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 examples, 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.
R1#show ip bgp
BGP table version is 2, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> i3.3.3.0/24 172.12.123.3 0 100 0 i
R2#show ip bgp
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.
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 now has the route to 3.3.3.0/24.
R2#show ip bgp
BGP table version is 2, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> i3.3.3.0/24 172.12.123.3 0 100 0 i
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.
Visit his blog and sign up for Cisco Certification Central, a daily newsletter packed with CCNA, Network+, Security+, A+, and CCNP certification exam practice questions! A free 7-part course, “How To Pass The CCNA", is also available, and you can attend an in-person or online CCNA boot camp with The Bryant Advantage!
Was this article helpful to you? Leave a Public Comment or
Question:
This Article has been viewed 1,075
times.
Article added to SearchWarp.com on Thursday, February 09, 2006 View other articles written by Chris Bryant CCIE 12933(13,682)
If you found this article interesting, you may want to check out:
Disclaimer: All information on this site is provided for informational purposes only! By no means is any
information presented herein intended to substitute for the advice provided to you by any health care or other professional
or organization.
