BGP nugacities (yes NUGACITIES)

CriscoSystems · ‎10-02-2008

I am taking the "Do I know this already" quiz in the BSCI Exam Certification Guide.

I will have you know I scored 12 out of 14, meaning I am eligible to skip the chapter; thanks for asking. However, dig this:

5. When are full routing updates sent in BGP?

a. At the beginning of each session

b. Whenever a fault is seen on the link

c. Every 30 minutes

d. At the startup of the BGP process.

Correct answer is given as "a" only. Are full routes not sent at the startup of the process? Not much point in starting up the process, otherwise.

12. What is the purpose of command "clear ip bgp" ?

a. To disconnect all sessions

b. To clear the BGP routing table

c. To end an administrative session

d. To clear all IGP entries from the routing table

I gave b, but the book says "a." Isn't that weird? I figured since clear ip route * clears the ip table....Was I really wrong or was the given answer a misprint? (Misprints are common in Cisco books.)

13. Which of the following are valid alternatives to using BGP when connecting to another autonomous system?

a. Using FTP

b. Default routes

c. Static routes

d. Merging the two autonomous systems into one.

Only b and c are considered correct. I wonder what the reasoning is in ruling out D?

See, the Cisco exams I've taken have lots of questions with these sorts of ambiguities; that's why I'm obsessing on them now. I've failed BSCI twice (which isn't uncommon, I know) but I'm a lot more than twice-shy about taking it again, especially when silly questions like these show up.

Istvan_Rabai · ‎10-02-2008

Hi Seth,

These are somewhat tricky questions, but the answers are right.

5. When you say " router bgp xxxx" and start configuring it, the bgp process starts. But at this time there may still be no neighbor relationships configured.

Full updates are sent only when the bgp process has started AND the neighbor relationship session has started. No updates are sent to non-existent neighbors.

12. You should test "clear ip bgp" command on 2 routers for yourself. It really disconnects 2 neighbors and the neighbor relationship then rebuilds. This is called hard reset and it stops data flow and routing as well while the neighbor relationship rebuilds and updates are sent. Don't use it on a production network.

Use the soft reset instead:

If you use the clear ip bgp x.x.x.x soft in | out then the routing updates are resent only, the neighbor session is not disconnected.

13. I can see they sometimes put confusing answers into the tests. This will ensure that you have the certainty of your knowledge.

Generally speaking, when routers are divided into separate autonomous systems, this is because there may be different routing policies within each of them. E.g. One belongs to an ISP, the other belongs to a customer. Big networks may be divided into different AS-es to create routing hierarchy. So it may not be a good solution to merge them.

Cheers:

Istvan

CriscoSystems · ‎10-07-2008

Thanks, Istvan. I was certain that they were just poorly worded questions, but you actually made them make sense! Well done.

Istvan_Rabai · ‎10-08-2008

You're welcome Seth and thanks for the rating!

Istvan