Dell C1048P Port Extender Networking Configuration Guide for the C9000 Series - Page 168

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 Route Reflection 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 20. 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 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 Border Gateway Protocol IPv4 (BGPv4) 168