ICND2 – EtherChannel Questions
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Question 1
[am4show have=’p2;’]Refer to the exhibit. While troubleshooting a switch, you executed the “show interface port-channel 1 etherchannel” command and it returned this output. Which information is provided by the Load value?
A. the percentage of use of the link
B. the preference of the link
C. the session count of the link
D. the number source-destination pairs on the link
Answer: D[/am4show]
Explanation
The way EtherChannel load balancing works is that the switch assigns a hash result from 0-7 based on the configured hash method ( load balancing algorithm ) for the type of traffic. This hash result is commonly called as Result Bundle Hash (RBH).
Now we need to convert Load value from Hexadecimal to Binary numbers. Therefore:
+ Gi1/1: 36 (Hex) = 00110110 (Bin) -> Bits 1, 2, 4, 5 are chosen (read from right to left, start from 0)
+ Gi1/2: 84 (Hex) = 10000100 (Bin) -> Bits 2, 7 are chosen
+ Gi1/3: 16 (Hex) = 00010110 (Bin) -> Bits 1, 2, 4 are chosen
Therefore if the RBH is 5, it will choose Gi1/1. If RBH is 4, it will choose Gi1/1 and Gi1/3 interfaces. If RBH is 2 it will choose all three above interfaces. And the bit sharing ratio is 3:3:2 (from “No of bits” column) hence two links has higher probability of getting utilized as compared to the third link.
Question 2
[am4show have=’p2;’]Which two actions must you take to configure a PAgP EtherChannel between two switches, S1 and S2? (Choose two)
A. Configure the channel-group 1 mode auto command on S1.
B. Configure the channel-group 1 mode desirable command on S2.
C. Configure the channel-group 1 mode active command on S2.
D. Configure the channel-group 1 mode on command on S2.
E. Configure the channel-group 1 mode active command on S1.
Answer: A B[/am4show]
Explanation
In PAgP we only have two modes: auto and desirable.
need more questions please.
Thanks Archie, that’s bloody wonderful. I’ll never forget you or your upload 🙂
[spam_suspect]@9tut
Math for Question 1 is not right! value should read from right to left as 76543210
Therefore: it should be:
+ Gi1/1: 36 (Hex 3 and 6) = 0011 0110 (Bin) -> Bits 1, 2, 4, 5 are chosen
+ Gi1/2: 84 (Hex 8 and 4) = 1000 0100 (Bin) -> Bits 2, 7 are chosen
+ Gi1/3: 16 (Hex 1 and 6) = 0001 0110 (Bin) -> Bits 1, 2, 4 are chosen
more detail
http://packetpushers.net/understand-etherchannel-load-balancing-catalyst-switches/
@9tut
Math for Question 1 is not right! value should read from right to left as 76543210
Therefore: it should be:
+ Gi1/1: 36 (Hex 3 and 6) = 0011 0110 (Bin) -> Bits 1, 2, 4, 5 are chosen
+ Gi1/2: 84 (Hex 8 and 4) = 1000 0100 (Bin) -> Bits 2, 7 are chosen
+ Gi1/3: 16 (Hex 1 and 6) = 0001 0110 (Bin) -> Bits 1, 2, 4 are chosen
more detail
packetpushers.net/understand-etherchannel-load-balancing-catalyst-switches/
@MN: Thanks for your information, we have just updated the explanation of this question!
Good stuff!
Yes, more questions, thanks.
Q2 “In PAgP we only have two modes: auto and desirable.” PAgP has Auto, Desirable and On. On/On works, Desirable/Auto & Desirable/Desirable works – A & B are correct.
Thanx MN. Entire thing was fuzzy at first, making distinction between 2 to the 0 power is 1 vs. 0 is the first bit, you know? thanks
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Scam!
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