BGP Path Selection

Author
Carole Warner Reece
Architect

I wanted to provide a few notes on BGP route selection criteria, specifically on weights and local preference. First a quick recap. The weight attribute is only local to a single router. This value is not propagated by the BGP protocol to other routers, so can be considered a routing policy local to the router.  However, local preference is assigned to a route as an attribute.

The local preference  value is carried by the BGP protocol with the route to all internal BGP sessions. With local preference, all other BGP speaking routers within the autonomous system (AS) receive the same information. Normally, a router assigns a local preference to a route that is received on an external BGP session before it is accepted and entered in the BGP table of the border router. Routers propagate the local preference attribute on internal BGP sessions only. This policy constitutes a routing policy for the entire AS.

The router can assign the weight attribute to a route in two ways:

    • All routes that are received from a specific neighbor can be assigned a default weight value. This weight value indicates that the neighbor is preferred over the other neighbors.
  • A route-map that is applied on incoming routes from a neighbor can be used to select some routes and assign them weight values. Remember that a route-map also acts as a filter and will silently drop the routes that are not permitted by any statement in the route-map.

If configured, the default weight assignment on routes that are received from a neighbor is applied first. All routes that are received from the neighbor are assigned a weight value as defined by the default weight.

Leave a Reply

 

Nick Kelly

Cybersecurity Engineer, Cisco

Nick has over 20 years of experience in Security Operations and Security Sales. He is an avid student of cybersecurity and regularly engages with the Infosec community at events like BSides, RVASec, Derbycon and more. The son of an FBI forensics director, Nick holds a B.S. in Criminal Justice and is one of Cisco’s Fire Jumper Elite members. When he’s not working, he writes cyberpunk and punches aliens on his Playstation.

 

Virgilio “BONG” dela Cruz Jr.

CCDP, CCNA V, CCNP, Cisco IPS Express Security for AM/EE
Field Solutions Architect, Tech Data

Virgilio “Bong” has sixteen years of professional experience in IT industry from academe, technical and customer support, pre-sales, post sales, project management, training and enablement. He has worked in Cisco Technical Assistance Center (TAC) as a member of the WAN and LAN Switching team. Bong now works for Tech Data as the Field Solutions Architect with a focus on Cisco Security and holds a few Cisco certifications including Fire Jumper Elite.

 

John Cavanaugh

CCIE #1066, CCDE #20070002, CCAr
Chief Technology Officer, Practice Lead Security Services, NetCraftsmen

John is our CTO and the practice lead for a talented team of consultants focused on designing and delivering scalable and secure infrastructure solutions to customers across multiple industry verticals and technologies. Previously he has held several positions including Executive Director/Chief Architect for Global Network Services at JPMorgan Chase. In that capacity, he led a team managing network architecture and services.  Prior to his role at JPMorgan Chase, John was a Distinguished Engineer at Cisco working across a number of verticals including Higher Education, Finance, Retail, Government, and Health Care.

He is an expert in working with groups to identify business needs, and align technology strategies to enable business strategies, building in agility and scalability to allow for future changes. John is experienced in the architecture and design of highly available, secure, network infrastructure and data centers, and has worked on projects worldwide. He has worked in both the business and regulatory environments for the design and deployment of complex IT infrastructures.