BGP Attributes

Attributes are one of the most important things concerning BGP. With attributes almost everything can be controlled.

There are diefferent types of attributes:

  • Welll-known-attributes
    • mandatory
      • Origin
      • AS_Path
      • Next_Hop
    • discretionary
      • Local_Preference
      • Atomic_Aggregate
  • Operational-attributes
    • transitiv
      • Aggregator
      • Community
    • non-transitiv
      • Multi_Exit_Descriminator
  • Propriatary-attributes
      • Weight (Cisco specific)

Description of Attributes:

      Origin

      • Type Code 1
      • Well known, mandatory
      • Identifies the route was originally learned from by BGP
        • 0 – by an IGP
        • 1 – by an EGP
        • 2- not complete /unrelaiable

      AS_Path

      • Type Code 2
      • Well known, mandatory
      • Lists ASs between local and destination

      Next_Hop

      • Well known, mandatory
      • Type Code 3
      • Identifies next hop to destination
      • Next_Hop = IP address of peer

      MED – Multi-Exit Discremitnator (MED)

      • Type Code 4
      • Optional, non-transitiv
      • AKA “metric”
      • Indicats preference of one route into an AS over another
      • Only shared with neighboring AS’s
      • Lowest MED is preferred

      Here in this example the MED 100 is preferred as the host with the better Path.

      BGP MED

      MED ist configured with the “default-metric [number]

      Local_Preference

      • Type Code 5
      • Well known, discretionary
      • Indicates preference of one route over another
      • Within AS only
      • Higher Local_Preferance preferred
      • Cisco’s default is 100
      • It’s setup with a route-map

      If there are two path’s from an AS to another and you can setup the local preference for your AS, you can set the Local_Preference to a lower value to become the one route more preferable. So this can be used to optimize the way routers communicate.

      Atomic_Aggregate

      • Type Code 6
      • Well known, discretionary
      • Automatically sent when two overlapping routes are received
        • Less specific route is chosen
        • More specific route is not chosen

      Aggregator

      • Type Code 7
      • Optional and transitiv
      • Any router that aggregates routes can use this attribute
      • Attribute contains
        • Aggregators AS number
        • Aggregators IP address

      Community

      • Definded in RFC 1997
      • Type Code 8
      • Optional and transitiv
      • Allows for grouping of destinations
      • Apply policies to the group
      • Transitiv but dropped by default
      • Predefined communities
        • No_Export – Do not advertise to EBGP peers
        • No_Advertise – Advertise to no one
        • Internet – Advertise to everyone

      Weight

      • Cisco defined
      • Non-transitiv
      • Wight assigned on peer-by-peer basis
      • Integer value (0-65535)
      • Higher values are preferred
      • Default is 32768 for originated local routes
      • Default for other routes is 0
      • BGP config command: neighbor [ip] weight [weight]
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