Hot Standby Router Protocol

So what problem are we trying to solve? We've looked at solutions that try to solve the problem of a network failure, but for various reasons don't necessarily fix the problem. Enter Hot Standby Router Protocol. HSRP addresses the problem caused by first-hop failures generally having static default gateway addresses on hosts. Previously, a failure at the default gateway address would leave the host unable to communicate outside of its own subnet. Now with HSRP, the default gateway is a virtual-router address. A failure of the active router would result in a switch to the standby router, and packets would continue to be forwarded.

Cisco routers use HSRP, which enables end stations to continue communicating throughout the network even when the default gateway becomes unavailable.

With HSRP, a set of routers works together to represent a single virtual standby router. The standby router group functions as a single router configured with a virtual IP and MAC address, distinct from the physical routers in the network.

Because the routers in the standby group route packets sent to a virtual address, packets are still routed through the network even when the router originally forwarding the packets fails.

HSRP allows one router to automatically assume the function of the second router if the second router fails. HSRP is particularly useful when the users on one subnet require continuous access to resources in the network.

If the primary or lead router of a group of HSRP routers fails, a standby router in the same group begins to forward traffic for the HSRP group.

The routers decide within the group which router forwards traffic for the virtual address. At regular intervals, the routers exchange information to determine which routers are still present and able to forward traffic.

When routers are configured to be part of an HSRP group, the routers recognize their own native MAC address, plus the HSRP group MAC address.

Routers whose Ethernet controllers only recognize a single MAC address will use the HSRP MAC address when performing as the active router and the burn-in address (BIA) when in standby mode or not speaking.

NOTE The router also sends a gratuitous ARP when it becomes active in order to make the end stations aware of the MAC address change.

HSRP Group Members

As shown in Figure 9-3, the HSRP group consists of the following members:

• Active router

• Standby router

• Virtual router

• Other routers

Figure 9-3 HSRP Group Members

Figure 9-3 HSRP Group Members

Each of these members will be discussed in detail later in this chapter.

To facilitate load sharing, a single router may be a member of multiple HSRP standby groups on a single segment. Each standby group emulates a single virtual router. There is a limit of 255 standby groups on any given LAN.

NOTE Some platforms do not support multiple HSRPs because of the single MAC address per interface restriction. This restriction can be lifted with the use of the standby use-bia command.

Figure 9-4 illustrates that both Router A and Router B are members of Groups 10 and 20. However, Router A is the active forwarding router for Group 10 and the standby router for Group 20. Router B is the active forwarding router for Group 20 and the standby router for Group 10. Both Groups are members of VLAN 100.

Figure 9-4 HSRP Group Members Example

Figure 9-4 HSRP Group Members Example

Addressing HSRP Groups Across ISL Links

As stated earlier, HSRP routers can provide for redundancy and load sharing across the same subnets, but what about different subnets?

As you are no doubt well aware, an ISL trunk provides the transit for multiple VLANs simultaneously.

NOTE HSRP over ISL was introduced in IOS 11.3 and is not possible with any earlier IOS version.

For each standby group, an IP address and a single well-known MAC address with a unique group identifier is allocated to the group.

The IP address of a group is in the range of addresses belonging to the subnet in use on the LAN. However, the IP address of the group must differ from the addresses allocated as interface addresses on all routers and hosts on the LAN, including virtual IP addresses assigned to other HSRP groups.

Running HSRP over ISL allows users to configure redundancy between multiple routers that are configured as front ends for VLAN IP subnets. By configuring HSRP over ISLs, users can eliminate situations in which a single point of failure causes traffic interruptions. This feature inherently provides some improvement in overall networking resilience by providing load balancing and redundancy capabilities between subnets and VLANs.

To configure HSRP over an ISL link between VLANs, perform the following tasks:

Step 1 Define the encapsulation type.

Step 2 Configure the IP address.

Step 3 Enable HSRP.

The first two steps were discussed in Chapter 7, "InterVLAN Routing." The steps to enable HSRP are discussed later in this chapter.

CAUTION Although a route processor can theoretically support up to 32,650 subinterfaces, practical usage is dictated by the robustness of the route processor and the number of VLANs. Please monitor the CPU utilization of the route processor to determine your practical limitations.

Multiple HSRP Groups

Routers can belong to multiple groups within multiple VLANs. As members of multiple HSRP groups, routers can simultaneously provide redundant backup and perform load sharing across different IP subnets. Some of the other characteristics of multiple HSRP groups include the following:

• Although multiple routers can exist in an HSRP group, only the active router forwards the packets sent to the virtual router.

• A separate HSRP group is configured for each separate VLAN.

• Multiple standby groups may coexist on a LAN segment. Each group operates independently of other groups. Each standby group emulates a single virtual router.

• Individual routers may participate in multiple groups. The router maintains separate state and timers for each group.

• For each standby group, an IP address and a single well-known MAC address with a unique group identifier is allocated to the group.

• There can be up to 255 standby groups on any LAN.

• If multiple groups are used on a single LAN, load splitting can be achieved by distributing hosts among different standby groups.

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