What is the recommended guideline for configuring DHCP service in PCM

Answer

1:  How do we modify dhcpd.conf so that XCAT will not overwrite it?     
Answer:
Normally dhcpd.conf does not get overwritten unless a node is removed/added/provisioned. However dhcp.leases file gets changed frequently.                                                             
                                                                        
2. How do we increase the lease duration?  
Answer:
Default lease duration is set to 12 hours. The duration can be changed by editing 'site' table in xcat.

• Steps                                                                   
  A. Edit xcat site table using following command on the head node     
  #tabedit site
  
  B. Change the following line to increase lease duration to 1 week (604800 seconds)   
  "dhcplease","604800",
  Optionally you can set the value to -1 which will set it to never expire.
  
  C. Update DHCP configuration files                                                   
  #makedhcp -n                               
  
  D. Restart dhcp service
  #service dhcpd restart
  
  E. Verify dhcpd.conf file
  #cat /etc/dhcp/dhcpd.conf |grep lease



 
•  

3.  Why is the dhclient constantly requesting for renewal (which is seen in syslog)?

2016-05-01T04:16:54.100050-05:00 server01 dhcpd: DHCPREQUEST for 172.24.14.26 from 7c:ae:8b:34:d4:20 via eth1
2016-05-01T04:16:54.100073-05:00 server01 dhcpd: DHCPACK on 172.24.14.26 to 7c:ae:8b:34:d4:20 via eth1
2016-05-01T04:16:54-05:00 server16 dhclient[2120]: DHCPACK from 172.24.14.11 (xid=0xd5f1b49)
2016-05-01T04:16:56-05:00 server16 dhclient[2120]: bound to 172.24.14.26 -- renewal in 17886 seconds.

Answer:
DHCP client determines the maximum lease time based on the algorithm that it uses (max lease time). At half time it checks with DHCP server with the current address information to see if it needs to renew.
What the logs shows is the typical behavior of DHCP architecture (based on the RFC2131). ie at every half time of the max lease time the client checks with dhcp server. The dhcp server then informs the dhclient that you are not due for renewal. So the dhclient says ok my next check in is (0.5* max_lease_time) value. By default, the lease time is 43200 seconds( 12 hours) in PCM/PHPC.