After determining how DHCP relaying works, I made a decision to try it out in a lab. netlab has no DHCP configuration module (in the meanwhile); the best method ahead appeared to be customized configuration templates mixed with a couple of additional attributes.
Lab Topology
That is how I arrange the lab:
- I created easy lab topology with DHCP server (IOSv), DHCP shopper (one other IOSv), and a relaying node that could possibly be something that helps DHCP relaying.
lab topology
Lab IP addressing
Interface IPv4 deal with Description
=========================================================
srv (10.0.0.1/32)
GigabitEthernet0/1 10.1.0.2/30 srv -> relay
relay (10.0.0.2/32)
GigabitEthernet0/1 10.1.0.1/30 relay -> srv
GigabitEthernet0/2 172.16.0.2/24 relay -> person
person (10.0.0.3/32)
GigabitEthernet0/1 172.16.0.3/24 person -> relay
- I used interface attribute dhcp.shopper (boolean) on the shopper and dhcp.server (node title, string) on the relay node. That is how I outlined these attributes:
Additional DHCP attributes
defaults.attributes:
hyperlink.dhcp:
shopper: bool
server: str
DHCP shopper, relay, and server teams
teams:
dhcp_server:
members: [ srv ]
module: [ ospf ]
config: [ dhcp-server ]
system: iosv
dhcp_client:
members: [ user ]
config: [ dhcp-client ]
system: iosv
swap:
members: [ relay ]
module: [ ospf ]
config: [ dhcp-relay ]
- I’m working OSPF between DHCP relay and DHCP server. Whereas that’s not the way you’d arrange a typical DHCP server, it permits me to relay DHCP requests to the DHCP server loopback interface.
- Lastly, I needed to outline the nodes and the hyperlinks:
Nodes and hyperlinks
nodes: [ srv, relay, user ]
hyperlinks:
- relay-srv
- person:
dhcp.shopper: True
relay:
dhcp.server: srv
kind: lan
I set the hyperlink kind on the hyperlink between person and relay swap to LAN to make sure it will get a /24 prefix. Doing DHCP on a /30 prefix is boring.
As all the time, you may discover the ultimate topology file on GitHub.
Configuration Templates
Now for the enjoyable half: customized configuration templates (additionally on GitHub). The shopper template was trivial:
- Discover interfaces with dhcp.shopper attribute
- Take away static IPv4 deal with from them
- Allow DHCP shopper on the interface
DHCP shopper configuration template
{% for intf in interfaces if intf.dhcp.shopper is outlined and intf.dhcp.shopper %}
interface {{ intf.ifname }}
no ip deal with
ip deal with dhcp
{% endfor %}
The relaying template was already a bit extra convoluted. I needed to discover the interfaces with dhcp.server attribute after which discover the loopback IP deal with of the DHCP server to make use of within the helper-address command. Apparently, I may use equivalent template for Cisco IOSv and Arista vEOS.
DHCP relay configuration template
{% for intf in interfaces if intf.dhcp.server is outlined %}
interface {{ intf.ifname }}
ip helper-address {ipaddr('deal with') }
{% endfor %}
Lastly the DHCP server template. This one is a beast:
- It iterates over all different nodes within the Ansible stock and finds interfaces with dhcp.server attribute (relaying interfaces)
- For every relaying interface, the template excludes its IPv4 deal with from the DHCP pool, and creates a corresponding pool with the relaying interface IPv4 deal with because the default router.
- I additionally turned on debugging within the configuration template so I may log into the DHCP server and examine the logs instantly after netlab up completes its job.
DHCP server configuration template
logging buffered
no service timestamp debug
!
do debug ip dhcp server packet
do debug ip dhcp server occasion
!
{% for h,v in hostvars.gadgets() %}
{% for intf in v.interfaces if intf.dhcp.server is outlined and intf.ipv4 is outlined %}
ip dhcp excluded-address {ipaddr('deal with') }
{% endfor %}
{% endfor %}
!
{% for h,v in hostvars.gadgets() %}
{% for intf in v.interfaces if intf.dhcp.server is outlined and intf.ipv4 is outlined %}
!
ip dhcp pool p_{ipaddr('community') }
community {ipaddr('community') } {ipaddr('netmask') }
default-router {ipaddr('deal with') }
{% endfor %}
{% endfor %}
Listed below are the additional configuration instructions generated by these templates:
Cisco IOS DHCP shopper configuration
interface GigabitEthernet0/1
no ip deal with
ip deal with dhcp
Cisco IOS DHCP relay configuration
interface GigabitEthernet0/2
ip helper-address 10.0.0.1
Cisco IOS DHCP server configuration (together with debugging instructions)
logging buffered
no service timestamp debug
!
do debug ip dhcp server packet
do debug ip dhcp server occasion
!
ip dhcp excluded-address 172.16.0.2
!
!
ip dhcp pool p_172.16.0.0
community 172.16.0.0 255.255.255.0
default-router 172.16.0.2
You will discover the last system configurations utilizing Arista EOS on the DHCP relay within the GitHub netlab-example repository.
Does It Work?
You guess. Right here’s the printout from the shopper router:
person#present dhcp lease
...
Temp IP addr: 172.16.0.3 for peer on Interface: GigabitEthernet0/1
Temp sub web masks: 255.255.255.0
DHCP Lease server: 10.1.0.2, state: 5 Certain
DHCP transaction id: EAB
Lease: 86400 secs, Renewal: 43200 secs, Rebind: 75600 secs
Temp default-gateway addr: 172.16.0.2
Subsequent timer fires after: 11:59:35
Retry rely: 0 Shopper-ID: cisco-5254.002c.2b7b-Gi0/1
Shopper-ID hex dump: 636973636F2D353235342E303032632E
326237622D4769302F31
Hostname: person
There appears to be a tiny glitch within the printout: the DHCP relay is forwarding DHCP requests to 10.0.0.1, however the DHCP shopper claims it’s speaking with DHCP server with IP deal with 10.1.0.2 – the LAN interface IPv4 deal with of the DHCP server. The change of IP deal with is an ideal implementation of RFC 2131 which says:
If the server has obtained a message by way of a DHCP relay agent, the server SHOULD select an deal with from the interface on which the message was recieved [sic] because the ‘server identifier’ (until the server has different, higher data on which to make its selection).
It’s good to see issues working precisely the best way they need to 
Combating Repeatability Disaster One Lab at a Time
Need to run this lab by yourself, or attempt it out with completely different gadgets? No drawback:
Coming Up Subsequent
Easy DHCP relaying works, however what about inter-VRF DHCP relaying? That’s the subject of the following weblog put up on this collection.
