Quick facts on BGP for interviews

Quick facts on BGP for interviews

BGP is an external Gateway protocol used on the internet to ease the exchange of routing information between various autonomous systems (AS). It is the protocol that supports global internet traffic routing. BGP is a route-vector protocol, which means it takes into account a variety of characteristics while determining the optimum way for data to transit across networks.

Fig 1.1- BGP

  • BGP uses TCP Port 179.
  • BGP uses path attributes (path vector protocol) to find best paths.
  • BGP has two versions: eBGP: AD of 20. Used to form adjacencies and advertise routes between different autonomous systems (AS). iBGP: AD of 200. Used to form adjacencies and advertise routes within an autonomous system
  • BGP Synchronization Rule: A route must be known by an Interior Gateway Protocol (IGP) before it is advertised to BGP peers
  • BGP uses 2 ranges of Autonomous System Numbers (ASNs), depending on whether you are using the 16bit ASN field or the newer 32bit ASN field. 0-65,535 (of which 64,512 – 65,535 are private ASNs). and 0-4,294,967,294 (of which 4,200,000,000 – 4,294,967,294 are private ASNs).
  • BGP’s loop prevention mechanism is the AS_PATH attribute: if an AS receives a route that includes its own ASN, it is discarded.
  • The Weight attribute is a Cisco proprietary attribute that is used only on the local router for path selection. (For locally originated routes= 32768, else=0.)

Path Vector attributes

  • Well-Known Mandatory 
    Origin: (IGP, EGP or ?)
    AS Path: List of autonomous systems (read Right to Left) the route has passed through.
    Next Hop: External peer in neighboring AS.
  • Well-Known Discretionary
    Local Preference: Indicates a preference for exiting the AS to the destination network prefix. Default=100.
    Atomic Aggregate: Includes ASNs which have been dropped due to route aggregation. Path attribute information may have been lost.
  • Optional Transitive
    Aggregator: ID and AS of summarizing router.
    Community: Route tag.
  • Optional Non-Transitive
    Multi-Exit Discriminator (MED): Metric used for ‘trying’ to suggest to external peers to use certain path to routes.
    Originator ID: The originator of a reflected route.
    Cluster List: List of cluster IDs.
    Cluster ID: Originating Cluster.