Route reflection divides iBGP peers into two groups: client peers and nonclient peers. A route reflectorand its client peers form a route reflection cluster. Because BGP speakers announce only the best routefor a given prefix, route reflector rules are applied after the router makes its best path decision.• If a route was received from a nonclient peer, reflect the route to all client peers.• If the route was received from a client peer, reflect the route to all nonclient and all client peers.To illustrate how these rules affect routing, refer to the following illustration and the following steps.Routers B, C, D, E, and G are members of the same AS (AS100). These routers are also in the same RouteReflection Cluster, where Router D is the Route Reflector. Router E and H are client peers of Router D;Routers B and C and nonclient peers of Router D.Figure 19. BGP Router Rules1. Router B receives an advertisement from Router A through eBGP. Because the route is learnedthrough 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 peeris 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 theroute 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 RouterD.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 adestination when multiple paths exist to a particular destination.These properties are referred to as BGP attributes, and an understanding of how BGP attributes influenceroute selection is required for the design of robust networks. This section describes the attributes thatBGP uses in the route selection process:• Weight• Local Preference• Multi-Exit Discriminators (MEDs)• Origin• AS PathBorder Gateway Protocol IPv4 (BGPv4) 169
