PASS ! HCIE interview experience sharing

Emily

First of all, I would like to thank BAERLAB for its continuous support , and especially thanks to classmate Zhoufor his help. He gave me a lot of test preparation materials and played a keyrole. At the same time, I would like to thank all the top colleges in the exampreparation group for their words and deeds, and here to respect them! Theinterview is finally over! ! !

  

With good luck, the interview questionsthat were picked up this time are very basic, and the questions are as follows:

1. The role and disadvantages of MPLSend-hop pop-up

2. Whether the gateway is placed inaggregation or access

3. ISIS, OSPF, BGP support for IPV6

The specific answer content can be answered in accordance with the content on the test preparation book. Here I will sharewith you the examiner's follow-up questions and my answering skills:

My order of answering questions is: 1-2-3

1. The role and disadvantages of MPLS end-hop pop-up

Answer ideas:

  This question is about the functions and shortcomings in 3 minutes. Talking too fast will definitely lead to a lot of questions from the examiner.So my idea is: First, start with the MPLS label format, introduce the role ofeach field, and talk about the label. When selecting the value range, itfocuses on explaining the functions and differences between 0 and 3. These twopieces took about 6 to 7 minutes, and then the advantages and disadvantages ofPHP and the lack of continuity in order to solve the problem of QOS service Asfor the number 0 label developed, 10 minutes have passed since I said this. Idon’t see any response from the examiner. I quickly introduced the MPLSforwarding process. 15 minutes have passed when the examiner started to ask:

The examiner began to ask:

1. The difference between label 0 and label3 (I suspect that the examiner did not listen to me at all...)

I answer; number 0 is to solve the problemthat the QOS service cannot get continuity after PHP. The label value assignedby the outgoing node to the penultimate hop node is 0, and the penultimate hopLSR needs to press the label with the value 0 normally. The top of the labelvalue of the message is forwarded to the last hop. When the last hop finds thatthe value of the label carried in the packet is 0, the label is ejected (noneed to look up the table), and then IP forwarding is performed. PHP is enabledby default. You can modify the Egress node in the mpls view to assign anexplicit empty label to the penultimate hop.

2. What are the fields for differentiatedservices on the second and third layers?

I said COS and TOS

The examiner also asked what is thedifference between IPP and DSCP?

I said that IPP is the first 3 bits of TOS,DSCP is the first 6 bits of TOS---the first 3 bits represent priority, and thelast 3 bits represent D delay, T throughput, and R reliability. IPP has 8categories, and DSCP has a total of 64 categories. Eight categories correspondto one IPP.

The examiner asked how DSCP has the valueof EF? (Fortunately, I remembered it before)

I said that the decimal system is 46 bitsand the value is 101. 110 represents low latency and high throughput. Theexaminer thought about it for a while and said it was right.

3. Do you know congestion management andcongestion avoidance?

I talked about the difference between thetwo, and then the examiner asked what is WRED

My answer: WRED is implemented based onRED. The flow queue supports WRED drop based on DSCP or IP priority. Eachpriority can independently set the upper and lower packet loss thresholds andpacket loss rate. When the packet reaches the lower limit, packet loss starts.As the threshold increases, The packet loss rate keeps increasing, and thehighest packet loss rate does not exceed the set packet loss rate until the highthreshold is reached, and all packets are discarded. In this way, packets inthe queue are actively discarded according to a certain drop probability,thereby avoiding congestion problems to a certain extent .

The examiner asks again, what is the differencebetween WRR and DRR?

I answer, WRR is a weighted round-robinscheduling, which performs round-robin scheduling between queues, and schedulesthe packet flow in each queue according to the weight of each queue. Thedisadvantage is that each queue does not have a fixed bandwidth. Under the samescheduling opportunity, the actual bandwidth obtained by large-size packets islarger than that obtained by small-size packets.

The DRR balance round-robin schedulingsolves the problem that WRR only cares about packets. Under the same schedulingopportunity, the actual bandwidth obtained by large-size packets is greaterthan the bandwidth obtained by small-size packets. The packet length factor isconsidered in the scheduling process. So as to achieve the fairness of thescheduling rate. In DRR scheduling, the bandwidth deficit of each queue isinitially 0. Before each scheduling, the system allocates bandwidth to eachqueue according to the weight, and calculates the deficit value. If the deficitvalue of the queue is greater than 0, it participates in this round ofscheduling, sends a message, and calculates the post-scheduling deficit valueaccording to the length of the sent message , As the basis for the next roundof scheduling; if the deficit value of the queue is less than 0, it will notparticipate in this round of scheduling, and the current deficit value is usedas the basis for the next round of scheduling.

After answering the first question, Ilooked at the time and spent 22 minutes, which was within controllable range;

2. whether the gateway is placed inaggregation or access

Answer ideas:

This question can be answered according tothe 6 aspects on the test preparation book:

1. Cost

2. Redundancy

3. Scalability

4. Security

5. Operation and maintenance management

6. Routing

I also talked about an existing networkcase to explain why the access layer is better than the convergence layer interms of security: for example, the wireless routers in the hotel industry havetheir own DHCP function, and usually directly plug the WAN port into the floorswitch If the floor switch does not support the DHCP snooping function, thereis a risk of counterfeiting the DHCP server, and the general access andaggregation are directly trunk roads, which may cause users on other floors toget the address assigned by this wireless router, resulting in failure Internetaccess will affect users on multiple floors. If the gateway is placed inaccess, only a single switch user will effectively isolate the broadcastdomain.

The examiner asked:

1. What technologies can VRRP be linkedwith?

I said routing or BFD, and he said he askedwhat kind of technical linkage BFD can link, I said static routing, dynamicrouting, he asked if there is any, I said there is an LDP.

He said yes. The examiner said that thiskind of linkage technology is very important in large-scale networks. In thescenario of active and standby links, it can speed up the switching speed ofthe active and standby links. It is a very important technology. You can studyit carefully when you go back. , I said yes.

It took almost 20 minutes to answer the sixmain questions of this question, and the examiner didn’t ask much, so continueto the next question.

3. ISIS, OSPF, BGP support for IPV6

Answer ideas:

This question is still the same, accordingto the routine, do not control the answering time, and do not leave too muchtime for the examiner to follow up.

ISIS: Two TLVs 232 and 232 also have anNLPID, and a new multi-topology function

OSPF: Hello message, LSA format, LSAincreased flooding range, link-local...

BGP: Introduced two new optionalnon-transition path attributes: MP_REACH_NLRI, MP_UNREACH_NLRI and 6 addresscluster views...

The answer to this question was thesmoothest, and it was basically completed in one go from beginning to end.

The examiner asked:

1. What is the multi-topology usage scenarioof ISIS?

I drew a mouth-shaped network and roughlyexplained it:

If A does not support multi-topology, theshortest path of A-D during SPF calculation: A-B-D, but because B does notsupport multi-topology, the IPv6 packets of A-D cannot be forwarded.

If multi-topology is enabled on A, thenRouterA will calculate the SPF separately according to different topologies atthis time, and the routes of IPV4 and IPV6 will take their respective optimalpaths.

The third question was completed in about58 minutes. At this time, the examiner said to comment:

The answers to the three-way questions arequite good and comprehensive. There is nothing to add. The linkage function ofVRPP, BFD and other protocols can be studied carefully, and the role inlarge-scale network projects is still very important.

  The questions in this interview were not difficult and relativelysimple. I also made careful planning for time control. I didn't have much timefor the examiner to ask questions. This is also a skill. In addition, duringthe interview preparation stage, students must work hard on the principles andfields of the agreement. With this basic examiner, they will be calm whenasking questions. It’s best to spend a certain amount of time every week formock interviews, find three questions to explain them thoroughly within anhour, and the logic must be clear, what to say first, then what to say, thedetails of knowledge must be considered, so repeated practice Improve yourability to speak.

I wish you all the prospective IEs everysuccess! ! !