Route reflectors have become an important part of any larger internet service provider's setup. Nowadays, it is difficult to imagine having multiple routers running BGP in a full mesh topology due to scalability concerns. The concept of route reflectors can be found in RFC4456, which was published in 2006. It is crucial to have a good understanding of the BGP protocol before working with route reflectors.

So, why are route reflectors so helpful for scalability? To answer this question, let's first look at how routes exchange information through the BGP protocol. In external BGP (when connecting different autonomous systems), there is a simple solution to avoid routing loops by using the autonomous system number, which indicates the origin of each route. When a router sees its own AS number in a route, it knows that it is coming from its own autonomous system and does not use it to prevent a loop. This mechanism works well for internet routes (eBGP), but it limits scalability with internal BGP peerings (iBGP). Since the AS number for iBGP peers is the same, the BGP protocol had to adopt a special rule. None of the routes received from one iBGP peer can be advertised or forwarded to another iBGP peer. In simple terms, all iBGP peers need to establish a neighborship with every other iBGP router, which results in a large number of BGP sessions.

This is the formula for calculating the number of sessions needed for a full mesh:

N(N -1)/2 [where N represents number of routers]

For 20 routers for instance: 20(20-1)/2 = 190 BGP sessions

Route reflector explained

iBGP full mesh topology

iBGP topology with route reflector

A Route reflector is like a central hub for all the iBGP routers. It helps in sharing routing information among these routers. It can send routes to other routers within the network.

This makes it easier to manage a large BGP network. It is recommended to have at least two route reflectors working together as a backup in case one of them fails. This is called a cluster and it ensures that the route reflector, which is an important component, remains functional even if one fails.

Route reflector configuration

We have set up a basic situation with one main router and two connected routers. All routers are running Cisco IOS. The Ethernet connections are set up with small networks within the private IP range of 10.0.0.0. Additionally, loopback interfaces are used for internal BGP connections.

R1 – 192.168.1.1

R2 – 192.168.2.1

RR – 192.168.0.3

We utilized the RIP protocol to share details about all IP prefixes and loopback interfaces. Specifically, the interface loopback-50 on R1 (IP 50.50.50.50) will only be promoted through the BGP protocol to ensure the proper functioning of our network structure. R1 will advertise its Loopback50 to the route reflector, which will then further advertise it to R2.

Topology with one route reflector in GNS3

Configuring route reflectors on Cisco devices

R1#show run | s bgp| s rip| s bgp| s rip| s bgp| s ripint eth 0/1

interface Ethernet0/1

ip address 10.0.0.1 255.255.255.252

full-duplex

end

R1#show run int lo 0

interface Loopback0

ip address 192.168.1.1 255.255.255.255

end

R1#show run int lo 50

interface Loopback50

ip address 50.50.50.50 255.255.255.0

end

R1#show run | s rip

router rip

network 10.0.0.0

network 192.168.1.0

no auto-summary

R1#show run | s bgp

router bgp 100

no synchronization

bgp log-neighbor-changes

network 50.50.50.0 mask 255.255.255.0

neighbor 192.168.0.3 remote-as 100

neighbor 192.168.0.3 update-source Loopback0

no auto-summary

R2#show run int e0/0

interface Ethernet0/0

ip address 10.0.0.6 255.255.255.255

full-duplex

end

R2#show run int lo0

interface Loopback0

ip address 192.168.2.1 255.255.255.255

end

R2#show run | s rip

router rip

network 10.0.0.0

network 192.168.2.0

no auto-summary

R2#show run | s bgp

router bgp 10

no synchronizatio

bgp log-neighbor-changes

neighbor 192.168.0.3 remote-as 100

neighbor 192.168.0.3 update-source Loopback0

no auto-summary

RR#show run int eth 0/1

interface Ethernet0/1

ip address 10.0.0.2 255.255.255.252

full-duplex

end

RR#show run int eth 0/0

interface Ethernet0/0

ip address 10.0.0.5 255.255.255.252

full-duplex

end

RR#show run int lo0

interface Loopback0

ip address 192.168.0.3 255.255.255.255

end

RR#show run | s rip

router rip

network 10.0.0.0

network 192.168.0.0

no auto-summary

RR#show run | s bgp

router bgp 100

no synchronization

bgp log-neighbor-changes

neighbor 192.168.1.1 remote-as 100

neighbor 192.168.1.1 update-source Loopback0

neighbor 192.168.1.1 route-reflector-client

neighbor 192.168.2.1 remote-as 100

neighbor 192.168.2.1 update-source Loopback0

neighbor 192.168.2.1 route-reflector-client

no auto-summary

Now we need to test that we can reach the Loopback 50 from R2, so let’s verify its routing table first:

R2#show ip route

***

     50.0.0.0/24 is subnetted, 1 subnets

B       50.50.50.0 [200/0] via 192.168.1.1, 00:14:12

     10.0.0.0/30 is subnetted, 2 subnets

R       10.0.0.0 [120/1] via 10.0.0.5, 00:00:07, Ethernet0/0

C       10.0.0.4 is directly connected, Ethernet0/0

R   192.168.0.0/24 [120/1] via 10.0.0.5, 00:00:07, Ethernet0/0

R   192.168.1.0/24 [120/1] via 10.0.0.5, 00:00:07, Ethernet0/0

     192.168.2.0/32 is subnetted, 1 subnets

C       192.168.2.1 is directly connected, Loopback0

R2#ping 50.50.50.50

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 50.50.50.50, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 32/56/76 ms

Great, we can see the path to the 50.50.50.0 network in R2's routing table, and it shows that we can reach it. If we want toadd another iBGP router, we just have to establish a connection with the route reflector and obtain all the networks. However, we must remember to advertise it by using the network command in the R1 configuration. It's important to note that Cisco routers don't automatically advertise anything.

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