Section 9.1: Traditional Redundancy Methods

There are various traditional redundancy methods that have been used, but for one reason or another are ineffective in some failure modes. These methods include default gateways, proxy ARP, RIP, and IRDP.

9.1.1: Default Gateways

In a network where a router is responsible for routing packets for a subnet, the subnet will become isolated should the router go down as the computer do not have the capability to collect and exchange routing information. These devices are configured with a single default gateway IP address. If the router that is the default gateway fails, the device is limited to communicating only on the local subnet and is disconnected from the rest of the network. Even if a redundant router exists that could serve as a default gateway, there is no way for the computer to dynamically switch to a new default gateway if the present default gateway fails.

9.1.2: Proxy ARP

Proxy ARP is used when the router is responding with its own MAC address as a proxy for some other host on a different subnet, thus, a host computer sending a packet to a remote station considers the destination to be on the same subnet as itself. Because the router knows where the destination MAC is located or via the routing table, the router forwards the packets to the real destination. However, should the router go down and the host computer continues to send packets destined for a remote station, the packets will be dropped. A reboot of the host computer will fix the problem by clearing the ARP cache, or a force of another ARP request will pick up a failover router. However, there would be a significant delay in communications, either to reboot the host or to initiate an ARP.

9.1.3: Routing Information Protocol (RIP)

The first routing protocol to discover available routers in networks was the Routing Information Protocol (RIP). In this method, the workstation holds a routing table, which lists routes and associated next hops that have a path to the destination. It is then up to the workstation to choose the best path. RIP is a distance vector routing protocol, which means it relies on the number of hops. However, RIP has a limitation of 15 hops and its network convergence is slow. In an unstable network, this can cause problems.

9.1.4: ICMP Router Discovery Protocol (IRDP)

Some newer IP hosts use the ICMP Router Discovery Protocol (IRDP) to find a new router when a route becomes unavailable. IRDP is not a routing protocol but is an extension to ICMP. It provides a mechanism for routers to advertise useful default routes. A host that runs IRDP listens for hello multicast messages from the preferred default router. As long as the host detects these hello messages, the MAC address for the router generating the hello messages is used as the destination MAC address by the host. The IRDP-based advertisements are considered valid only for a predefined lifetime value. If a new advertisement is not received during that lifetime, the router address is considered invalid and the host removes the corresponding default route. The host then uses an alternate router, which generally does not represent the shortest path to the destination station.