The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  TCP/IP Routing Protocols (Gateway Protocols)                 9  TCP/IP Exterior Gateway/Routing Protocols (BGP and EGP)                      9  TCP/IP Border Gateway Protocol (BGP/BGP-4)                           9  BGP Fundamentals and General Operation

BGP Route Storage and Advertisement, and BGP Routing Information Bases (RIBs) 1 2 3 BGP Route Determination and the BGP Decision Process

BGP Path Attribute Classes

The four formal classifications of path attributes are:

• Well-Known Mandatory: These are the most important path attributes; they must be included in every route description in Update messages, and must be processed by each BGP device receiving them.
  
  
• Well-Known Discretionary: These path attributes must be recognized by a BGP device if received, but they may or may not be included in an Update message. Thus, they are optional for a sender of information, but mandatory for a receiver to process.
  
  
• Optional Transitive: These path attributes may be recognized by a BGP router and may be included in an Update message. They must be passed on to other BGP speakers when the route is advertised, even if received by a device that does not recognize the attribute.
  
  
• Optional Non-Transitive: Optional attributes that may be recognized by a BGP device and may be included in an Update message. If received by a device that does not recognize the attribute, it is dropped and not passed on to the next router.

BGP Route Storage and Advertisement, and BGP Routing Information Bases (RIBs) 1 2 3 BGP Route Determination and the BGP Decision Process