Chapter 12: Hot Standby Routing Protocol

In Depth

Dynamic environments are constantly growing. I happen to work in one and I see the demand for 99.99 percent reliability increasing every day. However, even in a worldwide Enterprise network, high availability solutions are not prepared for various network failures. Here’s an example: I manage a few hundred servers at a Fortune 100 company. I’m trying to get to a local intranet site and my browser just hangs. Then I try to ping the server that is across subnets and the request times out. I can ping everything and anything within my subnet, but then I try the default gateway. Does this sound all too familiar? Think of the customer impact, the data warehouses that are unreachable, or online training sessions where students cannot connect. I find it hard to believe sometimes that redundancy is not in place.

The point I’m trying to make is that department budgets or deep pockets do not always solve the problem. Network and systems administrators need help, especially when it is usually a team of about four to six engineers that manage a dynamic networked enterprise environment. If this was a Cisco switched campus model, which would overcome these IP−related issues and provide network redundancy, every device would have a backup device standing by in case of a failure. Route processor devices (such as internal cards in multilayer switches and routers) would have been among the hardest devices to configure for fast convergence redundancy.

The Cisco switched campus model builds redundancy into the Layer 2 switch block level devices. However, Hot Standby Routing Protocol (HSRP) is designed to build redundancy into the Layer 3 routing devices found in the Distribution layer of a network. It also provides convergence in seconds without manual intervention from a network administrator while remaining transparent between other interfaces on the network. HSRP can be applied to almost any LAN environment.

One primary feature of HSRP, which allows it to be so transparent to users, is its use of priority schemes. These priorities are used to determine which router is set as the default active router. When a router is manually assigned a priority, the standby interface that has a higher priority is selected as the active router.

HSRP is one of the best solutions when host interfaces on a local LAN segment require continuous access to network resources.

Routing Problems

Within a standard client/server network, the ability to exchange routing information between segments is allowed by Layer 3 address translation. However, although the clients may route the packet to its default gateway, they cannot route beyond their local LAN segment.

Note The default gateway is the protocol address for the route processor to which data packets containing a destination address outside the local segment are sent.

In order for the client to route information out of its local LAN segment, it must use a manually configured IP stack; or, the client may be configured for Dynamic Host Configuration Protocol (DHCP) to determine a default gateway. In any event, if the Layer 3 routing device assigned as the default gateway fails or is offline, all devices located on that particular subnet or network will only be allowed to communicate with each other. The local collision or broadcast domain becomes the entire network in the eyes of each device. You can place another default gateway on the network, but there is no clear way to provide a secondary configuration to another default route without manually resetting the default gateway on either the client or server.

243

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