VRRP

VRRP stands for Virtual Router Redundancy Protocol. This protocol is used to allow multiple backup routers on the same segment to take over operation of each others’ IP addresses if the primary router fails. This is typically used to provide fault-tolerant gateways to hosts on the segment.

FRR implements VRRPv2 (RFC 3768) and VRRPv3 (RFC 5798). For VRRPv2, no authentication methods are supported; these are deprecated in the VRRPv2 specification as they do not provide any additional security over the base protocol.

Note

  • VRRP is supported on Linux 5.1+
  • VRRP does not implement Accept_Mode

Starting VRRP

The configuration file for vrrpd is vrrpd.conf. The typical location of vrrpd.conf is /etc/frr/vrrpd.conf.

If using integrated config, then vrrpd.conf need not be present and frr.conf is read instead.

VRRP supports all the common FRR daemon start options which are documented elsewhere.

Protocol Overview

From RFC 5798:

VRRP specifies an election protocol that dynamically assigns responsibility for a virtual router to one of the VRRP routers on a LAN. The VRRP router controlling the IPv4 or IPv6 address(es) associated with a virtual router is called the Master, and it forwards packets sent to these IPv4 or IPv6 addresses. VRRP Master routers are configured with virtual IPv4 or IPv6 addresses, and VRRP Backup routers infer the address family of the virtual addresses being carried based on the transport protocol. Within a VRRP router, the virtual routers in each of the IPv4 and IPv6 address families are a domain unto themselves and do not overlap. The election process provides dynamic failover in the forwarding responsibility should the Master become unavailable. For IPv4, the advantage gained from using VRRP is a higher-availability default path without requiring configuration of dynamic routing or router discovery protocols on every end-host. For IPv6, the advantage gained from using VRRP for IPv6 is a quicker switchover to Backup routers than can be obtained with standard IPv6 Neighbor Discovery mechanisms.

VRRP accomplishes these goals primarily by using a virtual MAC address shared between the physical routers participating in a VRRP virtual router. This reduces churn in the neighbor tables of hosts and downstream switches and makes router failover theoretically transparent to these devices.

FRR implements the election protocol and handles changing the operating system interface configuration in response to protocol state changes.

As a consequence of the shared virtual MAC requirement, VRRP is currently supported only on Linux, as Linux is the only operating system that provides the necessary features in its network stack to make implementing this protocol feasible.

When a VRRP router is acting as the Master router, FRR allows the interface(s) with the backed-up IP addresses to remain up and functional. When the router transitions to Backup state, these interfaces are set into protodown mode. This is an interface mode that is functionally equivalent to NO-CARRIER. Physical drivers typically use this state indication to drop traffic on an interface. In the case of VRRP, the interfaces in question are macvlan devices, which are virtual interfaces. Since the IP addresses managed by VRRP are on these interfaces, this has the same effect as removing these addresses from the interface, but is implemented as a state flag.

Configuring VRRP

VRRP is configured on a per-interface basis, with some global defaults accessible outside the interface context.

System Configuration

FRR’s VRRP implementation uses Linux macvlan devices to to implement the shared virtual MAC feature of the protocol. Currently, it does not create those system interfaces - they must be configured outside of FRR before VRRP can be enabled on them.

Each interface on which VRRP will be enabled must have at least one macvlan device configured with the virtual MAC and placed in the proper operation mode. The addresses backed up by VRRP are assigned to these interfaces.

Suppose you have an interface eth0 with the following configuration:

$ ip addr show eth0
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 02:17:45:00:aa:aa brd ff:ff:ff:ff:ff:ff
    inet 10.0.2.15/24 brd 10.0.2.255 scope global dynamic eth0
       valid_lft 72532sec preferred_lft 72532sec
    inet6 fe80::17:45ff:fe00:aaaa/64 scope link
       valid_lft forever preferred_lft forever

Suppose that the IPv4 and IPv6 addresses you want to back up are 10.0.2.16 and 2001:db8::370:7334, and that they will be managed by the virtual router with id 5. A macvlan device with the appropriate MAC address must be created before VRRP can begin to operate.

If you are using ifupdown2, the configuration is as follows:

iface eth0
 ...
 vrrp 5 10.0.2.16/24 2001:0db8::0370:7334/64

Applying this configuration with ifreload -a will create the appropriate macvlan device. If you are using iproute2, the equivalent configuration is:

ip link add vrrp4-2-1 link eth0 addrgenmode random type macvlan mode bridge
ip link set dev vrrp4-2-1 address 00:00:5e:00:01:05
ip addr add 10.0.2.16/24 dev vrrp4-2-1
ip link set dev vrrp4-2-1 up

ip link add vrrp6-2-1 link eth0 addrgenmode random type macvlan mode bridge
ip link set dev vrrp4-2-1 address 00:00:5e:00:02:05
ip addr add 2001:db8::370:7334/64 dev vrrp6-2-1
ip link set dev vrrp6-2-1 up

In either case, the created interfaces will look like this:

$ ip addr show vrrp4-2-1
5: [email protected]: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether 00:00:5e:00:01:05 brd ff:ff:ff:ff:ff:ff
    inet 10.0.2.16/24 scope global vrrp4-2-1
       valid_lft forever preferred_lft forever
    inet6 fe80::dc56:d11a:e69d:ea72/64 scope link stable-privacy
       valid_lft forever preferred_lft forever

$ ip addr show vrrp6-2-1
8: [email protected]: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
 link/ether 00:00:5e:00:02:05 brd ff:ff:ff:ff:ff:ff
 inet6 2001:db8::370:7334/64 scope global
    valid_lft forever preferred_lft forever
 inet6 fe80::f8b7:c9dd:a1e8:9844/64 scope link stable-privacy
    valid_lft forever preferred_lft forever

Using vrrp4-2-1 as an example, a few things to note about this interface:

  • It is slaved to eth0; any packets transmitted on this interface will egress via eth0
  • Its MAC address is set to the VRRP IPv4 virtual MAC specified by the RFC for VRID 5
  • The VIP address 10.0.2.16 must not be present on the parent interface eth0.
  • The link local address on the interface is not derived from the interface MAC

First to note is that packets transmitted on this interface will egress via eth0, but with their Ethernet source MAC set to the VRRP virtual MAC. This is how FRR’s VRRP implementation accomplishes the virtual MAC requirement on real hardware.

Ingress traffic is a more complicated matter. Macvlan devices have multiple operating modes that change how ingress traffic is handled. Of relevance to FRR’s implementation are the bridge and private modes. In private mode, any ingress traffic on eth0 (in our example) with a source MAC address equal to the MAC address on any of eth0’s macvlan devices will be placed only on that macvlan device. This curious behavior is undesirable, since FRR’s implementation of VRRP needs to be able to receive advertisements from neighbors while in Backup mode - i.e., while its macvlan devices are in protodown on. If the macvlan devices are instead set to bridge mode, all ingress traffic shows up on all interfaces - including eth0 - regardless of source MAC or any other factor. Consequently, macvlans used by FRR for VRRP must be set to bridge mode or the protocol will not function correctly.

As for the MAC address assigned to this interface, the last byte of the address holds the VRID, in this case 0x05. The second to last byte is 0x01, as specified by the RFC for IPv4 operation. The IPv6 MAC address is be identical except that the second to last byte is defined to be 0x02. Two things to note from this arrangement:

  1. There can only be up to 255 unique Virtual Routers on an interface (only 1 byte is available for the VRID)
  2. IPv4 and IPv6 addresses must be assigned to different macvlan devices, because they have different MAC addresses

Finally, take note of the generated IPv6 link local address on the interface. For interfaces on which VRRP will operate in IPv6 mode, this link local cannot be derived using the usual EUI-64 method. This is because VRRP advertisements are sent from the link local address of this interface, and VRRP uses the source address of received advertisements as part of its election algorithm. If the IPv6 link local of a router is equivalent to the IPv6 link local in a received advertisement, this can cause both routers to assume the Master role (very bad). ifupdown knows to set the addrgenmode of the interface properly, but when using iproute2 to create the macvlan devices, you must be careful to manually specify addrgenmode random.

A brief note on the Backup state

It is worth noting here that an alternate choice for the implementation of the Backup state, such as removing all the IP addresses assigned to the macvlan device or deleting their local routes instead of setting the device into protodown on, would allow the protocol to function regardless of whether the macvlan device(s) are set to private or bridge mode. Indeed, the strange behavior of the kernel macvlan driver in private mode, whereby it performs what may be thought of as a sort of interface-level layer 2 “NAT” based on source MAC, can be traced back to a patch clearly designed to accommodate a VRRP implementation from a different vendor. However, the protodown based implementation allows for a configuration model in which FRR does not dynamically manage the addresses assigned on a system, but instead just manages interface state. Such a scenario was in mind when this protocol implementation was initially built, which is why the other choices are not currently present. Since support for placing macvlan devices into protodown was not added to Linux until version 5.1, this also explains the relatively restrictive kernel versioning requirement.

In the future other methods of implementing Backup state may be added along with a configuration knob to choose between them.

Interface Configuration

Continuing with the example from the previous section, we assume the macvlan interfaces have been properly configured with the proper MAC addresses and the IPvX addresses assigned.

In FRR, a possible VRRPv3 configuration for this interface is:

interface eth0
 vrrp 5 version 3
 vrrp 5 priority 200
 vrrp 5 advertisement-interval 1500
 vrrp 5 ip 10.0.2.16
 vrrp 5 ipv6 2001:0db8::0370:7334

VRRP will activate as soon as the first IPvX address configuration line is encountered. If you do not want this behavior, use the vrrp (1-255) shutdown command, and apply the no form when you are ready to activate VRRP.

At this point executing show vrrp will display the following:

ubuntu-bionic# show vrrp

 Virtual Router ID                    5
 Protocol Version                     3
 Autoconfigured                       Yes
 Shutdown                             No
 Interface                            eth0
 VRRP interface (v4)                  vrrp4-2-5
 VRRP interface (v6)                  vrrp6-2-5
 Primary IP (v4)                      10.0.2.15
 Primary IP (v6)                      fe80::9b91:7155:bf6a:d386
 Virtual MAC (v4)                     00:00:5e:00:01:05
 Virtual MAC (v6)                     00:00:5e:00:02:05
 Status (v4)                          Master
 Status (v6)                          Master
 Priority                             200
 Effective Priority (v4)              200
 Effective Priority (v6)              200
 Preempt Mode                         Yes
 Accept Mode                          Yes
 Advertisement Interval               1500 ms
 Master Advertisement Interval (v4)   1000 ms
 Master Advertisement Interval (v6)   1000 ms
 Advertisements Tx (v4)               14
 Advertisements Tx (v6)               14
 Advertisements Rx (v4)               0
 Advertisements Rx (v6)               0
 Gratuitous ARP Tx (v4)               1
 Neigh. Adverts Tx (v6)               1
 State transitions (v4)               2
 State transitions (v6)               2
 Skew Time (v4)                       210 ms
 Skew Time (v6)                       210 ms
 Master Down Interval (v4)            3210 ms
 Master Down Interval (v6)            3210 ms
 IPv4 Addresses                       1
 ..................................   10.0.2.16
 IPv6 Addresses                       1
 ..................................   2001:db8::370:7334

At this point, VRRP has sent gratuitous ARP requests for the IPv4 address, Unsolicited Neighbor Advertisements for the IPv6 address, and has asked Zebra to send Router Advertisements on its behalf. It is also transmitting VRRPv3 advertisements on the macvlan interfaces.

The Primary IP fields are of some interest, as the behavior may be counterintuitive. These fields show the source address used for VRRP advertisements. Although VRRPv3 advertisements are always transmitted on the macvlan interfaces, in the IPv4 case the source address is set to the primary IPv4 address on the base interface, eth0 in this case. This is a protocol requirement, and IPv4 VRRP will not function unless the base interface has an IPv4 address assigned. In the IPv6 case the link local of the macvlan interface is used.

If any misconfiguration errors are detected, VRRP for the misconfigured address family will not come up and the configuration issue will be logged to FRR’s configured logging destination.

Per the RFC, IPv4 and IPv6 virtual routers are independent of each other. For instance, it is possible for the IPv4 router to be in Backup state while the IPv6 router is in Master state; or for either to be completely inoperative while the other is operative, etc. Instances sharing the same base interface and VRID are shown together in the show output for conceptual convenience.

To complete your VRRP deployment, configure other routers on the segment with the exact same system and FRR configuration as shown above. Provided each router receives the others’ VRRP advertisements, the Master election protocol will run, one Master will be elected, and the other routers will place their macvlan interfaces into protodown on until Master fails or priority values are changed to favor another router.

Switching the protocol version to VRRPv2 is accomplished simply by changing version 3 to version 2 in the VRID configuration line. Note that VRRPv2 does not support IPv6, so any IPv6 configuration will be rejected by FRR when using VRRPv2.

Note

All VRRP routers initially start in Backup state, and wait for the calculated Master Down Interval to pass before they assume Master status. This prevents downstream neighbor table churn if another router is already Master with higher priority, meaning this box will ultimately assume Backup status once the first advertisement is received. However, if the calculated Master Down Interval is high and this router is configured such that it will ultimately assume Master status, then it will take a while for this to happen. This is a known issue.

All interface configuration commands are documented below.

[no] vrrp (1-255) [version (2-3)]

Create a VRRP router with the specified VRID on the interface. Optionally specify the protocol version. If the protocol version is not specified, the default is VRRPv3.

[no] vrrp (1-255) advertisement-interval (10-40950)

Set the advertisement interval. This is the interval at which VRRP advertisements will be sent. Values are given in milliseconds, but must be multiples of 10, as VRRP itself uses centiseconds.

[no] vrrp (1-255) ip A.B.C.D

Add an IPv4 address to the router. This address must already be configured on the appropriate macvlan device. Adding an IP address to the router will implicitly activate the router; see [no] vrrp (1-255) shutdown to override this behavior.

[no] vrrp (1-255) ipv6 X:X::X:X

Add an IPv6 address to the router. This address must already be configured on the appropriate macvlan device. Adding an IP address to the router will implicitly activate the router; see [no] vrrp (1-255) shutdown to override this behavior.

This command will fail if the protocol version is set to VRRPv2, as VRRPv2 does not support IPv6.

[no] vrrp (1-255) preempt

Toggle preempt mode. When enabled, preemption allows Backup routers with higher priority to take over Master status from the existing Master. Enabled by default.

[no] vrrp (1-255) priority (1-254)

Set the router priority. The router with the highest priority is elected as the Master. If all routers in the VRRP virtual router are configured with the same priority, the router with the highest primary IP address is elected as the Master. Priority value 255 is reserved for the acting Master router.

[no] vrrp (1-255) shutdown

Place the router into administrative shutdown. VRRP will not activate for this router until this command is removed with the no form.

Global Configuration

Show commands, global defaults and debugging configuration commands.

show vrrp [interface INTERFACE] [(1-255)] [json]

Shows VRRP status for some or all configured VRRP routers. Specifying an interface will only show routers configured on that interface. Specifying a VRID will only show routers with that VRID. Specifying json will dump each router state in a JSON array.

[no] debug vrrp [{protocol|autoconfigure|packets|sockets|ndisc|arp|zebra}]

Toggle debugging logs for VRRP components. If no component is specified, debugging for all components are turned on/off.

protocol
Logs state changes, election protocol decisions, and interface status changes.
autoconfigure
Logs actions taken by the autoconfiguration procedures. See Autoconfiguration.
packets
Logs details of ingress and egress packets. Includes packet decodes and hex dumps.
sockets
Logs details of socket configuration and initialization.
ndisc
Logs actions taken by the Neighbor Discovery component of VRRP.
arp
Logs actions taken by the ARP component of VRRP.
zebra
Logs communications with Zebra.
[no] vrrp default <advertisement-interval (1-4096)|preempt|priority (1-254)|shutdown>

Configure defaults for new VRRP routers. These values will not affect already configured VRRP routers, but will be applied to newly configured ones.

Autoconfiguration

In light of the complicated configuration required on the base system before VRRP can be enabled, FRR has the ability to automatically configure VRRP sessions by inspecting the interfaces present on the system. Since it is quite unlikely that macvlan devices with VRRP virtual MACs will exist on systems not using VRRP, this can be a convenient shortcut to automatically generate FRR configuration.

After configuring the interfaces as described in System Configuration, and configuring any defaults you may want, execute the following command:

[no] vrrp autoconfigure [version (2-3)]

Generates VRRP configuration based on the interface configuration on the base system. If the protocol version is not specified, the default is VRRPv3. Any existing interfaces that are configured properly for VRRP - i.e. have the correct MAC address, link local address (when required), IPv4 and IPv6 addresses - are used to create a VRRP router on their parent interfaces, with VRRP IPvX addresses taken from the addresses assigned to the macvlan devices. The generated configuration appears in the output of show run, which can then be modified as needed and written to the config file. The version parameter controls the protocol version; if using VRRPv2, keep in mind that IPv6 is not supported and will not be configured.

The following configuration is then generated for you:

interface eth0
 vrrp 5
 vrrp 5 ip 10.0.2.16
 vrrp 5 ipv6 2001:db8::370:7334

VRRP is automatically activated. Global defaults, if set, are applied.

You can then edit this configuration with vtysh as needed, and commit it by writing to the configuration file.