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Cisco CCNP / BSCI Certification: BGP Route Reflector Tutorial |
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When you're studying for your BSCI exam and CCNP certification,
you quickly realize that BGP is a whole new world from anything
you've previously studies. One topic that sometimes confuses
CCNP candidates is when a BGP route reflector needs to be
configured. Route reflectors are an excellent workaround for
a basic rule of BGP route advertisement that we’ll review
in just a moment.
In the following example, the routers R1, R2, and R3 are
all in BGP AS 100. This is not a full mesh, however. There
are peer relationships between R1-R2 and R1-R3, but not between
R2 and R3. R3 is advertising network 3.3.3.0/24 via BGP, and
the route is seen on R1. R1's iBGP neighbor, R2 does not see
the route, nor would any other iBGP neighbors of R1. Why?
A basic rule of BGP is that a BGP speaker cannot advertise
a route to an iBGP neighbor if that route was learned from
another iBGP neighbor. Configuring R1 as a route reflector
will allow us to circumvent this rule. The entire route reflector
process is transparent to the clients, and no configuration
is necessary on those clients. We'll configure R1 as a route
reflector for both R2 and R3.
R1(config)#router bgp 100
R1(config-router)#neighbor 172.12.123.2 route-reflector-client
3d18h: %BGP-5-ADJCHANGE: neighbor 172.12.123.2 Down RR client
config change
R1(config-router)#neighbor 172.12.123.3 route-reflector-client
3d18h: %BGP-5-ADJCHANGE: neighbor 172.12.123.3 Down RR client
config change
The BGP adjacencies do come down when this configuration
is added, so this isn't something you want to do during a
peak traffic time.
Once the adjacencies come back up, R2 will have the route
to 3.3.3.0/24.
There are other possible solutions to this iBGP limitation,
such as configuring BGP confederations. Those solutions are
generally used on larger BGP deployments and with other concerns
in mind, though, and configuring route reflectors serves this
purpose just as well.
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage
(http://www.thebryantadvantage.com), home of free CCNA and
CCNP tutorials, The Ultimate CCNA Study Package, and Ultimate
CCNP Study Packages. Video courses and training, binary and
subnetting help, and corporate training are also available.
Pass the CCNA exam and CCNP exam with Chris Bryant, CCIE #12933!
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