Thursday, July 24, 2008

Welcome back! It's Thursday, July 24, and here are the answers to yesterday's free Cisco certification practice exam questions!

CCNA Certification:

In an OSPF hub-and-spoke NBMA network, which router(s) require the neighbor command?

A. The DR

B. The DROthers

C. The BDR

D. All of these

E. None of these

Answer: A. The DR requires the neighbor command. It will not hurt anything to have it configured on the DROthers in real-world networks, but I wouldn't put it there on exam day. There are no BDRs in an OSPF hub-and-spoke network.

CCENT Certification:

What mode is the following router in?


Answer: Global configuration mode.

CCNA Security Certification / CCNP ISCW Exam:

You're configuring IPS in SDM, and you just enabled the Fail Closed option. What's the net effect?

Answer: Here's the exact description of Fail Closed from SDM itself:

"By default, while IOS compiles a new signature for a particular engine, it allows packets to pass through without scanning for the corresponding engine. Enable this option to make IOS drop packets during the compilation process."

Fail Closed is disabled by default.

CCNP Certification / BSCI Exam:

Which of the following are Cisco recommendations for OSPF deployments?

A. No router in more than three areas.

B. No area should contain more than 50 routers.

C. No router should have more than 60 neighbors.

D. No ABR should run more than one OSPF process.

Answer: A, B, C, D. Those are all Cisco best practices for OSPF.

CCNP Certification / BCMSN Exam:

You're examining the lights on a Cisco Aironet card. The green light is blinking slowly, the amber light is off. What does this generally indicate?

Answer: Here's a quick review of what those lights are and what the different combinations indicate.

We have two lights on a Cisco Aironet card. The green light is the Status LED, and the amber light is the Activity LED. We've got quite a few combinations with those two lights, so let's take a look at what each of the following LED readouts indicates.

Status off, Activity off - Naturally, this means the card isn't getting power!

Status blinking slowly, Activity off - the adapter's in Power Save mode.

Status on, Activity off - adapter has come out of Power Save mode.

Both lights blinking in an alternating fashion - adapter is scanning for its network.

Both lights blinking slowly at the same time - adapter has successfully associated with an AP (or other client if you have an Ad Hoc network)

Both lights blinking quickly at the same time - adapter is associated and is sending or receiving data

CCNP / ONT Exam:

What is Global Synchronization? Is it a benefit or a detriment to network performance?

Answer: Here's a review of tail drop and how it can cause Global Synchronization. Usually synchronization is good, but this kind isn't!

When the queue is full, packets that are trying to queue up for transmission literally have nowhere to be put! These packets are then subject to tail drop, which is a fancy way of saying "you're being dropped because we have no place to put you".

You know that TCP has a detection and recovery scheme when it comes to missing segments, so tail drop is no big deal, right? Quite the opposite, it's a huge deal.

The problem starts innocently enough, as the senders realize their TCP packets are being dropped. As we'd expect, the senders then throttle back on their transmission speed. After doing so, the senders will then gradually speed their transmission rates back up.

As multiple senders increase their transmission rates, the queue will fill up again, and the senders will again almost simultaneously slow their tranmission rates, followed by another near-simultaneous increase.

As a result of this global synchronization, the links are perpetually in one of two states - congested or underused. Basically, the network ends up being either hammered or not being used to its full potential, and those are both circumstances we want to avoid.

One way to avoid global synchronization is though the use of Random Early Detection (RED).

More brand-new Cisco practice exam questions will be posted later today, along with the answers to my latest CCNA Security practice exam!

To your success,

Chris Bryant
CCIE #12933

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