This document discusses setting up a GRE overlay network to create a distributed switch. It outlines creating GRE tunnels between endpoints, disabling NetworkManager, configuring interfaces and bridges, populating the routing table, allowing GRE traffic through the firewall, and testing connectivity. Troubleshooting steps include checking interfaces, IP addresses, routing tables, and firewall rules.

1. GRE

2. Overlay network
 is built on top of another network
 underlying network

3. GRE Tunnel
 Create a gretap (ethernet over GRE) device
 ip link add ${interface name} type gretap local ${local
endpoint address} remote ${remote endpoint address}
 Create a GRE tunnel
 ip tunnel add ${interface name} mode gre local ${local
endpoint address} remote ${remote endpoint address}

4. Goal: Distributed switch
192.168.10.10/24 IP1/IP2 IP3/IP4 IP5/IP6 192.168.10.11/24
GRE tunnel

6. Steps
 Disable NetworkManager
 Setup interfaces (ethX, gretap1, br0)
 Populate routing table
 Allow GRE network traffic (firewall)
 Test network configuration (ping,tcpdump)

7. Key commands
 Bridge
 ip link add br0 type bridge
 ip link set eth2 master br0
 GRE
 ip link add gretap1 type gretap local 192.168.11.2
remote 192.168.12.2
 Interface
 ip link set up dev eth1
 Routing table
 ip route add 192.168.12.0/24 via 192.168.11.254

8. Troubleshooting steps
 Interfaces
 ip link
 IP address/subnet
 ip addr show
 Routing tables
 ip route
 Firewall
 firewalld/iptables