Route Reflector (RR)

What Is an RR?

A route reflector (RR), acting as a specialized IBGP router, can be implemented within a large-scale BGP network to address the issue of maintaining full-mesh IBGP connections. It can store routing entries for the entire network and efficiently distribute routing information to other IBGP routers.

Why Is an RR Needed?

In a conventional IBGP setup, to circumvent routing loops, an IBGP device refrains from advertising routes learned from one IBGP peer to others within the IBGP group. To maintain seamless connectivity between IBGP peers, it becomes essential to establish logical full-mesh connections among them.

In compliance with IBGP requirements, R2 refrains from advertising routes learned from R1 to R3. To facilitate bidirectional route advertisement between R1 and R3, an IBGP connection (illustrated as a dotted line) must be established between them, enabling R1 and R3 to function as IBGP peers. Similarly, for seamless connectivity among IBGP peers within the same autonomous system (AS), IBGP connections are also necessary between R1 and R4, as well as between R3 and R4. In an AS with N IBGP devices, the total number of required IBGP connections is calculated as N x (N–1)/2. However, in scenarios involving a large number of devices, this can lead to significant consumption of network and CPU resources. To mitigate this challenge, the use of a route reflector (RR) is advocated. The RR reduces the number of established connections to N–1, effectively conserving both network and device CPU resources.

Working Principles of an RR

In Figure 1-3, the roles within an AS with a deployed RR are as follows:

• RR: A BGP device tasked with reflecting routes learned from an IBGP peer to other IBGP peers within the network.

• Client: An IBGP device whose routes are reflected by the RR to other IBGP devices, facilitating streamlined routing within the network.

• Cluster: A grouping comprising the RR and its associated clients, representing a cohesive unit within the network architecture.

In a clustered setup, routing information exchange among clients exclusively occurs with the RR within the cluster. As a result, clients establish IBGP connections solely with the RR, effectively reducing the number of IBGP connections within the cluster. Illustrated in the previous diagram, within AS65000, R2 operates as the RR, with R1, R3, and R4 serving as clients, collectively forming Cluster1. For instance, R1 can effortlessly access routing information from R3 and R4 via the RR, with the same streamlined process applying to R3 and R4. Consequently, the number of IBGP connections within AS65000 is halved, decreasing from 6 to 3. This simplifies device configurations and alleviates network and CPU loads.

Furthermore, the RR functions to propagate routes learned from a client to all other clients within the cluster. This mechanism prevents an IBGP device from disseminating routes learned from one IBGP peer to other IBGP peers, ensuring network stability and integrity.

RR Application Scenarios

As BGP continues to evolve, its adoption has become widespread, with route reflectors (RRs) increasingly utilized across various scenarios, including software-defined wide area network (SD-WAN) implementations.

In Figure 1-4, the RR serves as an integral component of the SD-WAN control layer, tasked with overseeing and transmitting VPN routes and topology information throughout the network. Collaborating closely with the controller, the RR facilitates the dissemination of VPN routes and topology details across sites in accordance with user-defined policies. This collaborative effort enables the establishment of on-demand interconnections between sites, thereby enhancing the flexibility and efficiency of the SD-WAN infrastructure.

Below outlines the roles and responsibilities of the Route Reflector (RR) within an SD-WAN network.

1. 1. Devices located at the three sites initiate registration with the controller to establish management channels. Subsequently, the controller assigns a Route-Reflector (RR) to each device situated at Site1 and Site2 via the management channels. Notably, the RRs designated for Site 1 and Site 2 are affiliated with Site 3, acting as the designated RR site.

2. 2. Site1 and Site2 receive RR site information via the management channels and establish connections with the RR site accordingly.

3. 3. Upon connection establishment with the RRs, Site1 and Site2 proceed to advertise their respective routing information to the RRs. Concurrently, the RRs disseminate their routing information to Site1 and Site2. Subsequently, a control channel, commonly referred to as a BGP EVPN connection, is established between both ends.

4. 4. BGP peer relationships are established between the RR site and Site1, as well as between the RR site and Site2. Should the controller deem it necessary to establish a data channel between Site1 and Site2, it can instruct an RR to reflect routing information received from Site1 to Site2 and vice versa. This facilitates the mutual exchange of routing information between Site1 and Site2, enabling the establishment of a data tunnel between them. Consequently, when Site1 advertises a new route, Site2 can seamlessly learn about it through the RR.

5. 5. In SD-WAN scenarios, the RR mirrors routes acquired from a client to all other clients within the network. Consequently, the controller holds the capability to direct the RR to reflect routes to designated non-RR sites. For instance, to impede the establishment of a data channel between Site1 and Site2, the controller may adjust the networking topology to prevent the RR from reflecting routing information from Site1 to Site2. This strategic modification ensures precise control over the routing paths within the SD-WAN environment.

In conclusion, despite the expansion of BGP protocols and the increasing volume of routing information it handles, the role of a Route-Reflector (RR) remains pivotal as a centralized hub within a domain, reflecting routing information received from one device to others. This mechanism effectively mitigates network management and transmission expenses, enhances network management efficiency, and ensures robust network stability.