Cisco Systems RJ-45-to-AUX manual Routing Information Protocol, Proxy ARP, 244

The next sections look at some ways that administrators and engineers have devised to overcome problems related to assigning default gateways, along with the benefits and difficulties in using each technique.

Routing Information Protocol

Once solution designed to inject Layer 3 routing redundancy allows nodes utilizing IP to use Routing Information Protocol (RIP) to discover secondary routers located on the network. This method allows the end−user node to maintain a routing table that determines the shortest path by calculating the number of hops and using the router with the fewest hops.

However, RIP has a very slow convergence time when a change in the network topology occurs. It may take up to three times the update interval setting before RIP chooses another default route from the table.

Proxy ARP

Another solution to creating redundancy for Layer 3 uses Proxy Address Resolution Protocol (Proxy ARP). The Proxy ARP broadcasts an IP ARP request for the Media Access Control (MAC) address of the router. The router replies with the MAC address of the requesting node.

If a failure occurs, however, the node configured for the default gateway has two options:

∙Reboot the node.

∙Wait for the ARP update and the flush period of the ARP entry to expire.

This delay creates a very high convergence period.

ICMP Router Discovery Protocol

ICMP Router Discovery Protocol (IRDP) is probably one of the longest protocol names to come along. IRDP is one of the most commonly used solutions for locating a redundant router in the event of a gateway failure. It is available only when HSRP is not configured.

IRDP is an extension of Internet Control Message Protocol (ICMP); it allows network administrators to use such commands as ping and trace between interfaces to diagnose network−related problems. ICMP provides mechanisms to allow routers to advertise default routes available for any network.

This protocol has many features—unlike RIP and Proxy ARP—for discovering the addresses of neighboring routers. ICMP requires no additional configuration by an administrator.

Hosts that use IRDP listen for IRDP advertisements from the default router. In the event a host interface does not receive IRDP advertisements during a predefined value (sometimes referred to as the lifetime value), the host interface considers the default route void and will begin to choose another route to a remote network.

IRDP has a high convergence time in the event of a failure. The default lifetime of the default route is 30 minutes, and advertisements are sent every 7 to 10 minutes. The router controls the interval at which the advertisement messages are sent. IRDP requires you to configure two separate intervals on the router: the minimum advertising interval and the maximum advertising interval. All advertisements are sent during that window of time. So, a change in the network topology can go unnoticed for almost 30 minutes.

Cisco developed HSRP to address and resolve these problems faced by network administrators.

244