Figure 19. BGP Router Rules
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 reect 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.
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 inuence 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 specic 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 dierent 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.
Border Gateway Protocol IPv4 (BGPv4)
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