A Deep Dive into Route Reflectors in BGP

Introduction to Route Reflectors

The Border Gateway Protocol (BGP) is the routing protocol that is used to exchange routing information between autonomous systems (ASes) on the internet. BGP has two main flavors: eBGP and iBGP. eBGP is used between ASes, and iBGP is used within an AS.

The iBGP split-horizon rule states that a BGP router will not advertise a route that it has learned from an iBGP peer to another iBGP peer. This rule is used to prevent routing loops within an AS. However, it also means that all iBGP routers in an AS must be fully meshed. This can be a problem in large ASes, as the number of iBGP sessions can become very large.

Route reflectors are a solution to this problem. A route reflector is a BGP router that is allowed to advertise a route that it has learned from an iBGP peer to another iBGP peer. This breaks the iBGP split-horizon rule, but it also means that not all iBGP routers in an AS need to be fully meshed.

This article will provide a technical deep dive into BGP route reflectors, exploring how they work, how they are configured, and the benefits that they provide.

How Route Reflectors Work

A route reflector divides an AS into a number of clusters. Each cluster has a route reflector, and all of the other iBGP routers in the cluster are clients of the route reflector. The clients only need to peer with the route reflector. They do not need to peer with each other.

When a route reflector receives a route from a client, it advertises the route to all of the other clients in the cluster. It also advertises the route to all of the iBGP peers that are not in the cluster.

When a route reflector receives a route from an iBGP peer that is not in the cluster, it advertises the route to all of the clients in the cluster.

Route Reflector Attributes

Route reflectors use two new BGP attributes to prevent routing loops:

ORIGINATOR_ID: The ORIGINATOR_ID attribute is the router ID of the router that originated the route. A route reflector will not advertise a route back to the originator of the route.
CLUSTER_LIST: The CLUSTER_LIST attribute is a list of the cluster IDs of the route reflectors that a route has traversed. A route reflector will not advertise a route to a cluster if the cluster ID of the cluster is already in the CLUSTER_LIST.

Configuring Route Reflectors

Route reflectors are configured on a BGP router using the neighbor route-reflector-client command. The following is an example of how to configure a route reflector:

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router bgp 65000
 neighbor 10.0.0.1 remote-as 65000
 neighbor 10.0.0.1 route-reflector-client

This configuration will configure the local router to be a route reflector for the neighbor at 10.0.0.1.

Benefits of Route Reflectors

Route reflectors provide a number of benefits, including:

Scalability: Route reflectors allow iBGP networks to be scaled to a very large size.
Simplicity: Route reflectors simplify the configuration of iBGP networks.
Flexibility: Route reflectors provide a great deal of flexibility in the design of iBGP networks.

Conclusion

BGP route reflectors are a key component for scaling iBGP networks. They allow iBGP networks to be scaled to a very large size, and they simplify the configuration of iBGP networks. A deep understanding of BGP route reflectors is essential for anyone who is responsible for managing a large BGP network.