BGP - When To Disable Synchronization

[andrewmorris]

Hi All,

Please help. I am busy preparing for my CCNP Routing Exam and came across a question which is really bugging me.

When is it possible to turn off the default synchronization?

a. When confederations are used

b. When route reflectors are configured

c. When the iBGP neighbors are fully meshed

d. When all the routers in the AS are running iBGP.

I know "a" and "b" are definitely out and that the answer should be "c" and "d" although the Cisco Press Book(CCNP BSCI, 3rd Edition) states that the correct answer is "d" only.

Now does "all routers" mean just that...ALL routers in an AS including IGP and iBGP routers, or does ALL routers mean, "all iBGP routers"

I thought that if all iBGP routers are fully meshed, then this will warrant been able to turn off synchronization.

Any thoughts dudes?

[Georg Pauwen]

Hello Andrew,

answer D is indeed the correct answer. The fact that all iBGP routers are fully meshed (answer c) does not mean that there isn´t an IGB (like OSPF, EIGRP, or RIP) running between them. In answer D, ´all´ routers in the AS means exactly that, all physical routers, and if that is the case, turning off synchronization is possible.

This sounds like a typical Cisco exam question, you narrow it down to two options, and then you are stuck ... :)

Regards,

Georg

[andrewmorris]

Hi All,

Thanks very much for your help. I suspected that an IGP router may be inbetween the iBGP neighbors and hence all routers within the AS are to be meshed.

Going for the composite exam to recertify my CCNP and also attempt the CCIE routing and switching Beta exam which has just come out.

Wish me luck :))

[vcjones]

Answer D is the correct answer. Think about why synchronization is required in IBGP to begin with... (hint, consider a router not running IBGP on the path between two routers running IBGP, and what happens if one of the IBGP routers sends a packet for a destination which is unknown to the intermediate router, which in turn has a default path to the original IBGP router...).

Then consider under what conditions it is possible to have a router on the path between two routers running IBGP which is not running IBGP (and hence, where synchronization is required).

Good luck and have fun on your exam!

Vincent C Jones

www.networkingunlimited.com

[jkimcv60]

I believe, "All routers" means every single router in the AS hence this makes iBGP the IGP for the network. Remember that synchronization forces the router to only announce what it knows about via IGP so by turning off synchronization it's living it up to you to make sure some form of IGP is running whether is iBGP, OSPF...etc.

I agree with others, "D" is the only possible best answer.