Figure 19. BGP Router Rules1. Router B receives an advertisement from Router A through eBGP. Because the route is learned through eBGP, Router Badvertises 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 iBGPpeer, 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 camefrom 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.BGP AttributesRoutes learned using BGP have associated properties that are used to determine the best route to a destination when multiple pathsexist to a particular destination.These properties are referred to as BGP attributes, and an understanding of how BGP attributes influence route selection is requiredfor 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 HopNOTE: There are no hard coded limits on the number of attributes that are supported in the BGP. Taking into accountother constraints such as the Packet Size, maximum number of attributes are supported in BGP.CommunitiesBGP communities are sets of routes with one or more common attributes. Communities are a way to assign common attributes tomultiple routes at the same time.NOTE: Duplicate communities are not rejected.Best Path Selection CriteriaPaths for active routes are grouped in ascending order according to their neighboring external AS number (BGP best path selectionis 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 criteriaresults in more than one path, BGP moves on to the next option in the list. For example, two paths may have the same weights, butdifferent local preferences. BGP sees that the Weight criteria results in two potential “best paths” and moves to local preference toBorder Gateway Protocol IPv4 (BGPv4) 151
