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Computers >> Cisco CCNP / BSCI Certification: BGP Route Reflector Tutorial |
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2006-02-16
| Chris Bryant, CCIE #12933
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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.
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.
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.
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