Last week we explored the basics of unnumbered IPv4 Ethernet interfaces, and how you could use them to save IPv4 address space in routed access networks. I also mentioned that you could simplify the head-end router configuration if you’re using DHCP instead of per-host static routes.
Obviously you’d need a smart DHCP server/relay implementation to make this work. Simplistic local DHCP server would allocate an IP address to a client requesting one, send a response and move on. Likewise, a DHCP relay would forward a DHCP request to a remote DHCP server (adding enough information to allow the DHCP server to select the desired DHCP pool) and forward its response to the client.
What we need to make unnumbered IPv4 Ethernet interfaces work with DHCP is an extra step: as a DHCP reply (locally originated or relayed) is sent to a client connected over an unnumbered IPv4 interface, a static host route is added to the router’s routing table to make that client reachable through the router.
I’ve first seen static routes created from DHCP responses in DHCPv6 relays supporting IPv6 prefix delegation with IA_PD option. A few years later, DHCPv6 relays started adding host routes based on IA_NA (address allocation) responses. Similar behavior is now available in some IPv4 DHCP implementations1.
Does It Really Work?
A lab test always beats academic discussions, and we already have the lab built. We’ll use the same lab setup as the last time with a slightly different IP addressing. Instead of static IP addresses and static routes, we’ll configure DHCP server on the access router, and use DHCP-based address allocation on the clients.
To make our setup work we have to configure DHCP client on c1 and c2. I used Cisco IOS XE for the clients; I could have used Linux VMs or containers, but then I’d have to figure out how to configure DHCP client on a Linux box. Call me lazy…
interface GigabitEthernet2 description c1 -> rtr [dhcp] ip address dhcp
On the access router we have to configure:
- A DHCP pool;
- A loopback interface with an IP prefix exactly matching the DHCP pool IP prefix;
- Unnumbered access interfaces.
interface Loopback1 ip address 10.42.42.1 255.255.255.0 ! ip dhcp pool Access network 10.42.42.1 255.255.255.0 default-router 10.42.42.1 ! interface GigabitEthernet2 no ip address ip unnumbered Loopback1 interface GigabitEthernet3 no ip address ip unnumbered Loopback1
After a few seconds, we’ll see the DHCP bindings2:
rtr#show ip dhcp binding Bindings from all pools not associated with VRF: IP address Client-ID/ Lease expiration Interface Hardware address/ User name 10.42.42.2 0063.6973.636f.2d35. Jun 07 2021 07:27 AM GigabitEthernet3 3235.342e.3030.3438. 2e32.6462.362d.4769. 32 10.42.42.3 0063.6973.636f.2d35. Jun 07 2021 07:27 AM GigabitEthernet2 3235.342e.3030.6333. 2e61.3066.382d.4769. 32
At the same time, the static host routes pointing to allocated DHCP addresses are added to the IP routing table:
rtr#show ip route 10.42.42.0 longer-prefixes | begin Gateway Gateway of last resort is 192.168.121.1 to network 0.0.0.0 10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks C 10.0.0.3/32 is directly connected, Loopback0 C 10.42.42.0/24 is directly connected, Loopback1 L 10.42.42.1/32 is directly connected, Loopback1 S 10.42.42.2/32 is directly connected, GigabitEthernet3 S 10.42.42.3/32 is directly connected, GigabitEthernet2
From last week’s blog post we know that’s enough to make the IP routing work – we can ping c2 (10.42.42.2) and the other loopback interface of rtr (10.0.0.3) from c1.
c1#ping 10.42.42.2 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 10.42.42.2, timeout is 2 seconds: ..!!! Success rate is 60 percent (3/5), round-trip min/avg/max = 1/1/2 ms c1#ping 10.0.0.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 10.0.0.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms
Want to check my lab results? The final router configurations are on GitHub.
Want to build a lab in a few minutes and have it configured in seconds? You’ll have to invest a bit of time first:
- Install Vagrant and libvirt. If you’re running your lab on Windows or MacOS host, use a Ubuntu virtual machine.
- Build CSR 1000v box
- Install netsim-tools
After you got everything up and running:
- Copy my lab topology into dhcp.yml in an empty directory (or clone my netsim-examples repository)
- Execute **create-topology -t dhcp.yml -p -i -c ** to create Vagrantfile, Ansible inventory and Ansible configuration file
- Execute initial-config.ansible to configure the devices.
You could apply the configuration changes outlined in this blog post manually. I decided to automate them:
- Add client-side and server-side Jinja2 templates to your directory
- Execute config.ansible -e config=dhcp-server.j2 -l rtr3 to configure the DHCP server
- Execute config.ansible -e config=dhcp-client.j2 -l c1,c2 to configure DHCP clients
- Use connect.sh to connect to the routers and inspect the results.
At least in IOS XE release 16.6, pointers to other implementations are most welcome – please write a comment. ↩︎
I removed the type and state columns from the printout to squeeze it into available width. ↩︎
config.ansible script is an Ansible playbook and accepts the usual ansible-playbook parameters. The -e parameter sets an extra variable, the -l parameter limits the playbook run to a subset of devices. ↩︎