CCNP- BSCI (642-801) * BGP synchronization QUESTION

[FANCYpete]

Hello all, I am getting conflicting information from a testing engine that I'm studying with. The question is as follows:

No IGP is running in AS 65500 (routers B,C,D,E). Router B peers with Router A (AS 65000) via eBGP. Router A advertises (172.16.0.0) Router E peers with Router F(AS 64500) via eBGP. Which router(s) in AS 65500 will advertise a route to (172.16.0.0) if synchronization is OFF?
A. B only
B. E only
C. B and E only
D. B, C and D only
E. B, C, D, and E

THE BOOK SAYS THE Answer: E

*I'm thinking that the correct answer would be Answer: C

The reason I'm thinking this is that there is NO IGP running, and synchronization is turned OFF. Therefore, routers D, and C would not learn the eBGP route to (172.16.0.0). Router E would learn the eBGP route to (172.16.0.0) in that is an "internet" route, and Router E is doing eBGP with a different AS (AS 64500). Granted, the AS hop would be a little longer than if going through Router B.

* IF ANYONE can clear this up , please share!

Thanks.


I'm the Fanciest of the Fancy...INDEED

 

[FANCYpete]

A ROUTER-D ------- ROUTER-E >>>>> ROUTER-F (AS 64500)
S | eBGP
6 |
5 ROUTER-C ------- ROUTER-B >>>>> ROUTER-A (AS 65000)
5 eBGP
0
0

I'm the Fanciest of the Fancy...INDEED

 

[KiscoKid]

Hi

I think the answer is B as router B will not advertise any subnets back to the AS it originally learnt those routes from. This would introduce a routing loop.

 

[maturus]

I'm definitely no expert on this, but from what I can tell either the book is correct or the answer is D IMO. I believe this because A advertises to B, B advertises the route to C out of B-A, C advertises to D via B-C, etc. Where I get confused is if E would advertise to F. I suppose it would considering A advertised to B. That's my logic, but I could be completely off.

I believe the reason why C is the wrong answer is because D or E or F wouldn't have the advertisement to send out or otherwise. The route needs to be in every router, one way or the other. Hope that helped, and hope it's the right explanation.

 

[GM2005]

I believe B as well, because;

If iBGP was running full mesh with no other IGP all the routers would know the route.

As there is no iBGP running the opposite is true, and only Router E will learn the route. The answer could be none, but as its not an option we must accept that F has a route as well.

B is peering solely with the source of the advert.

There is no dynamic route learning going on inside AS65500.

This question is based on a BSCI training question i've seen, but in the training full mesh iBGP is running. Apart from that the whole question and diagram is identical.

I hope thats right......