>Let me preface this by saying that I am trying to learn more about large >scale BGP design and operation. This question is on route reflectors when >you have multiple POPs in seperate IGP domains. If you currently have one >POP and are going to move to 2 within the same AS, you can either run full >mesh (doesn't scale), reflectors, or confederations. Assuming you don't >currently have a central core that the POPs connect back to, how well does >reflection scale? I was reading Building Service Provider Networks >[Berkowitz], and it states that iBGP doesn't scale well once you go above >15-20 sessions per router.
Not an absolute number, but a fairly good one based on experience. There are a lot of variables involved. >It also states that most ISPs run reflectors >instead of confederations, but I believe that statement is being made under >the assumption that the ISP will have a central core to which the POPs will >connect. Do remember that you can create hierarchies of reflectors. I recognize that it can be argued that the upper levels of hierarchy, de facto, are a core. >This would indicate to me that assuming you don't have a central >core, one could only connect 6 or 7 POPs (dual reflectors for redundancy) >together using reflection before you would have to either create a central >core to reduce the amount of iBGP sessions, or turn to confederations. >Perhaps the best way to accomplish this would be to establish a "core" in >one of the POPs and run reflection from there, which is also presented as a >solution in the book? As you point out, the core doesn't have to be in the exact geographic center. Not every US carrier has its central point in Kansas! The likelihood is that one of the POPs will be colocated with your technical staff, infrastructure servers, etc., and the core might be no more than some LANs at that site. > Any opinions? I have made an attempt at ASCII >drawing below, to me the central core solution makes more sense. > > First Scenario, no central core > > POP 1 POP 2 > Cluster Cluster > o---------------o > Full Mesh Between Reflectors > o---------------o > > Second Scenario, central core establised at one of the POPs > > POP 1 POP 2 > Core Reflectors > Clients o----- -----o Clients > o----- \ / -----o > o o > POP 3 / \ POP 4 > Clients o----- -----o Clients > o----- -----o > > >Guy H. Lupi >CCIE No. 9275 Also, one of the philosophies in evolving core design with MPLS is that the core may essentially be IGP and MPLS only, with the distribution tier routers having the BGP knowledge. In other words, geography forces you to have lots of edge routers that still can distribute some of the policy processing load, while the core routers can be big, strong, and stupid. Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=48522&t=48509 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
