OpenStack Migrate Routers and DHCP

Neutron routers and DHCP agents run on network nodes. When you need to take a network node offline for maintenance, you must migrate these services to another network node first. This guide covers migrating L3 routers and DHCP agents in OpenStack 2024.2 Dalmatian.

When to Migrate

• Network node hardware maintenance
• OS patching that requires reboot
• Rebalancing load across network nodes
• Recovering from a failed network node

Prerequisites

Migrating L3 Routers

Step 1: List Routers and Their Agents

# List all L3 agents
openstack network agent list --agent-type l3

# List routers on a specific agent
neutron l3-agent-list-hosting-router <router-id>
# Or with newer CLI:
openstack network agent list --router <router-id>

Step 2: Identify the Source and Target Agent

# Source agent (the one being drained)
SOURCE_AGENT=$(openstack network agent list --agent-type l3 \
  --host network-node-01 -f value -c ID)

# Target agent
TARGET_AGENT=$(openstack network agent list --agent-type l3 \
  --host network-node-02 -f value -c ID)

Step 3: Move Each Router

# List all routers on the source agent
ROUTERS=$(openstack router list --agent $SOURCE_AGENT -f value -c ID)

# Remove from source and add to target
for ROUTER in $ROUTERS; do
  openstack network agent remove router $SOURCE_AGENT $ROUTER
  openstack network agent add router $TARGET_AGENT $ROUTER
  echo "Moved router $ROUTER"
done

Step 4: Verify

openstack network agent list --router <router-id>

The router should now show on network-node-02.

Migrating DHCP Agents

Step 1: List DHCP Agents and Networks

# List all DHCP agents
openstack network agent list --agent-type dhcp

# List networks on a specific DHCP agent
openstack network list --agent $SOURCE_DHCP_AGENT

Step 2: Identify Agents

SOURCE_DHCP=$(openstack network agent list --agent-type dhcp \
  --host network-node-01 -f value -c ID)

TARGET_DHCP=$(openstack network agent list --agent-type dhcp \
  --host network-node-02 -f value -c ID)

Step 3: Move Each Network

NETWORKS=$(openstack network list --agent $SOURCE_DHCP -f value -c ID)

for NET in $NETWORKS; do
  openstack network agent remove network $SOURCE_DHCP $NET
  openstack network agent add network $TARGET_DHCP $NET
  echo "Moved network $NET"
done

Step 4: Verify DHCP Migration

openstack network agent list --network <network-id>

Automating with a Script

Create a drain script for maintenance windows:

#!/bin/bash
SOURCE_HOST=$1
TARGET_HOST=$2

# Drain L3 routers
SRC_L3=$(openstack network agent list --agent-type l3 --host $SOURCE_HOST -f value -c ID)
TGT_L3=$(openstack network agent list --agent-type l3 --host $TARGET_HOST -f value -c ID)

for R in $(openstack router list --agent $SRC_L3 -f value -c ID); do
  openstack network agent remove router $SRC_L3 $R
  openstack network agent add router $TGT_L3 $R
done

# Drain DHCP
SRC_DHCP=$(openstack network agent list --agent-type dhcp --host $SOURCE_HOST -f value -c ID)
TGT_DHCP=$(openstack network agent list --agent-type dhcp --host $TARGET_HOST -f value -c ID)

for N in $(openstack network list --agent $SRC_DHCP -f value -c ID); do
  openstack network agent remove network $SRC_DHCP $N
  openstack network agent add network $TGT_DHCP $N
done

# Disable agents on source
openstack network agent set --disable $SRC_L3
openstack network agent set --disable $SRC_DHCP

echo "Drained $SOURCE_HOST. Safe to reboot."

Usage:

bash drain-network-node.sh network-node-01 network-node-02

Using HA Routers (Recommended)

For production, use HA routers to avoid manual migration. HA routers automatically failover:

# Enable HA routers globally in neutron.conf
[DEFAULT]
l3_ha = true
max_l3_agents_per_router = 2

# Create an HA router
openstack router create --ha ha-router

With HA routers, Neutron maintains a standby copy on another network node. If the primary fails, VRRP triggers automatic failover in seconds.

DHCP Agent Redundancy

Configure multiple DHCP agents per network:

# /etc/neutron/neutron.conf
[DEFAULT]
dhcp_agents_per_network = 2

This ensures DHCP continues working if one agent goes down.

Troubleshooting

Summary

Migrating Neutron routers and DHCP agents is essential for network node maintenance. For production environments, HA routers and multi-agent DHCP eliminate the need for manual migration during planned or unplanned outages.