5.4 Troubleshoot client- and router-based DHCP connectivity issues

Screenshot_2018-12-06_07-00-46

In order for PC-1 to to obtain a DHCP lease from R1 there needs to be L3 connectivity from PC-1 to R1.

Therefore the following needs to happen:

  1. R1 needs to be aware of the PC-1 subnet (I used RIPv2)
  2. R2 needs to be configured as a DHCP relay for the g1/0 interface
  3. R1 needs to be configured with a DHCP pool

To troubleshoot connectivity I used extended pings on R2:

Router#ping 
Protocol [ip]: 
Target IP address: 10.0.0.1
Repeat count [5]: 
Datagram size [100]: 
Timeout in seconds [2]: 
Extended commands [n]: y
Source address or interface: 172.16.100.1
Type of service [0]: 
Set DF bit in IP header? [no]: 
Validate reply data? [no]: 
Data pattern [0xABCD]: 
Loose, Strict, Record, Timestamp, Verbose[none]: 
Sweep range of sizes [n]: 
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.0.0.1, timeout is 2 seconds:
Packet sent with a source address of 172.16.100.1 
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/16/36 ms
Router#

I also used debug commands on R1/R2:

R2#debug ip dhcp server events 
DHCP server event debugging is on.
R2#
*Dec  6 07:44:16.947: DHCPD: Sending notification of DISCOVER:
*Dec  6 07:44:16.951:   DHCPD: htype 1 chaddr 0050.7966.6800
*Dec  6 07:44:16.951:   DHCPD: remote id 020a0000ac10640110000000
*Dec  6 07:44:16.951:   DHCPD: circuit id 00000000
*Dec  6 07:44:16.951: DHCPD: Seeing if there is an internally specified pool class:
*Dec  6 07:44:16.951:   DHCPD: htype 1 chaddr 0050.7966.6800
*Dec  6 07:44:16.955:   DHCPD: remote id 020a0000ac10640110000000
*Dec  6 07:44:16.955:   DHCPD: circuit id 00000000
*Dec  6 07:44:16.987: DHCPD: Forwarding reply on numbered intf
*Dec  6 07:44:17.955: DHCPD: Seeing if there is an internally specified pool class:
*Dec  6 07:44:17.959:   DHCPD: htype 1 chaddr 0050.7966.6800
*Dec  6 07:44:17.959:   DHCPD: remote id 020a0000ac10640110000000
*Dec  6 07:44:17.959:   DHCPD: circuit id 00000000
*Dec  6 07:44:17.959: DHCPD: there is no pool for 172.16.100.1.
*Dec  6 07:44:17.983: DHCPD: Forwarding reply on numbered intf
DHCP_SERVER#debug ip dhcp server events
DHCP server event debugging is on.
DHCP_SERVER#
*Dec  6 07:43:32.747: DHCPD: checking for expired leases.
*Dec  6 07:43:36.987: DHCPD: Sending notification of DISCOVER:
*Dec  6 07:43:36.991:   DHCPD: htype 1 chaddr 0050.7966.6800
*Dec  6 07:43:36.991:   DHCPD: remote id 020a00000a00000100000000
*Dec  6 07:43:36.991:   DHCPD: circuit id 00000000
*Dec  6 07:43:36.991: DHCPD: Seeing if there is an internally specified pool class:
*Dec  6 07:43:36.991:   DHCPD: htype 1 chaddr 0050.7966.6800
*Dec  6 07:43:36.995:   DHCPD: remote id 020a00000a00000100000000
*Dec  6 07:43:36.995:   DHCPD: circuit id 00000000
*Dec  6 07:43:37.987: DHCPD: Sending notification of ASSIGNMENT:
*Dec  6 07:43:37.987:  DHCPD: address 172.16.100.2 mask 255.255.255.0
*Dec  6 07:43:37.987:   DHCPD: htype 1 chaddr 0050.7966.6800
*Dec  6 07:43:37.987:   DHCPD: lease time remaining (secs) = 2678400

DHCP is a L3 protocol so anything effecting L3 connectivity will also effect DHCP:

  1. Missing/incorrect routes
  2. Interface issues
  3. ACLs

If the DHCP lease is successful but the server’s DHCP configuration is incorrect such as:

  1. Incorrect/missing default gateway
  2. Incorrect/missing DNS servers
  3. Subnet misconfiguration

This will result in a successful lease but will result in lack of connectivity.

PC-1> ip dhcp
DORA IP 172.16.100.2/24 GW 172.16.100.1

PC-1> ping 10.0.0.1
84 bytes from 10.0.0.1 icmp_seq=1 ttl=254 time=19.646 ms
84 bytes from 10.0.0.1 icmp_seq=2 ttl=254 time=19.753 ms
84 bytes from 10.0.0.1 icmp_seq=3 ttl=254 time=12.660 ms
84 bytes from 10.0.0.1 icmp_seq=4 ttl=254 time=12.738 ms
84 bytes from 10.0.0.1 icmp_seq=5 ttl=254 time=13.788 ms

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