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Routers, switches and other equipment that make up the core of the network have multiple MAC addresses, but this equipment can be modular meaning interface cards and their associated MAC addresses can be moved between devices. This would split the MAC address list which makes using the MAC address to uniquely identify a router difficult.
The same problem occurs with desktop machines when you switch the NIC but because there is only one MAC address Nomad would simply treat this as a new machine. Some machines can have multiple NIC's in which case Nomad would treat each MAC address as a separate machine.
Using IP addresses has the same problem, a subnet can easily be moved between routers, so Nomad uses the device name to identify equipment.
A device is a router, switch, managed hub or any other piece of kit that is not an end user machine or printer. Most switches are self contained units but for some kit like backbone routers are more modular, this means a card can be moved from found on the backbone
To keep things generic nomad only uses standard MIB variables to discover network details. rfc1213 MIB-II rfc1493 Definitions of Managed Objects for Bridges rfc1573 Evolution of the Interfaces Group of MIB-II Most network routers and switches support MIB-II if they have snmp support. This is what is used gather most network details. Switches and bridges also support rfc 1493 which defines a MIB specifically for layer 2 hardware and allow nomad to grab the bridging tables from devices. rfc 1573 defines a simple extension to the interface table as provided by MIB-II, nomad only uses this mib to discover shorter names for interfaces, everything will still work fine if this MIB is not supported.