Home > Ticket 13 – DHCP Helper-address

Ticket 13 – DHCP Helper-address

April 24th, 2018 in TSHOOT v2 Go to comments

Configuration on DSW1:

!
interface Vlan 10
 ip address 10.2.1.1 255.255.255.0
 ip helper-address 10.2.21.129
!

Note:
+ In this ticket you will find port-security configured on ASW1 but it is not the problem as the port-security is good (check with the “show interface fa1/0/1” command on ASW1 and you will see it is still in up/up state. Also if we use “ipconfig” command on Client1, we will see APIPA address (169.254.x.x).
+ 10.1.21.129 is the loopback1 interface of R4. The wrong IP address of 10.2.21.129 does not exist.

Ans1) DSW1
Ans2) IP DHCP Server (or DHCP)
Ans3) on DSW1 delete “ip helper-address 10.2.21.129” and apply “ip helper-address 10.1.21.129” command

Comments (9) Comments
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  1. how to dhcp
    April 26th, 2019

    Verify:
    ipconfig on Client 1 & 2
    ping 209.65.200.241

    on ASW1:
    show interfaces status – check if you have any down or in err-disabled statue
    show acces-list – this is routine just to check if something is blocking vlan 10 traffic and the ip subnets related to R4
    show vlan – verify that vlan 10 exists and ports are assigned

    on DSW1:
    show ip int brief – verify SVI 10 is up and has IP address
    show vlan – verify that VLAN 10 exists, otherwsie SVI won’t route for it
    show run int vlan 10 – check config on SVI
    ping 10.2.21.129 – try to ping current helper-address, if you can’t you have to check if it is the correct IP

    Remember that per topology Client 1 needs to obtain IP from 10.2.1.0/24 range and Client 2 from 10.2.2.0/24 range.
    We also have an arrow on the topology pointing to R4 for DHCP service.
    The idea of helper-address is to relay the DHCP packets to network device that is able to process discover/offer/request/ack packets
    So whatever IP you have in “ip helper-address” it needs to be reachable , i.e IP connectivity, we know that DSW1 has a default route via EIGRP towards R4
    – therefore the only requirement left is R4 to have active interface with an IP address of whatever is in the “ip helper-address” command
    – Our sim is configured in a way that DSW1 cannot ping 10.2.21.129 because that IP doesn’t exist on the network

    on R4:
    show run
    In our sim we see “network 10.2.1.0 255.255.255.0” and is the pool where Client 1 only will obtain his IP from
    We also have:
    ip dhcp excluded-address 10.2.1.1 10.2.1.2
    ip dhcp excluded-address 10.2.1.254
    – this is just a list of IP addresses that should not be assigned, this is not a range
    – these are the IP addresses of the SVIs for VLAN 10 on DSW1/2
    – you also have the exclude 10.2.1.254 which is the HSRP virutal IP on DSW1/2

    show ip int brief – check if there are any interfaces with 10.2.21.129, there aren’t
    – remember the default route in EIGRP is redistributed from OSPF, so ideally you are looking for a network that is reachable via OSPF on
    – this can aslo work with “ip helper-address 10.1.4.5”

    show ip ospf database – you can see LSA Type 1 for 10.1.21.129
    show ip int brief -> Loopback 1 has IP of 10.1.21.129

    on DSW1
    ping 10.1.21.129 -> success
    conf t
    int vlan 10
    no ip helper-address 10.2.21.129
    ip helper-address 10.1.21.129

    The configuration will work even if you point the helper-address to any of the other interface IPs of R4, as long as they are reachable.
    The only requirement is that R4 is able to process DHCP packets and is one-hop away from DSW1.

  2. Forever More
    May 11th, 2019

    Took the exam yesterday
    (This site is spot on) – use it to study

    This question took me almost an hour and cost me the time required to answer all the rest of the questions (I answered 18 of 24) and thus failed

    Don’t let time derail you

  3. Johnny
    May 13th, 2019

    Passed on May 10th 2019 with 946/1000.
    24 questions on exam: 10 MCQs, 12 tickets, BGP and HSRP simlet.
    Thanks mm for detailed review and you guys here as well as Networktut team.
    Chinese dump is valid however MCQs answers in it are not totally correct.
    You should read all recent comments here to get your own correct answers.
    Good luck!

  4. Johnny
    May 13th, 2019

    Contact: ballking242(at)gmail(dot)com

  5. abdou
    May 18th, 2019

    hi
    please sent dump :{email not allowed}

  6. abd
    May 18th, 2019

    please sent dump :rouam10(at)yahoo(dot)fr

  7. Anonymous
    July 24th, 2019

    Hey guys, this answer confused me. I was confused moreso becuase I couldnt understand why the loopback was picked (there is nothing in the configs that point anything dhcp related to the loopback interface, which really confused me).

    so here goes:

    BEST PRACTICE states that your helper address should be the LOOPBACK address, because its a virtual interface, and never goes down.

    so you have the option to pick between loopback 1, or loopback 0.
    The only correct answer shows Loopback 1 being the preferred ip-helper address.

    i hope this helps clear up anything.

  8. PJ
    August 18th, 2019

    This ticket in Packettracer is broken. Networktut please can you take a look. IPhelper is correct, but still no IP addr on client1

  9. eLMo
    August 27th, 2019

    Could we check this issue by another way? If both clients have AIPA IPs and they are in the same vlan, we could ping each other (fix dhcp). If not, ports security was applied…

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