Introduction to Split Ownership in IP Allocation

Split ownership occurs when two or more administrative domains claim authority over the same IP address space without a synchronized source of truth. Common causes include overlapping DHCP pools, static address assignments conflicting with central DHCP services, IPAM silos, and automation scripts lacking atomic reservation semantics.

Impact of Split Ownership on Network Infrastructure

When split ownership exists, the network exhibits duplicate MAC-to-IP bindings, intermittent reachability, increased broadcast traffic from gratuitous ARP storms, and difficult cleanup due to stale aliases and ARP caches.

Duplicate Addressing and Its Consequences

Duplicate addressing occurs when multiple devices claim the same IP address on the same broadcast domain. This can be reproduced in a Containerlab lab by launching two FRR routers with overlapping DHCP pools or static IP assignments.

Effects of Duplicate Addressing on Network Communication

Duplicate addressing causes ARP confusion, traffic blackholing, transport layer retransmissions, and application timeouts. These issues can be observed using ip and arp commands:

# On r1
sudo ip -4 addr show dev eth1
# On r2
sudo ip -4 addr show dev eth1
# From a host on the same segment
arp -n | grep 10.0.0.10

Gratuitous ARP Noise and Its Effects

Gratuitous ARP (GARP) is used to announce new IP-MAC bindings, detect duplicate address usage, and update neighboring caches. When duplicate addressing exists, each host emits a GARP, creating a rapid ARP ping-pong.

Mitigating Gratuitous ARP Noise

Mitigation focuses on eliminating the root cause (duplicate IP) rather than suppressing GARP. Temporary silencing can hide symptoms but does not resolve the underlying conflict. GARP noise can be observed using tcpdump:

sudo tcpdump -i any -nn -e arp

Brittle Cleanup and Its Challenges

Brittle cleanup refers to the fragile process of removing stale configuration or state after a network event. Causes include interface down/up cycles, persistent ARP cache entries, leftover DHCP client state files, and incomplete automation.

Consequences of Brittle Cleanup in Network Infrastructure

Brittle cleanup leads to persistent duplicate alerts, unpredictable forwarding, increased troubleshooting latency, and configuration drift.

Troubleshooting Split Ownership Issues

Troubleshooting involves identifying symptoms, verifying layer-2 and layer-3 configurations, and inspecting DHCP leases.

Tools and Techniques for Troubleshooting

Use arping, tcpdump, ip neigh flush, clab inspect, and FRR vtysh to troubleshoot split ownership issues:

arping -c 3 -I eth1 10.0.0.10
tcpdump -i any -nn -e arp host 10.0.0.10
sudo ip neigh flush all
clab inspect --nodes
vtysh -c "show ip route 10.0.0.0/24 longer-prefixes"

Repair Sequence to Avoid Exacerbating the Issue

To repair split ownership issues, follow these steps:

1. Document the duplicate address and all interfaces claiming it.
2. Identify the source of truth (e.g., DHCP or static assignment).
3. Schedule a maintenance window if the address is in use by production services.
4. Backup current configs (vtysh -c "show running-config" > r1.cfg.bak).

Minimal Repair Steps to Resolve Split Ownership

1. Choose the authoritative source (e.g., keep DHCP, remove static).
2. Remove the conflicting static address on the offending router(s):

sudo ip addr del 10.0.0.10/24 dev eth1

3. Flush ARP caches on all devices in the broadcast domain:

sudo ip neigh flush dev eth1

4. Restart networking services if needed (e.g., FRR, DHCP client):

sudo systemctl restart frr
sudo dhclient -r eth1 && sudo dhclient eth1

5. Verify that only one device now answers ARP for the address:

arping -c 2 -I eth1 10.0.0.10