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S2700, S3700, S5700, S6700, S7700, and S9700 Series Switches Interoperation and Replacement Guide

This document provides typical configuration examples for interoperation between Huawei switches and mainstream IP phones, firewalls, routers, Microsoft NLB servers, multi-NIC servers, Cisco switches, and SolarWinds.
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Overview of HSRP

Overview of HSRP

Background

As the Internet gains in popularity, people rely more and more on networks. To improve network stability, the device backup is used, which is similar to dual hard disks in a server to improve data security. Devices at the network core layer are central to the entire network. If a critical fault occurs on the core devices, the local network breaks down. If the backbone router becomes faulty, the impact will be significant. Therefore, it is inevitable that core devices work in hot standby mode to improve network reliability. When a core device fails, the backup device in the system will take over it until the faulty device is restored. The Hot Standby Router Protocol (HSRP) is used to address the preceding issue.

HSRP Principle

HSRP is a Cisco proprietary router redundancy protocol. It allows multiple routers to be deployed in hot standby mode to eliminate network interruption caused by a single device failure.

To achieve HSRP, two or more routers on a network form a hot standby group, which is a virtual router.

HSRP uses the priority to determine the active router. The HSRP priority can be set manually.

If a router has a higher priority than all other routers, the router becomes the active router in the corresponding standby group. When Hello messages sent from the active router fail to be received within the hold time, the standby router with the highest priority becomes the active router. None of the hosts on the network detects the packets exchanged between routers.

The following figure shows the working mechanism of HSRP which is similar to that of VRRP:
Figure 5-30  Working mechanism of HSRP

Basic Concepts

  • Standby group: indicates a group of devices which form a virtual router, which called HSRP router.
  • Active router: indicates a router in a standby group that forwards packets on behalf of the virtual router.
  • Standby router: indicates the first standby router in a standby group.
  • Hello Time: indicates the interval for a device to send Hello messages. If the value is not specified, the Hello time depends on the interval when the active router successfully sends two Hello messages. Otherwise, the default value (3s) is used.
  • Hold Time: indicates the interval for the HSRP router to declare an active router failure, which is characterized by being at least 3 times that of the Hello time
  • Standby priority: indicates the priority of routers in an HSRP group, which is 100 by default. If the routers have the same priority, the router with the largest IP address becomes the active router. This address is the IP address of the interface have a HSRP configured.
  • Virtual MAC address: indicates the MAC address of the virtual router. 00.00.0c.07.ac.2f is used as an example.
    • Vendor ID: indicates the first three bytes. 00.00.0c indicates the Cisco device.
    • HSRP code: indicates that the MAC address is used to identify one HSRP virtual router, which is always 07.ac.
    • HSRP group number: indicates the group ID, which identifies the number of the HSRP backup group. In this example, 2f is a hexadecimal value, which equals to the decimal value 47.

HSRP Message

A router configured with HSRP has the following three types of multicast messages:
  • Hello: is sent when HSRP is running on the router that can become an active router or standby router. By default, HSRP routers send a Hello message every three seconds.
  • Coup: is sent by a standby router when it becomes an active router.
  • Resign: is sent by the active router when the active router wants to go Down or when a router with a higher priority sends a Hello message. This message indicates that the active router does not want to be the active router again.

HSRP messages are encapsulated in UDP packets using the UDP port number 1985. The destination IP address is the multicast IP address 224.0.0.2 (means all-router) with a TTL value of 1.

HSRP State

HSRP defines six possible states of an HSRP-enabled router.
  • Initial: indicates the state of HSRP upon startup. HSRP is not running at this time. A router enters this state when the configuration is changed or the interface is just started.
  • Learn: indicates that a router is waiting for messages from the active router. At this time, the router has not received Hello messages from the active router and has not learned the virtual router IP address.
  • Listen: indicates that the router is listening to Hello messages. When the virtual IP address is obtained, routers (but not the active and standby routers) remain in Listen state.
  • Speak: indicates that the router sends Hello messages periodically and participates in the election of the active router or standby router.
  • Standby: indicates the state of a router in a standby group. Standby group members monitor the active router, and are ready to take over services on the active router when it fails. Additionally, a group member periodically sends Hello messages to other members to notify its own state.
  • Active: indicates the state of the active router (responsible for data transmission) in a standby group.
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Updated: 2019-05-15

Document ID: EDOC1000114005

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