Orhan Ergun 2 Comments

BGP Route Reflector in Plain English, in this post, I will explain you the BGP Route Reflector basics, after you read this post, you will be able to answer many questions regarding BGP Route Reflectors.

 

 

Outline of this post is as below.

 

  • What is BGP Route Reflector ?
  • Why BGP Route Reflector is used ?
  • What is the alternate methods ?
  • Different type of BGP Route Reflectors
  • Benefits of BGP Route Reflector
  • Problems with the BGP Route Reflector
  • BGP Route Reflector Redundancy

 

What is BGP Route Reflector ? 

 

A route reflector (RR) is a network routing component for BGP (RFC 4456). It offers an alternative to the logical full-mesh requirement of internal border gateway protocol (IBGP).

 

Above is the wikipedia definition of BGP Route Reflector. Let’s extend the definition a bit.

 

BGP Route Reflector is only used in IBGP.

 

There are two BGP methods. EBGP and IBGP. In EBGP, BGP neighborship is created between two routers which have different BGP Autonomous System number.

 

In IBGP, BGP neighborship is created between the routers which have same BGP Autonomous System number.

 

BGP Route Reflector as I said above, only used in IBGP. Route Reflector is not a special device, it is a regular router, which can run any routing protocol, it can be physical or virtual router which needs to have BGP code, that’s all.

 

BGP Route Reflector is an alternative to BGP full mesh. What does it mean ?

 

The requirement with IBGP, every BGP router needs to have a BGP neighborship with all the other BGP routers in one particular BGP Autonomous System.

 

ibgp full mesh

Figure 1 –  IBGP Full Mesh 

 

Above topology shows IBGP Full Mesh design. As you can see, from each router, there is a BGP connection to each and every other BGP router inside the network.

 

RR simplifies the logical topology of the network, reduces the total number of BGP session in the network and reduces the configuration on the BGP routers.

 

bgp route reflector topologyFigure 2 – BGP Route Reflector Topology

 

 

As you can see in the above topology, BGP Route Reflector is a central node which all the other BGP routers in the network build an IBGP neighborship with.

 

Routers which have a BGP session to Route Reflector is called as BGP Route Reflector Client.

 

Very important and less known definition is BGP Route Reflector Cluster.

 

BGP Route Reflector Cluster is the BGP Route Reflector and its all clients. So if you have an RR and the RR clients, you have a BGP Cluster. We will talk about BGP RR Cluster in the BGP RR Redundancy section.

 

Between the figure -1 and the figure -2, did you realize something ?

 

Topology changed from full mesh to hub-and-spoke. Thus, we conclude this part by saying that BGP Route Reflector changes the logical BGP topology of the network from full-mesh to hub-and-spoke.

 

Why BGP Route Reflector is used ? 

 

Almost every Service Provider today use BGP Route Reflector. As you might know, because of my job, I work with probably more than hundred operators. Very few of them use alternate method which I will mention throughout the post.

 

For those who deploy BGP Route Reflector in their network, main reason is scaling. When you don’t need to have a IBGP neighborship to each and every BGP router in the network, you have less amount of BGP session in the network which mean less connection to troubleshoot and maintain.

 

Configuration on the devices reduce greatly too.

 

In the figure 2, each RR client, only have one BGP session. Without BGP Route Reflector in that topology, as there are 6 routers, each router would have 5 IBGP session.

 

 

What is the alternate method for BGP Route Reflector ? 

 

Many people believe that BGP Confederation is an alternate method for BGO Route Reflector. In fact not exactly.

 

Although I don’t want to cover the BGP Confederation in this post as it requires a separate detailed post, you need to understand the very basic idea with it.

 

Let’s look at the below topology.

 

bgp confederation

Figure 3 – BGP Confederation (Source : www.potaroo.net)

 

With BGP Confederation, BGP topology is divided to smaller parts. Each part is called as sub-autonomous system.

 

As yo can see from the Figure -3, there is only one Global Autonomous System which represent this network to the outside world. So when this network has EBGP connection with the other BGP network, global/public Autonomous system is used. In this example Global Autonomous system number is 1.

 

There are two other Autonomous System numbers in the figure-3 topology. 65010 and 65011.

 

These are sub-autonomous system numbers. In each sub-autonomous system there might be multiple routers and all have same sub-autonomous system number. That’s why IBGP neighborship is created between each and every number inside the sub-autonomous systems.

 

Here is the critical part, ‘ Inside each sub-autonomous system, since IBGP is setup between the routers, either full-mesh BGP connections or BGP Route Reflector is required ‘

 

So, if BGP Route Reflector is still required in BGP Confederation, how it is an alternate method for BGP Route Reflection ?

 

Alternative to BGP Route Reflector is BGP Full Mesh IBGP.

 

 

Different type of BGP Route Reflectors

 

Many application , many services run on today networks. IP and MPLS are the most common services in the Enterprise and Service Provider networks.

 

When BGP Route Reflector is used for the IP address family, then BGP Route Reflector is called as an IP Route Reflector or for short, IP RR.

 

When BGP Route Reflector is used in MPLS network for the VPN service, BGP Route Reflector is called as VPN Route Reflector or for short, VPN RR.

 

There are different design requirements for the IP RR and the VPN RR. I will mention them in the BGP Route Reflector Problems section in this article.

 

 

Benefits of BGP Route Reflector 

 

Main benefit of BGP Route Reflector is Scalability.

 

  • BGP Route Reflector reduces the total number of BGP sessions in the network and also reduces the number of BGP session per router.

 

  • BGP RR simplifies the configuration of the BGP routers.

 

But, there is one more thing which I will explain with the below topology.

 

  • Route Reflector hides the available paths. This is benefit for some networks, problem with the others. I will explain how it is good and bad at the same time.

 

 

unique rd per pe

Figure 4 – BGP Route Reflectors hide the available path/next hop to the destination

 

In Figure-4, Customer site is connected to two different PE devices in the operator network.

 

BGP Route Reflector , both in IP and MPLS environment, selects only one path for the destination, install only this path in the routing table and advertise this path to Route Reflector Clients.

 

Which mean, in Figure-4, even though Route Reflector learns both path/next hop (PE1 and PE2) for the customer site, Route Reflector Selects either PE1 or PE2 as a best path and advertise it to PE3.

 

PE3 only receives one path for all the customer site prefixes. This is Route Reflector benefit from device resource usage point of view. If PE3 would receive both PE1 and PE2 as next hop, it would have more route in the routing table, thus memory and CPU usage would increase.

 

Having only one path for the destinations on the PE3 has disadvantages as well which I will explain in the BGP Route Reflector Problems section in this post.

 

Note: In Figure-4, Unique RD per PE per VRF technique is used to advertise both PE1 and PE2 from BGP Route Reflector to PE3.  But this is not the topic of this post.

 

 

Problems with the BGP Route Reflector

 

In the above example (Figure 4), PE3 had only one path for the customer site prefixes as BGP Route Reflector advertises it to best path only.

 

  • BGP Route Reflectors and Active-Active Customer Link Utilization

 

What if customer wants to use both of the links as an active-active manner. If BGP Route Reflector always chooses PE1 let’s say as a best path, PE2 link would be always idle.

 

But how PE3 can receive both of the available next hops/paths for the customer site prefixes while BGP Route Reflectors is in place ?

 

There are multiple techniques such as BGP Add-Path, BGP Shadow Sessions, BGP Shadow RRs but the best one for the MPLS networks is using Unique RD per PE per VRF/VPN. I promised to keep this post simple thus I won’t start explaining all of them.

 

So, hiding the available paths can create many problems. In the above example, active-active link utilization was the requirement , so from the PE3 IBGP Multipath would be enabled if PE3 receives both of the available paths for the customer sites.

 

  • What if the requirement is an optimal routing, How BGP Route Reflector affects optimal routing?

 

If available paths are hidden, and Route Reflector is sending only the best path to the RR Clients, can it create a sub-optimal routing problem ?

 

Yes, it does. Let’s look at the below topology.

 

BGP RR and Optimal Routing

Figure 5 – BGP Route Reflector and Sub Optimal Routing Problem

 

Numbers show the IGP costs. R1, R2 and R3 are the RR Clients. In this topology, for the 192.168.0.0/24 customer prefix, both R1 and R2 advertises the prefix to RR.

 

RR based on the IGP cost , selects one best path. Since R2’s IGP cost to 192.168.0.0/24 is smaller than R1, BGP RR, selects R2 as best path for the customer prefix and advertise best path to all its clients.

 

From R3’s point of view, R1 is the best path, because R1 has smaller IGP cost than R2 , for this customer network.

 

But R3 will not receive R1 as an available next hop for this customer site, because BGP RR will not send R1 as next-hop to R3.

 

Remember, what was the RR rule ? RR will select only one best path for given destination and advertise it to the RR Clients.

 

From R3’s point of view, using R2 is sending traffic to the suboptimal path.

 

 

  • Route Reflector prevents BGP Fast Reroute

 

BGP Route Reflector prevents fast reroute. For the fast reroute, at least two path needs to be installed on the device which will take an action. In the Figure-5, If R3 would receive the both R1 and R2 as next-hops for the customer site, R3 would be able to install , R1 in to routing  table as primary path and R2 as hot-standby path.

 

I explained the BGP Fast Reroute detail earlier in the website thus I recommend BGP PIC (BGP FRR) article if you want to understand this section more.

 

 

  • Another problem with the BGP Route Reflector is, BGP Route Reflector increases convergence time.

 

Without BGP Route Reflector, we would have Full Mesh IBGP. Every BGP Router would have a direct IBGP session with each other.

 

They would have all available next-hop/exit point for the prefixes.

 

In the absence of IBGP Multipath, only one of them would be installed in the IP Routing Table but all available next-hops would be in the BGP Routing Table of the routers.

 

Just look at above , Figure-5 topology.

 

Without BGP RR, R3 would have an IBGP session with both R1 and R2.

 

R3 would install R1 as best path into the routing table due to lower IGP cost. (IGP cost would be tie breaker in the best path selection process)

 

In case of R1 failure, R3 would be notified and start installing R2 as best path.

 

When Route Reflector is deployed, in case of R1 failure in the Figure-5 topology, BGP RR would declare that R2 is no longer best path for this customer site, then RR would choose R1 as best path after finishing its BGP Best Path Selection process, would wait for the MRAI (Minimum Route Advertisement Interval) and send the new best path to R3. When R3 receives new best path, it would spend time to install the prefixes with next-hop to the IP Routing and Forwarding Table.

 

As you can see, BGP Route Reflector increases convergence time !

 

 

BGP Route Reflector Redundancy

 

BGP Route Reflector is an important node. Failure of it, collapses entire BGP topology. Thus, BGP RR Redundancy is important..

 

More than one RR is important. And as a general design rule I always recommend you to have two , right ? Not three, four.

 

What was the motto ? Two is company, three is crowded..

 

As I mentioned above in the article, There might be different services in the network. Internet , VPNs etc.

 

For the Internet service, IP Route Reflector, for the VPNs, VPN Route Reflector is used.

 

But should you use the same router for both IP and VPN RR functionoalities ?

 

Answer is it depends. If the deployment is small enough , yes you can. But always consider the impact of having problem in one service to another. If there is an attack towards IP RR, it will affects VPN customers as well. This concept is called fate-sharing.

 

Last but not least, when you deploy more than one BGP Route Reflector, what should be the cluster ID on the Route Reflectors. I recommend you to read my ‘ BGP Route Reflector Clusters ‘ post for this.

 

Hope you enjoyed and learned something new. Please put your thoughts and comments in the comment box section.

 

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