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Access-list Questions 2

January 27th, 2021 Go to comments

Question 1

Explanation

The established keyword is only applicable to TCP access list entries to match TCP segments that have the ACK and/or RST control bit set (regardless of the source and destination ports), which assumes that a TCP connection has already been established in one direction only. Let’s see an example below:

access-list_established.jpgSuppose you only want to allow the hosts inside your company to telnet to an outside server but not vice versa, you can simply use an “established” access-list like this:

access-list 100 permit tcp any any established
access-list 101 permit tcp any any eq telnet
!
interface S0/0
ip access-group 100 in
ip access-group 101 out

Note:

Suppose host A wants to start communicating with host B using TCP. Before they can send real data, a three-way handshake must be established first. Let’s see how this process takes place:

TCP_Three_way_handshake.jpg

1. First host A will send a SYN message (a TCP segment with SYN flag set to 1, SYN is short for SYNchronize) to indicate it wants to setup a connection with host B. This message includes a sequence (SEQ) number for tracking purpose. This sequence number can be any 32-bit number (range from 0 to 232) so we use “x” to represent it.

2. After receiving SYN message from host A, host B replies with SYN-ACK message (some books may call it “SYN/ACK” or “SYN, ACK” message. ACK is short for ACKnowledge). This message includes a SYN sequence number and an ACK number:
+ SYN sequence number (let’s called it “y”) is a random number and does not have any relationship with Host A’s SYN SEQ number.
+ ACK number is the next number of Host A’s SYN sequence number it received, so we represent it with “x+1”. It means “I received your part. Now send me the next part (x + 1)”.

The SYN-ACK message indicates host B accepts to talk to host A (via ACK part). And ask if host A still wants to talk to it as well (via SYN part).

3. After Host A received the SYN-ACK message from host B, it sends an ACK message with ACK number “y+1” to host B. This confirms host A still wants to talk to host B.

Question 2

Comments
  1. nhema
    April 19th, 2021

    Answer should be Option A. Option B shows line vty 0 15, that is not matched with exabit, line vty 5 15

  2. ldnl
    April 27th, 2021

    May I ask why is letter C not also the correct answer for Q1? The only difference is the access-list number used

  3. nottooshabby
    April 30th, 2021

    ldnl – The access-list number determines if it is a standard or extended access-list. A standard access-list (1-99) does not support protocol based rules. You can only pick a source and destination address. Extended access lists allow this, and that is why it is correct. I hope that helps.

  4. nottooshabby
    April 30th, 2021

    nhema – vty 0 15 includes all vty lines from the exhibit, so that is why it is the correct answer. Option A only includes the first 5 vty lines, and ignores the rest. That is why it is incorrect.

  5. CC
    May 30th, 2021

    @idnl access-list 10 is not extended that’s why the correct answer is D.

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