Rate and give feedback:  Feedback Received. Thank You!

Rate and give feedback: 

X

This document helped resolve my issue

Yes No

Additional Comments

800 characters remaining

May we contact you if necessary?

Name:  

E-mail: 

Enter a valid Email ID

Need product assistance? Contact Juniper Support

Submit

This site is protected by hCaptcha and its Privacy Policy and Terms of Service apply.

English  

English

Understanding BGP Route Reflectors

Because of the internal BGP (IBGP) full-mesh requirement, most networks use route reflectors to simplify configuration. The formula to compute the number of sessions required for a full mesh is v * (v - 1)/2, where v is the number of BGP-enabled devices. The full-mesh model does not scale well. Using a route reflector, you group routers into clusters, which are identified by numeric identifiers unique to the autonomous system (AS). Within the cluster, you must configure a BGP session from a single router (the route reflector) to each internal peer. With this configuration, the IBGP full-mesh requirement is met.

To use route reflection in an AS, you designate one or more routers as a route reflector—typically, one per point of presence (POP). Route reflectors have the special BGP ability to readvertise routes learned from an internal peer to other internal peers. So rather than requiring all internal peers to be fully meshed with each other, route reflection requires only that the route reflector be fully meshed with all internal peers. The route reflector and all of its internal peers form a cluster, as shown in Figure 1.

Note: For some Juniper Networks devices, you must have an Advanced BGP Feature license installed on each device that uses a route reflector. For license details, see the Junos OS Initial Configuration Guide for Security Devices.

Figure 1: Simple Route Reflector Topology (One Cluster)

Figure 1 shows Router RR configured as the route reflector for Cluster 127. The other routers are designated internal peers within the cluster. BGP routes are advertised to Router RR by any of the internal peers. RR then readvertises those routes to all other peers within the cluster.

You can configure multiple clusters and link them by configuring a full mesh of route reflectors (see Figure 2).

Figure 2: Basic Route Reflection (Multiple Clusters)

Figure 2 shows Route Reflectors RR 1, RR 2, RR 3, and RR 4 as fully meshed internal peers. When a router advertises a route to RR 1, RR 1 readvertises the route to the other route reflectors, which, in turn, readvertise the route to the remaining routers within the AS. Route reflection allows the route to be propagated throughout the AS without the scaling problems created by the full mesh requirement.

However, as clusters become large, a full mesh with a route reflector becomes difficult to scale, as does a full mesh between route reflectors. To help offset this problem, you can group clusters of routers together into clusters of clusters for hierarchical route reflection (see Figure 3).

Figure 3: Hierarchical Route Reflection (Clusters of Clusters)

Figure 3 shows RR 2, RR 3, and RR 4 as the route reflectors for Clusters 127, 19, and 45, respectively. Rather than fully mesh those route reflectors, the network administrator has configured them as part of another cluster (Cluster 6) for which RR 1 is the route reflector. When a router advertises a route to RR 2, RR 2 readvertises the route to all the routers within its own cluster, and then readvertises the route to RR 1. RR 1 readvertises the route to the routers in its cluster, and those routers propagate the route down through their clusters.