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| 6.2 |  | Implementing Router Redundancy in a Switched Network |
| 6.2.1 |  | Router redundancy operation |

IP routing redundancy is a key component of high-availability strategy for any network as it is a critical component in delivering reliable data, voice, and video integration. The goal of IP routing redundancy is to protect against first-hop router failure when a source host is unable to dynamically learn the IP address of an alternate first-hop router.

Issues with establishing redundancy

Proxy Address Resolution Protocol
Some IP hosts use proxy Address Resolution Protocol (ARP) to select a router. When a host runs proxy ARP an ARP request is sent with the IP address of the remote host. A router on the network replies on behalf of the remote host and provides its own MAC address. With proxy ARP, the host assumes it is connected to the same segment of the network as the remote host. If Router A fails, the host will continue to send packets to the MAC address of Router A. Those packets will be lost until the host ARP table is updated by way of a new ARP request acquiring the MAC address of another router on the local segment. It is possible that for a significant period of time the host will be unable to communicate with the remote host, even though the routing protocol has converged, and Router B is prepared to transfer packets that would otherwise go through Router A.

Default Gateways
Specifying a default gateway on a host will allow the workstation to simply send the packet to the default gateway. There may only be one default gateway specified in some operating systems and when this gateway fails, the workstation will not be able to reach remote subnets.

Although the network infrastructure can quickly recover and reconverge, the host will not know about a new default gateway. With operating systems that support multiple default gateway entries such as Microsoft Windows, failover between the entries is not dynamic. The operating system will send an echo to each gateway entry in order, on bootup. The first gateway to reply will be used as the default gateway. To use another listed gateway, in the event of a router failure, requires a restart of the host.

Dynamic Routing Protocol
Some IP hosts run a dynamic routing protocol such as the RIP or OSPF to discover routes. Running a dynamic routing protocol on every host may not be feasible for numerous reasons. These reasons include administrative and processing overhead, security issues, or lack of a protocol implementation for some platforms.

Dynamic Host Configuration Protocol
DHCP provides a mechanism for passing IP configuration information to hosts on a TCP/IP network. A host joining the network will request IP configuration information from the DHCP server. The configuration information typically consists of an IP address and a default gateway.

| Interactive Media Activity Point and Click: Router Redundancy Operation After completing this activity, the students will be able to understand router redundancy operation.