Dynamic routing protocols such as RIP adapt the entries in dynamic routing tables when, for example, a route is no longer available. When using VRRP, hosts in the LAN can use a static routing table even though the gateway IP address may change, for example, when a device fails due to a defect and another device takes over its functions. VRRP uses "virtual routers" in the routing tables so that the network users always find the right gateway nevertheless. A virtual router is announced on the network with the IP address '192.168.1.100' in the same way as a "normal" router would be and takes over the function of a gateway to certain remote sites. The actual work of data transfer is carried out by the physical routers behind the virtual router.
- Under normal operating conditions, for example, router A with the IP address '192.168.1.1' establishes the connection to the Internet.
- If router A fails, then router B with the IP address '192.168.1.2' takes over the functions of router A. The network clients do not notice this change; for them, the "virtual" router '192.168.1.100' is still the gateway.
From a more technical standpoint, a router in a network requires a unique MAC address in addition to an IP address. Therefore, when defining a virtual router, a virtual MAC address is defined simultaneously which the virtual router reacts to. The virtual MAC address is formed as '00-00-54-00-01-xx', whereby 'xx' stands for the unique router ID.
In order to determine which physical router reacts to the combination of virtual IP and MAC address, priorities are used for the physical routers. For this purpose, every physical router is assigned a priority. The router with the highest priority takes over the functions of the virtual router as master and thus reacts to the virtual IP and MAC addresses. If two physical routers have the same priority, then the router with the "higher" physical IP address is considered to be the master.
All physical routers report their availability on a regular basis so that, should the current master fail, the router with the next highest priority can take over the routing function at the end of this interval at the latest. If a device determines that it cannot complete the tasks required, it can actively log off before the end of the interval thereby triggering the transfer of the master role to the router with the next priority.
The major advantage of virtual routers is that they enable very flexible scenarios with backup and load balancing functions which remain virtually undetected by the LAN. Clients in the local network randomly select a DHCP server from those available and retrieve the required address information from this server.
Address assignment via DHCP with more than one DHCP server in the LAN
Several DHCP servers can be operated parallel to each other in a LAN without disrupting one another's functionality. Upon establishing a network connection, the DHCP clients request an IP address selecting one of the available DHCP servers. The DHCP server receiving the request checks to determine whether the address requested is available or already in use within the LAN before assigning the address. This check prevents address conflicts even when several DHCP servers are in use.
For the clients, it is irrelevant which physical router subsequently establishes the data connection. Similarly, the LAN clients do not notice when a router or WAN interface fails due to the fact that, in this case, another router steps in and is available under the same virtual addresses as before.
