Information Technology Reference
In-Depth Information
Route redundancy within and between network components
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Link media redundancy in the access layer
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The following sections discuss each type of redundancy.
Workstation-to-Router Redundancy and LAN High Availability Protocols
When a workstation has traffic to send to a station that is not local, the workstation has
many possible ways to discover the address of a router on its network segment, including
the following:
ARP
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Explicit configuration
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ICMP Router Discovery Protocol (RDP)
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RIP
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HSRP
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VRRP
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GLBP
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VSS
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The following sections cover each of these methods.
ARP
Some IP workstations send an ARP frame to find a remote station. A router running proxy
ARP can respond with its data link layer address. Cisco routers run proxy ARP by default.
Explicit Configuration
Most IP workstations must be configured with the IP address of a default router, which is
sometimes called the default gateway.
In an IP environment, the most common method for a workstation to find a server is via
explicit configuration (a default router). If the workstation's default router becomes un-
available, you must reconfigure the workstation with the address of a different router.
Some IP stacks enable you to configure multiple default routers, but many other IP imple-
mentations support only one default router.
RDP
RFC 1256 specifies an extension to Internet Control Message Protocol (ICMP) that allows
an IP workstation and router to run RDP to let the workstation learn a router's address.
RIP
An IP workstation can run RIP to learn about routers, although this is not common prac-
