Default Gateway Redundancy

The default gateway facilitates communication between hosts on different networks. The source host sends the data to its default gateway. The default gateway is an IP address on a router (or Layer 3 switch), connected to the same subnet the source host is on, that forwards the data to the destination host. End hosts are typically configured with a single default gateway IP address that does not change when the network topology changes. If the default gateway cannot be reached, the local device is unable to send packets off the local network segment. Even if a redundant router exists that could serve as a default gateway for that segment, there is no dynamic method by which end hosts can determine the address of a new default gateway.

Figure 3-1 illustrates the default gateway’s limitations. Router A is configured as the default gateway for Host A. If Router A becomes unavailable, the routing protocols can quickly and dynamically converge and determine that Router B can transfer packets to the destination server that would otherwise have gone through Router A. However, most workstations, servers, and printers do not receive this dynamic routing information.

  

Figure 3-1 Default Gateway Limitations

The default gateway limitation discussed earlier can be resolved by using router redundancy. In router redundancy, multiple routers are configured to work together to present the illusion of a single virtual router to the hosts on a particular IP segment. This is achieved by sharing a virtual IP (Layer 3) address and a virtual MAC (Layer 2) address between multiple routers. The IP address of the virtual router is configured as the default gateway for the hosts on that particular IP segment.

In the beginning, before the end host can send any packets to a different network than its own, the end host uses Address Resolution Protocol (ARP) to resolve the MAC address that is associated with the IP address of the default gateway. The ARP resolution returns the MAC address of the virtual router. Frames that are sent to the MAC address of the virtual router can be physically processed by an active router or standby router that is part of that virtual router group, depending on the first-hop redundancy protocol used.

Figure 3-2 illustrates the router redundancy concept.

  

Figure 3-2 Router Redundancy

Host devices send traffic to the address of the virtual router. The physical router that forwards this traffic is transparent to the end stations. The redundancy protocol provides the mechanism for determining which router should take the active role in forwarding traffic and determining when a standby router must assume that role. In short, first-hop redundancy provides a network the ability to dynamically recover from the failure of a device acting as a default gateway.

In Figure 3-3, when the forwarding router fails, the standby router stops receiving hello messages from the forwarding router. We will discuss the concept of hello messages in router redundancy protocols later in this chapter. The standby router assumes the role of the forwarding router and assumes the IP address and the MAC address of the virtual router. During this failover, the end hosts see no disruption in service, as the end hosts are still sending the data packets to the same virtual IP and MAC address of the default gateway configured on them.

  

Figure 3-3 Router Failover