Concept Guide

Item Default
Distance
external distance = 20
internal distance = 200
local distance = 200
Timers
keepalive = 60 seconds
holdtime = 180 seconds
Add-path Disabled
BGP Attributes for selecting Best Path
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 inuence 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 specic 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 dierent 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 EMC Networking OS choosing dierent 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 EMC Networking OS compares MED between the adjacent paths within an AS group because all paths in the AS group are
from the same AS.
Border Gateway Protocol (BGP)
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