Hsrp

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Hot Standby Router Protocol Features and Functionality
Document ID: 9234
Introduction Prerequisites Requirements Components Used Conventions HSRP Background and Operations Dynamic Router Discovery Mechanisms HSRP Operation HSRP Addressing Cisco IOS Release and HSRP Functionality Matrix Cisco IOS Boot Images and HSRP Functionality HSRP Features Preemption Interface Tracking Use Burned−In AddressMultiple HSRP Groups Configurable MAC Address Syslog Support HSRP Debugging Enhanced HSRP Debugging Authentication IP Redundancy SNMP Management Information Base HSRP Support for Multiprotocol Label Switching Virtual Private Networks HSRP Support for ICMP Redirects HSRP Interface and Media Support Ethernet Token Ring 802.1Q ISL FDDI MAC Refresh Bridge Group Virtual Interface Subinterfaces NetProDiscussion Forums − Featured Conversations Related Information

Introduction
This document describes the features and functionality of Hot Standby Router Protocol (HSRP).

Prerequisites

Requirements
There are no specific requirements for this document.

Components Used
This document is not restricted to specific software and hardware versions.

Conventions
For more information ondocument conventions, refer to Cisco Technical Tips Conventions.

HSRP Background and Operations
One way to achieve near−100 percent network uptime is to use HSRP, which provides network redundancy for IP networks, ensuring that user traffic immediately and transparently recovers from first hop failures in network edge devices or access circuits. By sharing an IP address and a MAC (Layer 2)address, two or more routers can act as a single "virtual" router. The members of the virtual router group continually exchange status messages. This way, one router can assume the routing responsibility of another, should it go out of commission for either planned or unplanned reasons. Hosts continue to forward IP packets to a consistent IP and MAC address, and the changeover of devices doing therouting is transparent.

Dynamic Router Discovery Mechanisms
Below are descriptions of dynamic router discovery mechanisms that are available to hosts. Many of these mechanisms don't provide the network resiliency required by network administrators. This may be because the protocol wasn't initially intended to provide network resiliency or because it isn't feasible for every host on a network tobe running the protocol. In addition to what is listed below, it is important to note that many hosts only allow you to configure a default−gateway. Proxy Address Resolution Protocol Some IP hosts use proxy Address Resolution Protocol (ARP) to select a router. When a host runs proxy ARP, it sends an ARP request for the IP address of the remote host it wants to contact. A router, Router A, on thenetwork replies on behalf of the remote host and provides its own MAC address. With proxy ARP, the host behaves as if the remote host were connected to the same segment of the network. If Router A fails, the host continues to send packets destined for the remote host to the MAC address of Router A even though those packets have nowhere to go and are lost. You can either wait for ARP to acquire theMAC address of another router, Router B, on the local segment by sending another ARP request, or reboot the host to force it to send an ARP request. In either case, for a significant period of time, the host can't 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. Dynamic RoutingProtocol Some IP hosts run (or snoop) a dynamic routing protocol such as the Routing Information Protocol (RIP) or Open Shortes Path First (OSPF) to discover routers. The drawback of using RIP is that it is slow to adapt to changes in the topology. Running a dynamic routing protocol on every host may not be feasible for a number of reasons, including administrative overhead, processing overhead,...
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