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

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 are 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 the system 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, the
system compares MED between the adjacent paths within an AS group because all paths in the AS group
are from the same AS.
NOTE: In the Dell Networking OS version 8.3.11.4, the bgp bestpath as-path multipath-
relax command is disabled by default, preventing BGP from load-balancing a learned route across
two or more eBGP peers. To enable load-balancing across different eBGP peers, enable the bgp
bestpath as-path multipath-relax command. A system error results if you configure the
bgp bestpath as-path ignore command and the bgp bestpath as-path multipath-
relax command at the same time. Only enable one command at a time.
The following illustration shows that the decisions BGP goes through to select the best path. The list
following the illustration details the path selection criteria.
180 Border Gateway Protocol IPv4 (BGPv4)