First Hop Redundancy Protocols Configuration Guide, Cisco IOS Release 12.4T
Last Updated: December 20, 2011
The Virtual Router Redundancy Protocol (VRRP) is an election protocol that dynamically assigns responsibility for one or more virtual routers to the VRRP routers on a LAN, allowing several routers on a multiaccess link to utilize the same virtual IP address. A VRRP router is configured to run the VRRP protocol in conjunction with one or more other routers attached to a LAN. In a VRRP configuration, one router is elected as the virtual router master, with the other routers acting as backups in case the virtual router master fails.
This module explains the concepts related to VRRP and describes how to configure VRRP in a network.
Finding Feature Information
Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the Feature Information Table at the end of this document.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Restrictions for VRRP
Information About VRRP
There are several ways a LAN client can determine which router should be the first hop to a particular remote destination. The client can use a dynamic process or static configuration. Examples of dynamic router discovery are as follows:
The drawback to dynamic discovery protocols is that they incur some configuration and processing overhead on the LAN client. Also, in the event of a router failure, the process of switching to another router can be slow.
An alternative to dynamic discovery protocols is to statically configure a default router on the client. This approach simplifies client configuration and processing, but creates a single point of failure. If the default gateway fails, the LAN client is limited to communicating only on the local IP network segment and is cut off from the rest of the network.
VRRP can solve the static configuration problem. VRRP enables a group of routers to form a single virtual router. The LAN clients can then be configured with the virtual router as their default gateway. The virtual router, representing a group of routers, is also known as a VRRP group.
VRRP is supported on Ethernet, Fast Ethernet, BVI, and Gigabit Ethernet interfaces, and on MPLS VPNs, VRF-aware MPLS VPNs, and VLANs.
The figure below shows a LAN topology in which VRRP is configured. In this example, Routers A, B, and C are VRRP routers (routers running VRRP) that comprise a virtual router. The IP address of the virtual router is the same as that configured for the Ethernet interface of Router A (10.0.0.1).
Because the virtual router uses the IP address of the physical Ethernet interface of Router A, Router A assumes the role of the virtual router master and is also known as the IP address owner. As the virtual router master, Router A controls the IP address of the virtual router and is responsible for forwarding packets sent to this IP address. Clients 1 through 3 are configured with the default gateway IP address of 10.0.0.1.
Routers B and C function as virtual router backups. If the virtual router master fails, the router configured with the higher priority will become the virtual router master and provide uninterrupted service for the LAN hosts. When Router A recovers, it becomes the virtual router master again. For more detail on the roles that VRRP routers play and what happens if the virtual router master fails, see the VRRP Router Priority and Preemption section.
The figure below shows a LAN topology in which VRRP is configured so that Routers A and B share the traffic to and from clients 1 through 4 and that Routers A and B act as virtual router backups to each other if either router fails.
In this topology, two virtual routers are configured. (For more information, see the Multiple Virtual Router Support section.) For virtual router 1, Router A is the owner of IP address 10.0.0.1 and virtual router master, and Router B is the virtual router backup to Router A. Clients 1 and 2 are configured with the default gateway IP address of 10.0.0.1.
For virtual router 2, Router B is the owner of IP address 10.0.0.2 and virtual router master, and Router A is the virtual router backup to Router B. Clients 3 and 4 are configured with the default gateway IP address of 10.0.0.2.
VRRP enables you to configure multiple routers as the default gateway router, which reduces the possibility of a single point of failure in a network.
You can configure VRRP in such a way that traffic to and from LAN clients can be shared by multiple routers, thereby sharing the traffic load more equitably among available routers.
Multiple Virtual Routers
VRRP supports up to 255 virtual routers (VRRP groups) on a router physical interface, subject to the platform supporting multiple MAC addresses. Multiple virtual router support enables you to implement redundancy and load sharing in your LAN topology.
Multiple IP Addresses
The virtual router can manage multiple IP addresses, including secondary IP addresses. Therefore, if you have multiple subnets configured on an Ethernet interface, you can configure VRRP on each subnet.
The redundancy scheme of VRRP enables you to preempt a virtual router backup that has taken over for a failing virtual router master with a higher priority virtual router backup that has become available.
VRRP message digest 5 (MD5) algorithm authentication protects against VRRP-spoofing software and uses the industry-standard MD5 algorithm for improved reliability and security.
VRRP uses a dedicated Internet Assigned Numbers Authority (IANA) standard multicast address (220.127.116.11) for VRRP advertisements. This addressing scheme minimizes the number of routers that must service the multicasts and allows test equipment to accurately identify VRRP packets on a segment. The IANA assigned VRRP the IP protocol number 112.
Multiple Virtual Router Support
You can configure up to 255 virtual routers on a router physical interface. The actual number of virtual routers that a router interface can support depends on the following factors:
In a topology where multiple virtual routers are configured on a router interface, the interface can act as a master for one virtual router and as a backup for one or more virtual routers.
VRRP Router Priority and Preemption
An important aspect of the VRRP redundancy scheme is VRRP router priority. Priority determines the role that each VRRP router plays and what happens if the virtual router master fails.
If a VRRP router owns the IP address of the virtual router and the IP address of the physical interface, this router will function as a virtual router master.
Priority also determines if a VRRP router functions as a virtual router backup and the order of ascendancy to becoming a virtual router master if the virtual router master fails. You can configure the priority of each virtual router backup with a value of 1 through 254 using the vrrp priority command.
For example, if Router A, the virtual router master in a LAN topology, fails, an election process takes place to determine if virtual router backups B or C should take over. If Routers B and C are configured with the priorities of 101 and 100, respectively, Router B is elected to become virtual router master because it has the higher priority. If Routers B and C are both configured with the priority of 100, the virtual router backup with the higher IP address is elected to become the virtual router master.
By default, a preemptive scheme is enabled whereby a higher priority virtual router backup that becomes available takes over for the virtual router backup that was elected to become virtual router master. You can disable this preemptive scheme using the no vrrp preempt command. If preemption is disabled, the virtual router backup that is elected to become virtual router master remains the master until the original virtual router master recovers and becomes master again.
The virtual router master sends VRRP advertisements to other VRRP routers in the same group. The advertisements communicate the priority and state of the virtual router master. The VRRP advertisements are encapsulated in IP packets and sent to the IP Version 4 multicast address assigned to the VRRP group. The advertisements are sent every second by default; the interval is configurable.
Although the VRRP protocol as per RFC 3768 does not support millisecond timers, Cisco routers allow you to configure millisecond timers. You need to manually configure the millisecond timer values on both the primary and the backup routers. The master advertisement value displayed in the show vrrp command output on the backup routers is always 1 second because the packets on the backup routers do not accept millisecond values.
You must use millisecond timers where absolutely necessary and with careful consideration and testing. Millisecond values work only under favorable circumstances, and you must be aware that the use of the millisecond timer values restricts VRRP operation to Cisco devices only.
VRRP Object Tracking
Object tracking is an independent process that manages creating, monitoring, and removing tracked objects such as the state of the line protocol of an interface. Clients such as the Hot Standby Router Protocol (HSRP), Gateway Load Balancing Protocol (GLBP), and VRRP register their interest with specific tracked objects and act when the state of an object changes.
Each tracked object is identified by a unique number that is specified on the tracking CLI. Client processes such as VRRP use this number to track a specific object.
The tracking process periodically polls the tracked objects and notes any change of value. The changes in the tracked object are communicated to interested client processes, either immediately or after a specified delay. The object values are reported as either up or down.
VRRP object tracking gives VRRP access to all the objects available through the tracking process. The tracking process provides the ability to track individual objects such as a the state of an interface line protocol, state of an IP route, or the reachability of a route.
VRRP provides an interface to the tracking process. Each VRRP group can track multiple objects that may affect the priority of the VRRP router. You specify the object number to be tracked and VRRP will be notified of any change to the object. VRRP increments (or decrements) the priority of the virtual router based on the state of the object being tracked.
How Object Tracking Affects the Priority of a VRRP Router
The priority of a device can change dynamically if it has been configured for object tracking and the object that is being tracked goes down. The tracking process periodically polls the tracked objects and notes any change of value. The changes in the tracked object are communicated to VRRP, either immediately or after a specified delay. The object values are reported as either up or down. Examples of objects that can be tracked are the line protocol state of an interface or the reachability of an IP route. If the specified object goes down, the VRRP priority is reduced. The VRRP router with the higher priority can now become the virtual router master if it has the vrrp preempt command configured. See the VRRP Object Tracking section for more information on object tracking.
VRRP ignores unauthenticated VRRP protocol messages. The default authentication type is text authentication.
You can configure VRRP text authentication, authentication using a simple MD5 key string, or MD5 key chains for authentication.
MD5 authentication provides greater security than the alternative plain text authentication scheme. MD5 authentication allows each VRRP group member to use a secret key to generate a keyed MD5 hash of the packet that is part of the outgoing packet. A keyed hash of an incoming packet is generated and if the generated hash does not match the hash within the incoming packet, the packet is ignored.
The key for the MD5 hash can either be given directly in the configuration using a key string or supplied indirectly through a key chain.
A router will ignore incoming VRRP packets from routers that do not have the same authentication configuration for a VRRP group. VRRP has three authentication schemes:
VRRP packets will be rejected in any of the following cases:
In Service Software Upgrade--VRRP
VRRP supports In Service Software Upgrade (ISSU). In Service Software Upgrade (ISSU) allows a high-availability (HA) system to run in stateful switchover (SSO) mode even when different versions of Cisco IOS software are running on the active and standby Route Processors (RPs) or line cards.
ISSU provides the ability to upgrade or downgrade from one supported Cisco IOSrelease to another while continuing to forward packets and maintain sessions, thereby reducing planned outage time. The ability to upgrade or downgrade is achieved by running different software versions on the active RP and standby RP for a short period of time to maintain state information between RPs. This feature allows the system to switch over to a secondary RP running upgraded (or downgraded) software and continue forwarding packets without session loss and with minimal or no packet loss. This feature is enabled by default.
For detailed information about ISSU, see theCisco IOS In Service Software Upgrade Process document in the Cisco IOS High Availability Configuration Guide.
VRRP Support for Stateful Switchover
With the introduction of the VRRP Support for Stateful Switchover feature, VRRP is SSO aware. VRRP can detect when a router is failing over to the secondary RP and continue in its current group state.
SSO functions in networking devices (usually edge devices) that support dual Route Processors (RPs). SSO provides RP redundancy by establishing one of the RPs as the active processor and the other RP as the standby processor. SSO also synchronizes critical state information between the RPs so that network state information is dynamically maintained between RPs.
Prior to being SSO aware, if VRRP was deployed on a router with redundant RPs, a switchover of roles between the active RP and the standby RP would result in the router relinquishing its activity as a VRRP group member and then rejoining the group as if it had been reloaded. The SSO--VRRP feature enables VRRP to continue its activities as a group member during a switchover. VRRP state information between redundant RPs is maintained so that the standby RP can continue the router's activities within the VRRP during and after a switchover.
This feature is enabled by default. To disable this feature, use the no vrrp sso command in global configuration mode.
For more information, see the Stateful Switchover document.
How to Configure VRRP
Customizing the behavior of VRRP is optional. Be aware that as soon as you enable a VRRP group, that group is operating. It is possible that if you first enable a VRRP group before customizing VRRP, the router could take over control of the group and become the virtual router master before you have finished customizing the feature. Therefore, if you plan to customize VRRP, it is a good idea to do so before enabling VRRP.
Disabling a VRRP Group on an Interface
Disabling a VRRP group on an interface allows the protocol to be disabled, but the configuration to be retained. This ability was added with the introduction of the VRRP MIB, RFC 2787, Definitions of Managed Objects for the Virtual Router Redundancy Protocol .
You can use a Simple Network Management Protocol (SNMP) management tool to enable or disable VRRP on an interface. Because of the SNMP management capability, the vrrp shutdown command was introduced to represent a method via the command line interface (CLI) for VRRP to show the state that had been configured using SNMP.
When the show running-config command is entered, you can see immediately if the VRRP group has been configured and set to enabled or disabled. This is the same functionality that is enabled within the MIB.
The no form of the command enables the same operation that is performed within the MIB. If the vrrp shutdown command is specified using the SNMP interface, then entering the no vrrp shutdown command reenables the VRRP group.
Configuring VRRP Object Tracking
Configuring VRRP MD5 Authentication Using a Key String
[0 | 7] key-string [timeout seconds]
8. Repeat Steps 1 through 7 on each router that will communicate.
Configuring VRRP MD5 Authentication Using a Key Chain
Perform this task to configure VRRP MD5 authentication using a key chain. Key chains allow a different key string to be used at different times according to the key chain configuration. VRRP will query the appropriate key chain to obtain the current live key and key ID for the specified key chain.
12. Repeat Steps 1 through 11 on each router that will communicate.
Verifying the VRRP MD5 Authentication Configuration
Configuring VRRP Text Authentication
Before You BeginSUMMARY STEPS
Interoperability with vendors that may have implemented the RFC 2338 method is not enabled.
Text authentication cannot be combined with MD5 authentication for a VRRP group at any one time. When MD5 authentication is configured, the text authentication field in VRRP hello messages is set to all zeros on transmit and ignored on receipt, provided the receiving router also has MD5 authentication enabled.
Enabling the Router to Send SNMP VRRP Notifications
The VRRP MIB supports SNMP Get operations, which allow network devices to get reports about VRRP groups in a network from the network management station.
Enabling VRRP MIB trap support is performed through the CLI, and the MIB is used for getting the reports. A trap notifies the network management station when a router becomes a Master or backup router. When an entry is configured from the CLI, the RowStatus for that group in the MIB immediately goes to the active state.
Configuration Examples for VRRP
Example: Configuring VRRP
In the following example, Router A and Router B each belong to three VRRP groups.
In the configuration, each group has the following properties:
Router(config)# interface GigabitEthernet 1/0/0 Router(config-if)# ip address 10.1.0.2 255.0.0.0 Router(config-if)# vrrp 1 priority 120 Router(config-if)# vrrp 1 authentication cisco Router(config-if)# vrrp 1 timers advertise 3 Router(config-if)# vrrp 1 timers learn Router(config-if)# vrrp 1 ip 10.1.0.10 Router(config-if)# vrrp 5 priority 100 Router(config-if)# vrrp 5 timers advertise 30 Router(config-if)# vrrp 5 timers learn Router(config-if)# vrrp 5 ip 10.1.0.50 Router(config-if)# vrrp 100 timers learn Router(config-if)# no vrrp 100 preempt Router(config-if)# vrrp 100 ip 10.1.0.100 Router(config-if)# no shutdown
Router(config)# interface GigabitEthernet 1/0/0 Router(config-if)# ip address 10.1.0.1 255.0.0.0 Router(config-if)# vrrp 1 priority 100 Router(config-if)# vrrp 1 authentication cisco Router(config-if)# vrrp 1 timers advertise 3 Router(config-if)# vrrp 1 timers learn Router(config-if)# vrrp 1 ip 10.1.0.10 Router(config-if)# vrrp 5 priority 200 Router(config-if)# vrrp 5 timers advertise 30 Router(config-if)# vrrp 5 timers learn Router(config-if)# vrrp 5 ip 10.1.0.50 Router(config-if)# vrrp 100 timers learn Router(config-if)# no vrrp 100 preempt Router(config-if)# vrrp 100 ip 10.1.0.100 Router(config-if)# no shutdown
Example: VRRP Object Tracking
In the following example, the tracking process is configured to track the state of the line protocol on serial interface 0/1. VRRP on Ethernet interface 1/0 then registers with the tracking process to be informed of any changes to the line protocol state of serial interface 0/1. If the line protocol state on serial interface 0/1 goes down, then the priority of the VRRP group is reduced by 15.
Router(config)# track 1 interface Serial 0/1 line-protocol Router(config-track)# exit Router(config)# interface Ethernet 1/0 Router(config-if)# ip address 10.0.0.2 255.0.0.0 Router(config-if)# vrrp 1 ip 10.0.0.3 Router(config-if)# vrrp 1 priority 120 Router(config-if)# vrrp 1 track 1 decrement 15
Example: VRRP Object Tracking Verification
The following examples verify the configuration shown in the Example: VRRP Object Tracking section:
Router# show vrrp Ethernet1/0 - Group 1 State is Master Virtual IP address is 10.0.0.3 Virtual MAC address is 0000.5e00.0101 Advertisement interval is 1.000 sec Preemption is enabled min delay is 0.000 sec Priority is 105 Track object 1 state Down decrement 15 Master Router is 10.0.0.2 (local), priority is 105 Master Advertisement interval is 1.000 sec Master Down interval is 3.531 sec Router# show track Track 1 Interface Serial0/1 line-protocol Line protocol is Down (hw down) 1 change, last change 00:06:53 Tracked by: VRRP Ethernet1/0 1
Example: VRRP MD5 Authentication Configuration Using a Key String
The following example shows how to configure MD5 authentication using a key string and timeout of 30 seconds:
Router(config)# interface Ethernet0/1 Router(config-if)# description ed1-cat5a-7/10 Router(config-if)# vrrp 1 ip 10.21.0.10 Router(config-if)# vrrp 1 priority 110 Router(config-if)# vrrp 1 authentication md5 key-string f00c4s timeout 30 Router(config-if)# exit
Example: VRRP MD5 Authentication Configuration Using a Key Chain
The following example shows how to configure MD5 authentication using a key chain:
Router(config)# key chain vrrp1 Router(config-keychain)# key 1 Router(config-keychain-key)# key-string f00c4s Router(config-keychain-key)# exit Router(config)#interface ethernet0/1 Router(config-if)# description ed1-cat5a-7/10 Router(config-if)# vrrp 1 priority 110 Router(config-if)# vrrp 1 authentication md5 key-chain vrrp1 Router(config-if)# vrrp 1 ip 10.21.0.10
In this example, VRRP queries the key chain to obtain the current live key and key ID for the specified key chain.
Example: VRRP Text Authentication
Example: Disabling a VRRP Group on an Interface
The following example shows how to disable one VRRP group on GigabitEthernet interface 0/0/0 while retaining VRRP for group 2 on GigabitEthernet interface 1/0/0:
Router(config)# interface GigabitEthernet 0/0/0 Router(config-if)# ip address 10.24.1.1 255.255.255.0 Router(config-if)# vrrp 1 ip 10.24.1.254 Router(config-if)# vrrp 1 shutdown Router(config-if)# exit Router(config)# interface GigabitEthernet 1/0/0 Router(config-if)# ip address 10.168.42.1 255.255.255.0 Router(config-if)# vrrp 2 ip 10.168.42.254
Feature Information for VRRP
The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
virtual IP address owner --The VRRP router that owns the IP address of the virtual router. The owner is the router that has the virtual router address as its physical interface address.
virtual router --One or more VRRP routers that form a group. The virtual router acts as the default gateway router for LAN clients. Also known as a VRRP group.
virtual router backup --One or more VRRP routers that are available to assume the role of forwarding packets if the virtual router master fails.
virtual router master --The VRRP router that is currently responsible for forwarding packets sent to the IP addresses of the virtual router. Usually the virtual router master also functions as the IP address owner.
VRRP router --A router that is running VRRP.
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