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1. Router B receives an advertisement from Router A through eBGP. Because the route is learned through eBGP, Router B
advertises it to all its iBGP peers: Routers C and D.
2. Router C receives the advertisement but does not advertise it to any peer because its only other peer is Router D, an iBGP
peer, and Router D has already learned it through iBGP from Router B.
3. Router D does not advertise the route to Router C because Router C is a nonclient peer and the route advertisement came
from Router B who is also a nonclient peer.
4. Router D does reflect the advertisement to Routers E and G because they are client peers of Router D.
5. Routers E and G then advertise this iBGP learned route to their eBGP peers Routers F and H.
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.
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.
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Border Gateway Protocol IPv4 (BGPv4)