Dell S4820T, Configuration Manual

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BGP Attributes
Routes learned using BGP have associated properties that are used to determine the best route to a
destination when multiple paths exist to a particular destination.
These properties are referred to as BGP attributes, and an understanding of how BGP attributes influence
route selection is required for the design of robust networks. This section describes the attributes that
BGP uses in the route selection process:
• Weight
• Local Preference
• Multi-Exit Discriminators (MEDs)
• Origin
• AS Path
• Next Hop
NOTE: There are no hard coded limits on the number of attributes that are supported in the BGP.
Taking into account other constraints such as the Packet Size, maximum number of attributes are
supported in BGP.
Communities
BGP communities are sets of routes with one or more common attributes. Communities are a way to
assign common attributes to multiple routes at the same time.
NOTE: Duplicate communities are not rejected.
Best Path Selection Criteria
Paths for active routes are grouped in ascending order according to their neighboring external AS
number (BGP best path selection is deterministic by default, which means the
bgp non-
deterministic-med
command is NOT applied).
The best path in each group is selected based on specific criteria. Only one “best path” is selected at a
time. If any of the criteria results in more than one path, BGP moves on to the next option in the list. For
example, two paths may have the same weights, but different local preferences. BGP sees that the Weight
criteria results in two potential “best paths” and moves to local preference to reduce the options. If a
number of best paths is determined, this selection criteria is applied to group’s best to determine the
ultimate best path.
In non-deterministic mode (the
bgp non-deterministic-med
command is applied), paths are
compared in the order in which they arrive. This method can lead to Dell Networking OS choosing
different best paths from a set of paths, depending on the order in which they were received from the
neighbors because MED may or may not get compared between the adjacent paths. In deterministic
mode, Dell Networking OS compares MED between the adjacent paths within an AS group because all
paths in the AS group are from the same AS.
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Border Gateway Protocol IPv4 (BGPv4)