BGP - The BGP Weight Attribute

BGP - The BGP Weight Attribute

The video titled “BGP - The BGP Weight Attribute” by Lazarus at telecomTech provides a clear and practical explanation of the BGP weight attribute, focusing on its role in BGP path selection and configuration on Cisco routers.

Key Technological Concepts and Features

• BGP Overview Border Gateway Protocol (BGP) is a path vector routing protocol used on the Internet and large enterprise networks. It selects the best path among multiple routes by evaluating a sequence of attributes in a predefined order.

• Weight Attribute

  • A Cisco proprietary attribute, not part of the official BGP standard but widely adopted.
  • It is local to the router where it is configured and not advertised to BGP peers.
  • Represented as a 32-bit value, with a default of 0 for routes learned from peers and 32768 for locally originated routes.
  • BGP prefers paths with higher weight values during best path selection.

• Best Path Selection Process Weight is the first attribute checked. If weights are equal, BGP examines other attributes like AS_Path, etc.

• Practical Example The video demonstrates a topology with two paths to the same destination (30.30.30.0/24) learned via two neighbors (R1 and R2). By default, BGP prefers the path with the shorter AS_Path (via R1). Using the weight attribute, the router is configured to assign a weight of 500 to all routes learned from R2, influencing BGP to prefer the R2 path despite the longer AS_Path. Commands shown include setting the weight on a neighbor and clearing BGP sessions (clear ip bgp *).

• Granular Weight Assignment Instead of applying weight to all routes from a neighbor, a route-map can be used to assign weight to specific routes only. This allows fine-tuned control over path selection.

• Use Cases and Benefits Weight is commonly used to influence outbound routing decisions on a local router. Because it is local and not propagated, it is a safe and easy method to control routing without impacting other routers.

Tutorials and Guides Provided

• Explanation of the BGP weight attribute concept.
• Step-by-step example of configuring weight on Cisco routers.
• Demonstration of how weight affects BGP best path selection.
• Mention of route-maps for more granular weight application (though detailed configuration is not shown).

Main Speaker

• Lazarus from telecomTech

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This summary captures the educational content focused on the BGP weight attribute, its configuration, and impact on routing decisions within Cisco-based BGP environments.