Route reflectors are used for breaking full mesh iBGP rule. In this post I am evaluating how to synchronize both the route reflectors especially in case of MPLSVPN network. In my previous post I have described the problems can come if proper route updates are missing. Most of the service providers are using peer-group in BGP. If you are using peer-group then might face the cisco ios bug which is CSCsj09838. According to the bug
BGP sending incomplete updates when using update-groups
BGP fails to send complete update to peers who are part of an update group with more than one member. Soft clear of peer in question will fix the issue.
I have seen the problem which is coming in 12.4.11T4 as well as with 12.4 15T1.
Consider a figure in the last post in which PE1 and PE2 is having VPN-V4 peering with both route reflectors. Any of the vpn which is advertised on PE1 should go to both route reflectors and there on wards replcicated to whole cloud. Assume if a route of vpn is advertised by PE1 to both the route reflectots and being received by route reflectors. But RR1 is only sending the updates to PE2 no updates are being sent by RR2. In this case if RR1 goes down then the sla of vpn will be breached which is really not a good thing for service provider. This arise of this type of problem is nothing except a ios bug.
We can get rid from this problem by making both RRs of different clusters and then use the iBGP between them. By implementing this both RRs with share the routes with each other. In the above mentioned problem if iBGP is configured between the RRs then the vpn route will be advertised to RR2 by RR1 with next hop as PE1. Cluster list will play an important role in that and definitely stop the routing loop problems.