference. If multiple routes have the same weight, prefer the route with the highest local preference. ...
Origin. ...
AS-Path. ...
Origin Code. ...
MED (Multi-Exit Discriminator) ...
eBGP over iBGP Path. ...
Shortest IGP Path.

How does BGP determine best path?
What is the criteria in path selection in router?
What is weight in BGP path selection?
What attributes determine route and best path selection?
What are BGP path attributes?
What is the golden rule of BGP?
What does the route 0.0 0.0 0.0 0.0 mean?
How to manipulate traffic in BGP?
What is path selection?
What is weight vs local preference?
What is the difference between BGP weight and local preference?
What is the difference between med and weight in BGP?
How do routing protocols determine the best path?
Does BGP advertise only best routes?
How does BGP influence routes?
How does BGP detect looping paths?
Which layer is responsible for selecting the best path?

How does BGP determine best path?

If there are no specific settings that can affect the outcome, BGP Best Path Selection Algorithm determines the best route by selecting the shortest path to the destination. An Autonomous System is a single network or a set of networks and routers, which are under the control of one administrative entity.

What is the criteria in path selection in router?

The best path is selected by a routing protocol based on the value or metric it uses to determine the distance to reach a network. A metric is the quantitative value used to measure the distance to a given network. The best path to a network is the path with the lowest metric.

What is weight in BGP path selection?

The BGP Weight Attribute

Weight is a value that is assigned to our prefixes as a locally significant value. Weight is a simple number in the range of 0 through 65535, and the higher the weight value, the higher the preference for that path. When the prefix is locally generated, it will get a weight of 32768.

What attributes determine route and best path selection?

The three attributes for Route Preference are the Route Specificity, the Administrative Distance, and the Metric. If multiple routes exist and all three of these attributes are identical, the Router will load balance across the available paths.

What are BGP path attributes?

A path attribute is a list of AS numbers that a route has traversed to reach a destination. BGP uses path attributes to provide more information about each route and to help prevent routing loops in an arbitrary topology. You can also use path attributes to determine administrative preferences.

What is the golden rule of BGP?

The golden rule of BGP

You have control over your AS and you can influence routing in your AS, thats it. You cannot ultimately influence other ASes.

What does the route 0.0 0.0 0.0 0.0 mean?

In IPv4-based routing, 0.0. 0.0 serves as a default route. This means no particular address has been designated in the routing table as the next hop in the packet's path to its final destination.

How to manipulate traffic in BGP?

To influence the inbound traffic path, customers can use certain attributes (such as MED, AS-PATH, BGP communities) in the updates sent to their providers. Another method is based on the longest prefix-matching behavior and can be accomplished by the BGP conditional route injection.

What is path selection?

Path selection ensures the right traffic travels the right path, by choosing a predefined WAN gateway for certain traffic flows in real time, based on availability. You define a path, called an uplink, by specifying a WAN egress point and providing a direction for the egressing packets to take.

What is weight vs local preference?

The weight attribute is not advertised to neighboring routers. If the router learns about more than one route to the same destination, the route with the highest weight will be preferred. The local preference attribute is used to prefer an exit point from the local autonomous system (AS).

What is the difference between BGP weight and local preference?

BGP local preference vs Weight

This means if you have a BGP Policy, the Weight attribute value is not exchanged/agreed between the routers but BGP Local Preference is an AS-wide attribute, thus it is exchanged/agreed by every BGP speaking device in the Internal BGP network.

What is the difference between med and weight in BGP?

Weight is used to influence traffic outbound from your AS ie. you apply weight to routes you receive from other ASs. MED is used to influence traffic inbound to your AS ie. you apply MED to routes you advertise to a neighboring AS.

How do routing protocols determine the best path?

Each of these metric calculation mechanisms are specific to the routing protocol. According to the metric mechanism of a routing protocol, each path has a metric. Between these paths, the path that has lower metric, is selected as the best route.

Does BGP advertise only best routes?

BGP routers only advertise the best path to their neighbors. When a better path is found, it replaces the current path. Advertising a path and replacing it with a new path is called an implicit withdraw. Since we only advertise the best path, a lot of other possible paths are unknown to some of the routers.

How does BGP influence routes?

How does BGP detect looping paths?

Based on RFC 4271, BGP AS loop detection is done by scanning the full AS path (as specified in the AS_PATH attribute), and checking that the autonomous system number of the local system does not appear in the AS path.

Which layer is responsible for selecting the best path?

The Layer 3, or Network Layer, is responsible for finding the right path for the data packet to reach its destination based on Logical Addresses (means addresses not really present on the network node).