standby arp gratuitous through standby version

To configure the number of gratuitous Address Resolution Protocol (ARP) packets sent by a Hot Standby Router Protocol (HSRP) group when it transitions to the active state, and how often the ARP packets are sent, use the standby arp gratuitous command in interface configuration mode. To configure HSRP to send the default number of gratuitous of ARP packets at the default interval when an HSRP group changes to the active state, use the no form of this command.

Syntax Description

Command Default

HSRP sends one gratuitous ARP packet when a group becomes active, and then another two and four seconds later.

Command Modes

Interface configuration (config-if)

Command History

Usage Guidelines

You can configure HSRP to send a gratuitous ARP packet from one or more HSRP active groups. By default, HSRP sends one gratuitous ARP packet when a group becomes active, and then another two and four seconds later.

Use the standby arp gratuitous command in interface configuration mode to configure the number of gratuitous ARP packets sent by an Active HSRP group, and how often they are sent. The count and interval keywords can be specified in any order. If both the count and interval keywords are set to their default values, the standby arp gratuitous command does not appear in the running configuration.

Use the standby send arp command in EXEC mode to configure HSRP to send a single gratuitous ARP packet when an HSRP group becomes active.

Related Commands

To configure an authentication string for the Hot Standby Router Protocol (HSRP), use the standby authentication command in interface configuration mode. To delete an authentication string, use the no form of this command.

Syntax Description

Command Default

No text authentication string is configured.

Command Modes

Interface configuration (config-if)

Command History

Usage Guidelines

The authentication string is sent unencrypted in all HSRP messages when using the standby authentication text string option. The same authentication string must be configured on all routers and access servers on a cable to ensure interoperation. Authentication mismatch prevents a device from learning the designated Hot Standby IP address and the Hot Standby timer values from other routers configured with HSRP.

When group number 0 is used, no group number is written to NVRAM, providing backward compatibility.

If password encryption is configured with the service password-encryption command, the software saves the key string as encrypted text.

The timeout seconds is the duration that the HSRP group will accept message digests based on both the old and new keys. This allows time for configuration of all routers in a group with the new key. HSRP route flapping can be minimized by changing the keys on all the routers, provided that the active router is changed last. The active router should have its key string changed no later than one holdtime period, specified by the standby timers interface configuration command, after the non-active routers. This procedure ensures that the non-active routers do not time out the active router.

Related Commands

To reenable Hot Standby Router Protocol (HSRP) Bidirectional Forwarding Detection (BFD) peering if it has been disabled on an interface, use the standby bfd command in interface configuration mode. To disable HSRP support for BFD, use the no form of this command.

Syntax Description

This command has no arguments or keywords.

Command Default

HSRP support for BFD is enabled.

Command Modes

Interface configuration

Command History

Usage Guidelines

HSRP BFD peering is enabled by default when the router is configured for BFD. Use this command to reenable HSRP BFD peering on the specified interface when it has previously been manually disabled.

To enable HSRP BFD peering globally on the router, use the standby bfd all-interfaces command in global configuration mode.

Related Commands

To reenable Hot Standby Router Protocol (HSRP) Bidirectional Forwarding Detection (BFD) peering on all interfaces if it has been disabled, use the standby bfd all-interfaces command in global configuration mode. To disable HSRP support for BFD peering, use the no form of this command.

Syntax Description

This command has no arguments or keywords.

Command Default

HSRP BFD peering is enabled.

Command Modes

Global configuration

Command History

Usage Guidelines

The HSRP BFD peering feature introduces BFD in the HSRP group member health monitoring system. Previously, group member monitoring relied exclusively on HSRP multicast messages, which are relatively large and consume CPU memory to produce and check. In architectures where a single interface hosts a large number of groups, there is a need for a protocol with low CPU memory consumption and processing overhead. BFD addresses this issue and offers subsecond health monitoring (failure detection in milliseconds) with a relatively low CPU impact. This command is enabled by default.

To enable HSRP support for BFD on a per-interface basis, use the standby bfd command in interface configuration mode.

Related Commands

To configure the delay period before the initialization of Hot Standby Router Protocol (HSRP) groups, use the standby delay minimum reload command in interface configuration mode. To disable the delay period, use the no form of this command.

Syntax Description

Command Default

HSRP group initialization is not delayed.

Command Modes

Interface configuration (config-if)

Command History

Usage Guidelines

If the active router fails or is removed from the network, then the standby router will automatically become the new active router. If the former active router comes back online, you can control whether it takes over as the active router by using the standby preempt command.

However, in some cases, even if the standby preempt command is not configured, the former active router will resume the active role after it reloads and comes back online. Use the standby delay minimum reload command to set a delay period for HSRP group initialization. This command allows time for the packets to get through before the router resumes the active role.

We recommend that all HSRP routers have the standby delay minimum reload configured with a minimum delay time of 30 seconds and a minimum reload time of 60 seconds.

The delay will be cancelled if an HSRP packet is received on an interface.

The standby delay minimum reload interface configuration command delays HSRP groups from initializing for the specified time after the interface comes up.

This command is separate from the standby preempt delay interface configuration command, which enables HSRP preemption delay.

Related Commands

To configure a Hot Standby Router Protocol (HSRP) group to become an IP redundancy client of another HSRP group, use thestandby follow command in interface configuration mode. To remove the configuration of an HSRP group as a client group, use the no form of this command.

Syntax Description

Command Default

HSRP groups are not configured as client groups.

Command Modes

Interface configuration (config-if)

Command History

Usage Guidelines

The standby follow command configures an HSRP group to become an IP redundancy client of another HSRP group.

Client or slave groups must be on the same physical interface as the master group.

A client group takes its state from the master group it is following. Therefore, the client group does not use its timer, priority, or preemption settings. A warning is displayed if these settings are configured on a client group:

Router(config-if)# standby 1 priority 110
%Warning: This setting has no effect while following another group.
Router(config-if)# standby 1 timers 5 15
% Warning: This setting has no effect while following another group.
Router(config-if)# standby 1 preempt delay minimum 300
% Warning: This setting has no effect while following another group.

HSRP client groups follow the master HSRP with a slight, random delay so that all client groups do not change at the same time.

You cannot configure an HSRP group to follow another HSRP group if that group is itself being followed by another HSRP group.

Use theshow standby command to display complete information about an HSRP client group.

Related Commands

To activate the Hot Standby Router Protocol (HSRP), use the standby ip command in interface configuration mode. To disable HSRP, use the no form of this command.

Syntax Description

Command Default

The default group number is 0. HSRP is disabled by default.

Command Modes

Interface configuration (config-if)

Command History

Usage Guidelines

The standby ip command activates HSRP on the configured interface. If an IP address is specified, that address is used as the designated address for the Hot Standby group. If no IP address is specified, the designated address is learned through the standby function. For HSRP to elect a designated router, at least one router on the cable must have been configured with, or have learned, the designated address. Configuration of the designated address on the active router always overrides a designated address that is currently in use.

When the standby ip command is enabled on an interface, the handling of proxy Address Resolution Protocol (ARP) requests is changed (unless proxy ARP was disabled). If the Hot Standby state of the interface is active, proxy ARP requests are answered using the MAC address of the Hot Standby group. If the interface is in a different state, proxy ARP responses are suppressed.

When group number 0 is used, no group number is written to NVRAM, providing backward compatibility.

HSRP version 2 permits an expanded group number range from 0 to 4095. The increased group number range does not imply that an interface can, or should, support that many HSRP groups. The expanded group number range was changed to allow the group number to match the VLAN number on subinterfaces.

To ac tivate the Hot Standby Router Protocol (HSRP) in IPv6, use the standby ipv6 command in interface configuration mode. To disable HSRP, use the noform of this command.

Syntax Description

Command Default

The default group number is 0. HSRP is disabled by default.

Command Modes

Interface configuration

Command History

Usage Guidelines

An Ethernet or FDDI type interface must be used for HSRP for IPv6. HSRP version 2 must be enabled on an interface before HSRP IPv6 can be configured.

The standby ipv6 command enables an HSRP group for IPv6 operation. If the autoconfig keyword is used, then a link-local address will be generated from the link-local prefix and a modified EUI-64 format interface identifier, where the EUI-64 interface identifier is created from the relevant HSRP virtual MAC address.

If an IPv6 global address is used, it must include an IPv6 prefix length. If a link-local address is used, it does not have a prefix.

Related Commands

To specify a virtual Media Access Control (MAC) address for the Hot Standby Router Protocol (HSRP), use the standby mac-address command in interface configuration mode. To revert to the standard virtual MAC address (000.0C07.ACxy), use the no form of this command.

Syntax Description

Command Default

If this command is not configured, and the standby use-bia command is not configured, the standard virtual MAC address is used: 0000.0C07.ACxy, where xy is the group number in hexadecimal. This address is specified in RFC 2281, Cisco Hot Standby Router Protocol (HSRP).

Command Modes

Interface configuration (config-if)

Command History

Usage Guidelines

This command cannot be used on a Token Ring interface.

HSRP is used to help end stations locate the first-hop gateway for IP routing. The end stations are configured with a default gateway. However, HSRP can provide first-hop redundancy for other protocols. Some protocols, such as Advanced Peer-to-Peer Networking (APN), use the MAC address to identify the first hop for outing purposes. In this case, it is often necessary to be able to specify the virtual MAC address; the virtual IP address is unimportant for these protocols. Use the standby mac-address command to specify the virtual MAC address.

The MAC address specified is used as the virtual MAC address when the router is active.

This command is intended for certain APPN configurations. The parallel terms are shown in the table below.

In an APPN network, an end node is typically configured with the MAC address of the adjacent network node. Use the standby mac-address command in the routers to set the virtual MAC address to the value used in the end nodes.

Related Commands

To change the interval at which packets are sent to refresh the Media Access Control (MAC) cache when the Hot Standby Router Protocol (HSRP) is running over FDDI, use the standby mac-refresh command in interface configuration mode. To restore the default value, use the no form of this command.

Syntax Description

Command Default

The standby MAC refresh interval is 10 seconds.

Command Modes

Interface configuration (config-if)

Command History

Usage Guidelines

This command applies to HSRP running over FDDI only. Packets are sent every 10 seconds to refresh the MAC cache on learning bridges or switches. By default, the MAC cache entries age out in 300 seconds (5 minutes).

All other routers participating in HSRP on the FDDI ring receive the refresh packets, although the packets are intended only for the learning bridge or switch. Use this command to change the interval. Set the interval to 0 if you want to prevent refresh packets (if you have FDDI but do not have a learning bridge or switch).

To specify the name of the HSRP standby group, use the standby name command in interface configuration mode. To remove the name, use the no form of this command.

Syntax Description

Command Default

The Hot Standby Router Protocol (HSRP) is disabled.

Command Modes

Interface configuration (config-if)

Command History

Usage Guidelines

This command specifies the HSRP group used. The HSRP group name must be unique on the router. HSRP group names must not exceed 25 characters in length in releases after Cisco IOS Release 12.2SR. If the name is longer than 25 characters, it must be truncated by the user.

Related Commands

To configure Hot Standby Router Protocol (HSRP) preemption and preemption delay, use the standby preempt command in interface configuration mode. To restore the default values, use the no form of this command.

Syntax Description

Command Default

The default group number is 0. The default delay is 0 seconds; if the router wants to preempt, it will do so immediately. By default, the router that comes up later becomes the standby.

Command Modes

Interface configuration (config-if)

Command History

Usage Guidelines

Note

Cisco IOS 12.2SX software releases earlier than Cisco IOS Release 12.2(33)SXH use the syntax from Cisco IOS Release 12.1, which supports preempt as a keyword for the standby priority command. Cisco IOS Release 12.2(33)SXH and later releases use Cisco IOS Release 12.2 syntax, which requires standby preempt and standby priority to be entered as separate commands.

When the standby preempt command is configured, the router is configured to preempt, which means that when the local router has a Hot Standby priority higher than the current active router, the local router should attempt to assume control as the active router. If preemption is not configured, the local router assumes control as the active router only if it receives information indicating no router is in the active state (acting as the designated router).

This command is separate from the standby delay minimum reload interface configuration command, which delays HSRP groups from initializing for the specified time after the interface comes up.

When a router first comes up, it does not have a complete routing table. If it is configured to preempt, it will become the active router, yet it is unable to provide adequate routing services. Solve this problem by configuring a delay before the preempting router actually preempts the currently active router.

When group number 0 is used, no group number is written to NVRAM, providing backward compatibility.

IP redundancy clients can prevent preemption from taking place. The standby preempt delay sync seconds command specifies a maximum number of seconds to allow IP redundancy clients to prevent preemption. When this expires, then preemption takes place regardless of the state of the IP redundancy clients.

The standby preempt delay reload seconds command allows preemption to occur only after a router reloads. This provides stabilization of the router at startup. After this initial delay at startup, the operation returns to the default behavior.

The no standby preempt delay command will disable the preemption delay but preemption will remain enabled. The no standby preempt delay minimum seconds command will disable the minimum delay but leave any synchronization delay if it was configured.

When the standby follow command is used to configure an HSRP group to become an IP redundancy client of another HSRP group, the client group takes its state from the master group it is following. Therefore, the client group does not use its timer, priority, or preemption settings. A warning is displayed if these settings are configured on a client group:

Router(config-if)# standby 1 preempt delay minimum 300
    % Warning: This setting has no effect while following another group.

To configure Hot Standby Router Protocol (HSRP) priority, use the standby priority command in interface configuration mode. To restore the default values, use the no form of this command.

Syntax Description

Command Default

The default group number is 0. The default priority is 100.

Command Modes

Interface configuration (config-if)

Command History

Usage Guidelines

Note

Cisco IOS 12.2SX software releases earlier than Cisco IOS Release 12.2(33)SXH use the syntax from Cisco IOS Release 12.1, which supports preempt as a keyword for the standby priority command. Cisco IOS Release 12.2(33)SXH and later releases use Cisco IOS Release 12.2 syntax, which requires standby preempt andstandby priority to be entered as separate commands.

When group number 0 is used, the number 0 is written to NVRAM, providing backward compatibility.

The assigned priority is used to help select the active and standby routers. Assuming that preemption is enabled, the router with the highest priority becomes the designated active router. In case of ties, the primary IP addresses are compared, and the higher IP address has priority.

Note that the priority of the device can change dynamically if an interface is configured with the standby track command and another interface on the router or a tracked object goes down.

When the standby follow command is used to configure an HSRP group to become an IP redundancy client of another HSRP group, the client group takes its state from the master group it is following. Therefore, the client group does not use its timer, priority, or preemption settings. A warning is displayed if these settings are configured on a client group:

Router(config-if)# standby 1 priority 110

%Warning: This setting has no effect while following another group.

Related Commands

To enable Hot Standby Router Protocol (HSRP) filtering of Internet Control Message Protocol (ICMP) redirect messages, use the standby redirect command in interface configuration mode. To disable the HSRP filtering of ICMP redirect messages, use the no form of this command.

Syntax Description

Command Default

HSRP filtering of ICMP redirect messages is enabled if HSRP is configured on an interface.

Command Modes

Interface configuration (config-if)

Command History

Usage Guidelines

The standby redirect command can be configured globally or on a per-interface basis. When HSRP is first configured on an interface, the setting for that interface will inherit the global value. If the filtering of ICMP redirects is explicitly disabled on an interface, then the global command cannot reenable this functionality.

With the standby redirect command enabled, the real IP address of a router can be replaced with a virtual IP address in the next hop address or gateway field of the redirect packet. HSRP looks up the next hop IP address in its table of real IP addresses versus virtual IP addresses. If HSRP does not find a match, the HSRP router allows the redirect packet to go out unchanged. The host HSRP router is redirected to a router that is unknown, that is, a router with no active HSRP groups. You can specify the no standby redirect unknown command to stop these redirects from being sent.

Related Commands

To configure Internet Control Message Protocol (ICMP) redirect messages with a Hot Standby Router Protocol (HSRP) virtual IP address as the gateway IP address, use the standby redirects command in global configuration mode. To disable the configuration, use the no form of this command.

Syntax Description

Command Default

The HSRP virtual IP address is configured as the gateway IP address.

Command Modes

Global configuration (config)

Command History

Related Commands

To configure Hot Standby Router Protocol (HSRP) to send a single gratuitous ARP packet for each active HSRP group, use the standby send arp command in user EXEC or privileged EXEC mode.

Syntax Description

Command Default

HSRP sends gratuitous ARP packets from an HSRP group when it changes to the Active state.

Command Modes

User EXEC Privileged EXEC(#)

Command History

Usage Guidelines

Use the standby send arp command to cause a single gratuitous ARP packet to be sent for each active group. HSRP checks that the virtual IP address is entered correctly in the ARP cache prior to sending a gratuitous ARP packet. If the ARP entry is incorrect then HSRP will try to re-add it. This enables you to ensure that a host ARP cache is updated prior to starting heavy CPU-usage processes or configurations.

Static or alias ARP entries cannot be overwritten by HSRP.

You can use the standby arp gratuitous command in interface configuration mode to configure the number of gratuitous ARP packets sent by an active HSRP group, and how often they are sent.

Related Commands

To enable Hot Standby Router Protocol (HSRP) Stateful Switchover (SSO), use the standby sso command in global configuration mode. To disable HSRP SSO, use the no form of this command.

Syntax Description

This command has no arguments or keywords.

Command Default

HSRP SSO is enabled when redundancy mode SSO is configured.

Command Modes

Global configuration (config)

Command History

Usage Guidelines

Use the standby sso command to enable HSRP SSO. This is the default when redundancy mode SSO is configured. When standby SSO is enabled, traffic sent using an HSRP virtual IP address continues through the HSRP group member using the current path while a Route Processor (RP) switchover occurs. The HSRP state is maintained and kept synchronized across the redundant RPs within the chassis.

If you want the traffic to switch to a redundant device (another chassis) even though the redundant RP is capable of taking over, then the feature can be disabled by using the no form of the command. If the command is disabled and if the primary HSRP router fails, the HSRP state is not maintained across RP switchover and traffic targeted to the HSRP virtual IP address is handled by the standby HSRP router.

Related Commands

To configure the time between hello packets and the time before other routers declare the active Hot Standby or standby router to be down, use the standby timers command in interface configuration mode. To restore the timers to their default values, use the no form of this command.

Syntax Description

Command Default

The default group number is 0. The default hello interval is 3 seconds. The default hold time is 10 seconds.

Command Modes

Interface configuration (config-if)

Command History

Usage Guidelines

The standby timers command configures the time between standby hello packets and the time before other routers declare the active or standby router to be down. Routers or access servers on which timer values are not configured can learn timer values from the active or standby router. The timers configured on the active router always override any other timer settings. All routers in a Hot Standby group should use the same timer values. Normally, holdtime is greater than or equal to 3 times the value of hellotime. The range of values for holdtime force the holdtime to be greater than the hellotime. If the timer values are specified in milliseconds, the holdtime is required to be at least three times the hellotime value and not less than 50 milliseconds.

Some HSRP state flapping can occasionally occur if the holdtime is set to less than 250 milliseconds, and the processor is busy. It is recommended that holdtime values less than 250 milliseconds be used on Cisco 7200 platforms or better, and on Fast-Ethernet or FDDI interfaces or better. Setting the process-max-time command to a suitable value may also help with flapping.

The value of the standby timer will not be learned through HSRP hellos if it is less than 1 second.

When group number 0 is used, no group number is written to NVRAM, providing backward compatibility.

When the standby follow command is used to configure an HSRP group to become an IP redundancy client of another HSRP group, the client group takes its state from the master group it is following. Therefore, the client group does not use its timer, priority, or preemption settings. A warning is displayed if these settings are configured on a client group:

Router(config-if)# standby 1 timers 5 15
    % Warning: This setting has no effect while following another group.

To configure the Hot Standby Router Protocol (HSRP) to track an object and change the Hot Standby priority on the basis of the state of the object, use the standby track command in interface configuration mode. To remove the tracking, use the no form of this command.

Syntax Description

Command Default

There is no tracking.

Command Modes

Interface configuration (config-if)

Command History

Usage Guidelines

This command ties the Hot Standby priority of the router to the availability of its tracked objects. Use the track interface command or track ip route command to track an interface object or an IP-route object. The HSRP client can register its interest in the tracking process by using the standby track command and take action when the object changes.

When a tracked object goes down, the Hot Standby priority decreases by 10. If an object is not tracked, its state changes do not affect the Hot Standby priority. For each object configured for Hot Standby, you can configure a separate list of objects to be tracked.

The optional priority-decrement and interface-priority arguments specify how much to decrement the Hot Standby priority when a tracked object goes down. When the tracked object comes back up, the priority is incremented by the same amount.

When multiple tracked objects are down, the decrements are cumulative, whether configured with priority-decrement or interface-priority values or not.

The optional shutdown keyword configures the HSRP group to change to the Init state and become disabled rather than having its priority decremented when a tracked object goes down.

Use the no standby group-number track command to delete all tracking configuration for a group.

When group number 0 is used, no group number is written to NVRAM, providing backward compatibility.

The standby track command syntax prior to Cisco IOS Release 12.2(15)T is still supported. Using the older form of the command syntax will cause a tracked object to be created in the new tracking process. This tracking information can be displayed using the show track command.

Note	Using the command syntax of standby track prior to Cisco IOS Release 12.2(15)T results in the same performance as using the new standby track command syntax.

If you configure HSRP to track an interface, and that interface is physically removed as in the case of an Online Insertion and Removal (OIR) operation, then HSRP regards the interface as always down. You cannot remove the HSRP interface-tracking configuration. To prevent this situation, use the no standby track command before you physically remove the interface.

If an object is already being tracked by an HSRP group, you cannot change the configuration to use the HSRP Group Shutdown feature that disables the HSRP group. You must first remove the tracking configuration using the no standby track command and then reconfigure it using the standby track command with the shutdown keyword.

As of Cisco IOS Release 15.1(3)T, 15.1(1)S, and 12.2(50)SY, a maximum of 1000 objects can be tracked. Although 1000 tracked objects can be configured, each tracked object uses CPU resources. The amount of available CPU resources on a router is dependent upon variables such as traffic load and how other protocols are configured and run. The ability to use 1000 tracked objects is dependent upon the available CPU. Testing should be conducted on site to ensure that the service works under the specific site traffic conditions.

Related Commands

To configure Hot Standby Router Protocol (HSRP) to use the burned-in address of the interface as its virtual MAC address, instead of the preassigned MAC address (on Ethernet and FDDI) or the functional address (on Token Ring), use the standby use-bia command in interface configuration mode. To restore the default virtual MAC address, use the no form of this command.

Syntax Description

Command Default

HSRP uses the preassigned MAC address on Ethernet and FDDI, or the functional address on Token Ring.

Command Modes

Interface configuration (config-if)

Command History

Usage Guidelines

Note

This command is not supported on Cisco 7600 series routers that are configured with a Policy Feature Card, version 2 (PFC2). The PFC2 supports a maximum of 16 unique HSRP-group numbers. You can use the same HSRP-group numbers in different VLANs. If you configure more than 16 HSRP groups, this restriction prevents use of the VLAN number as the HSRP-group number.

For an interface with this command configured, multiple standby groups can be configured. Hosts on the interface must have a default gateway configured. We recommend that you set the no ip proxy-arp command on the interface. It is desirable to configure the standby use-bia command on a Token Ring interface if there are devices that reject ARP replies with source hardware addresses set to a functional address.

When HSRP runs on a multiple-ring, source-routed bridging environment and the HRSP routers reside on different rings, configuring the standby use-bia command can prevent confusion about the routing information field.

Without the scope interface keywords, the standby use-bia command applies to all subinterfaces on the major interface. The standby use-bia command may not be configured both with and without the scope interface keywords at the same time.

Note	Identically numbered HSRP groups use the same virtual MAC address, which might cause errors if you configure bridge groups.

To change the version of the Hot Standby Router Protocol (HSRP), use the standby version command in interface configuration mode. To change to the default version, use the no form of this command.

Syntax Description

Command Default

HSRP version 1 is the default HSRP version.

Command Modes

Interface configuration (config-if)

Command History

Usage Guidelines

HSRP version 2 addresses limitations of HSRP version 1 by providing an expanded group number range of 0 to 4095.

HSRP version 2 does not interoperate with HSRP version 1. An interface cannot operate both version 1 and version 2 because both versions are mutually exclusive. However, the different versions can be run on different physical interfaces of the same router. The group number range is from 0 to 255 for HSRP version 1 and from 0 to 4095 for HSRP version 2. You cannot change from version 2 to version 1 if you have configured groups above 255. Use the no standby version command to set the HSRP version to the default version, version 1.

If an HSRP version is changed, each group will reinitialize because it now has a new virtual MAC address.

Related Commands