Unicast Support for the Virtual Router Redundancy Protocol (VRRP)
draft-abinabraham-vrrp-unicast-00
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| Last updated | 2026-04-19 | ||
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draft-abinabraham-vrrp-unicast-00
Network Working Group A. Dogra
Internet-Draft A. Abraham
Updates: 9568 (if approved) S. Krishnamurthy
Intended status: Informational Cisco Systems
Expires: 21 October 2026 19 April 2026
Unicast Support for the Virtual Router Redundancy Protocol (VRRP)
draft-abinabraham-vrrp-unicast-00
Abstract
The Virtual Router Redundancy Protocol (VRRP) is specified for
multicast operation on a shared LAN in RFC 9568. Some deployments,
including virtualized and cloud environments, require VRRP-like
first-hop redundancy but cannot use multicast delivery for VRRP
advertisements. This document updates RFC 9568 by defining an
optional unicast mode for VRRP.
In unicast mode, VRRP advertisements are sent to configured peer
addresses rather than to the VRRP multicast group. This document
preserves the VRRP packet format, protocol number, virtual IP
semantics, and host-facing forwarding behavior defined in RFC 9568,
while adding explicit peer configuration and receive-side source
validation for unicast operation.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 21 October 2026.
Copyright Notice
Copyright (c) 2026 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Scope and Applicability . . . . . . . . . . . . . . . . . . . 3
4. Use Cases and Deployment Drivers . . . . . . . . . . . . . . 3
5. Additional Definitions . . . . . . . . . . . . . . . . . . . 4
6. Unicast VRRP Overview . . . . . . . . . . . . . . . . . . . . 4
7. Peer Configuration . . . . . . . . . . . . . . . . . . . . . 5
8. Updates to RFC 9568 . . . . . . . . . . . . . . . . . . . . . 5
8.1. VRRP Overview . . . . . . . . . . . . . . . . . . . . . . 5
8.2. Protocol Processing . . . . . . . . . . . . . . . . . . . 6
8.3. IPv4 Field Descriptions . . . . . . . . . . . . . . . . . 6
8.4. IPv6 Field Descriptions . . . . . . . . . . . . . . . . . 6
8.5. Transmitting VRRP Packets . . . . . . . . . . . . . . . . 6
8.6. Receiving VRRP Packets . . . . . . . . . . . . . . . . . 7
8.7. Host-Facing Behavior . . . . . . . . . . . . . . . . . . 7
9. Operational Considerations . . . . . . . . . . . . . . . . . 7
10. Implementation Status . . . . . . . . . . . . . . . . . . . . 8
11. Security Considerations . . . . . . . . . . . . . . . . . . . 10
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
13.1. Normative References . . . . . . . . . . . . . . . . . . 10
13.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
[RFC9568] specifies VRRP for IPv4 and IPv6 and assumes multicast
operation on a shared LAN. In a number of modern deployments,
redundant routers still need fast active/backup failover for a
virtual default gateway, but the environment does not provide usable
multicast support for VRRP advertisements.
The primary deployment driver is the continued need for the classic
VRRP function of protecting a virtual IPv4 or IPv6 first-hop gateway
in environments where multicast delivery is unavailable, undesirable,
or operationally constrained. This includes virtualized, cloud,
overlay, and other deployments in which Virtual Routers still provide
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a common host-facing gateway service, but control traffic between the
Virtual Routers is exchanged through explicit peer connectivity
instead of a simple multicast-capable LAN.
The intended use case for this document is not a generic active/
backup role-election mechanism. Rather, it is a narrow extension of
VRRP for deployments that want to preserve the familiar VRRP state
machine, protocol number, virtual IP address semantics, Virtual
Router MAC behavior, and host-facing forwarding model while replacing
only the advertisement delivery method.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Scope and Applicability
This document updates [RFC9568] by defining an optional unicast mode
of operation for VRRP. The unicast mode is intended for deployments
that still want the classic VRRP model of a Virtual Router protecting
one or more virtual IPv4 or IPv6 addresses, but that cannot rely on
multicast delivery of advertisements.
The unicast mode defined here is limited to deployments in which the
participating VRRP Routers can exchange advertisements without
traversing a router that decrements the IPv4 TTL or IPv6 Hop Limit.
This preserves the receive-side validation model of [RFC9568] and
keeps the security and topology assumptions close to those of the
base protocol.
This document does not define multi-hop operation. If a deployment
requires routed multi-hop active/backup election or transport
encapsulation other than IP protocol 112, that deployment is outside
the scope of this specification.
4. Use Cases and Deployment Drivers
The operational motivation for unicast VRRP is straightforward:
operators still need the classic VRRP function of protecting a
virtual IPv4 or IPv6 default gateway, but some modern deployments
cannot rely on multicast delivery for VRRP advertisements.
Typical use cases include:
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1. Cloud and virtualized environments in which multicast is not
available or is not exposed in a way that is operationally
equivalent to a simple shared LAN.
2. Overlay or virtualized edge deployments in which the protected
routers still present a common host-facing gateway service, but
the control traffic between the routers is exchanged using
explicit peer connectivity rather than a multicast-capable LAN.
3. Service-provider and data-center designs in which operators want
to preserve the familiar VRRP packet format, state machine, and
virtual IP semantics of [RFC9568] while replacing only the
advertisement delivery method.
This document addresses those use cases by defining a unicast
delivery mode that stays close to the original VRRP model. It does
not attempt to standardize broader routed active/backup election
mechanisms that are no longer centered on protecting a virtual first-
hop gateway.
5. Additional Definitions
Unicast Mode A mode of VRRP operation in which
advertisements for a Virtual Router are sent
as unicast IPv4 or IPv6 packets to configured
peer addresses instead of to the VRRP
multicast destination address.
Unicast Peer A configured VRRP Router participating in the
same Virtual Router and address family whose
address is used as a permitted source and
destination for unicast VRRP advertisements.
Unicast Peer List The configured set of all other VRRP Routers
that participate in a unicast-mode Virtual
Router for a given address family.
6. Unicast VRRP Overview
A Virtual Router defined by this document operates in exactly one of
two modes:
* multicast mode, as specified in [RFC9568], or
* unicast mode, as specified in this document.
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A VRRP Router operating a given Virtual Router in unicast mode MUST
NOT send VRRP advertisements for that Virtual Router to the VRRP
multicast destination address. Instead, it MUST send a copy of each
advertisement to each address in the configured Unicast Peer List.
Except as updated by this document, the VRRP packet format, VRRP
state machine, timer calculations, preemption behavior, Virtual
Router semantics, virtual IP address behavior, and host-facing
forwarding behavior remain as specified in [RFC9568].
Unicast mode is configured per Virtual Router. A VRRP Router MUST
NOT mix multicast mode and unicast mode for the same Virtual Router
instance.
7. Peer Configuration
A Virtual Router operating in unicast mode MUST be configured with
one or more Unicast Peers. A configuration that enables unicast mode
without at least one peer is invalid, and the Virtual Router MUST NOT
operate in unicast mode until corrected.
Each VRRP Router participating in a unicast-mode Virtual Router MUST
be configured with the addresses of all other participating VRRP
Routers for that Virtual Router and address family.
For IPv4 operation, each configured peer address MUST be an IPv4
address that the receiving peer uses as the source address for VRRP
advertisements, as described in Section 8.3. For IPv6 operation,
each configured peer address MUST be an IPv6 link-local address used
by the receiving peer as the source address for VRRP advertisements,
as described in Section 8.4.
The local router's own address MUST NOT appear in its Unicast Peer
List.
8. Updates to RFC 9568
8.1. VRRP Overview
The references to multicast-only operation in Section 3 of [RFC9568]
are updated to allow an advertisement to be delivered either to the
VRRP multicast destination address, as specified in [RFC9568], or to
configured Unicast Peers, as specified in this document.
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8.2. Protocol Processing
Section 5 of [RFC9568] is updated so that a Virtual Router operating
in unicast mode sends and receives VRRP advertisements only through
the configured Unicast Peer List for that Virtual Router and address
family.
8.3. IPv4 Field Descriptions
For a Virtual Router operating in unicast mode, the IPv4 field
descriptions in Section 5.1.1 of [RFC9568] are updated as follows:
1. The IPv4 source address MUST be the primary IPv4 address of the
sending interface, as specified in [RFC9568].
2. The IPv4 destination address MUST be the IPv4 address of the
configured Unicast Peer to which the copy of the advertisement is
being sent.
3. The IPv4 TTL MUST be set to 255, and a received packet whose IPv4
TTL is not 255 MUST be discarded.
4. The IPv4 Protocol field MUST remain 112.
8.4. IPv6 Field Descriptions
For a Virtual Router operating in unicast mode, the IPv6 field
descriptions in Section 5.1.2 of [RFC9568] are updated as follows:
1. The IPv6 source address MUST be the link-local address of the
sending interface, as specified in [RFC9568].
2. The IPv6 destination address MUST be the configured IPv6 link-
local address of the Unicast Peer to which the copy of the
advertisement is being sent.
3. The IPv6 Hop Limit MUST be set to 255, and a received packet
whose Hop Limit is not 255 MUST be discarded.
4. The IPv6 Next Header field MUST remain 112.
8.5. Transmitting VRRP Packets
Section 7.2 of [RFC9568] is updated so that a VRRP Router operating a
Virtual Router in unicast mode sends one copy of each VRRP
advertisement to each configured Unicast Peer instead of sending the
advertisement to the VRRP multicast group.
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Other than the destination address, the packet contents MUST be the
same for each transmitted copy.
8.6. Receiving VRRP Packets
A VRRP Router operating a Virtual Router in unicast mode MUST process
only advertisements whose source address matches an address in the
configured Unicast Peer List for that Virtual Router and address
family.
A received VRRP packet for a unicast-mode Virtual Router MUST be
discarded if:
* the source address is not a configured Unicast Peer,
* the IPv4 TTL or IPv6 Hop Limit is not 255,
* the packet is received for the wrong address family, or
* the packet is otherwise invalid according to [RFC9568].
A VRRP Router operating a Virtual Router in unicast mode MUST ignore
VRRP advertisements for that same Virtual Router received through
multicast delivery.
8.7. Host-Facing Behavior
This document changes only advertisement delivery. The Active
Router's behavior with respect to the Virtual Router MAC address,
ARP, gratuitous ARP, IPv6 Neighbor Discovery, Router Advertisements,
Unsolicited Neighbor Advertisements, and forwarding responsibility
remains as specified in [RFC9568].
In particular, unicast mode does not replace the Virtual Router MAC
with a multicast MAC. The Virtual Router MAC remains the well-known
unicast VRRP MAC associated with the VRID, as specified in [RFC9568].
9. Operational Considerations
A deployment using unicast mode SHOULD ensure that all routers in a
given Virtual Router are configured with a consistent peer inventory.
Inconsistent peer lists can create asymmetric reachability and can
lead to multiple routers independently deciding that the Active
Router has failed.
A deployment using unicast mode SHOULD continue to use distinct
priority values as recommended in [RFC9568] so that Backup Routers do
not transition to Active state simultaneously after a failure.
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This document does not update the VRRP management model. Future work
may standardize YANG or other management objects for peer lists,
source validation policy, and related unicast-mode configuration.
10. Implementation Status
This section records publicly documented implementation experience
for the benefit of reviewers. It is not part of the protocol
specification.
Public documentation reviewed by the authors shows that unicast VRRP
support already exists in multiple products and open-source
implementations, although not all of them match the protocol profile
defined in this document.
One implementation detail that varies across products is the
interaction between unicast advertisement delivery and Virtual Router
MAC behavior. Implementations that remain close to classic VRRP tend
to preserve the standard unicast Virtual Router MAC and change only
the control-packet delivery method, while more divergent "unicast
VRRP" modes move away from the classic virtual-gateway and neighbor
handling model.
+================+============+====================================+
| Implementation | Support | Notes |
| | status | |
+================+============+====================================+
| Cisco IOS XR | Documented | Documents a unicast peer model |
| | | that preserves VRRP first-hop |
| | | redundancy semantics; published |
| | | operational output still shows the |
| | | standard VRRP Virtual MAC while |
| | | unicast transport is enabled, |
| | | indicating that only advertisement |
| | | delivery changes [CISCO-XR-VRRP] |
+----------------+------------+------------------------------------+
| Keepalived | Documented | Documents unicast peers, source |
| | | validation, and TTL controls; |
| | | documentation and reviewed source |
| | | show that VMAC behavior remains |
| | | available, but that vmac_xmit_base |
| | | is required when VMAC is combined |
| | | with unicast peers [KEEPALIVED] |
| | | [KEEPALIVED-SRC] |
+----------------+------------+------------------------------------+
| VyOS | Documented | Documents peer-based unicast VRRP |
| | | configuration and a separate |
| | | rfc3768-compatibility mode that |
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| | | creates a VRRP interface and |
| | | assigns the virtual MAC and |
| | | virtual IP, illustrating that |
| | | classic VRRP VMAC behavior is a |
| | | compatibility choice in some |
| | | unicast deployments [VYOS-HA] |
+----------------+------------+------------------------------------+
| Huawei VRP | Documented | Documents a proprietary |
| | | VRRPv2-based unicast mode, |
| | | including configurable transport |
| | | details, that is related to but |
| | | broader than the scoped profile in |
| | | this document [HUAWEI-UC-VRRP] |
| | | [HUAWEI-UC-VRRP-PORT] |
+----------------+------------+------------------------------------+
| Juniper Cloud- | Documented | Documents unicast VRRP use in |
| Native Router | | cloud workflows and route-table |
| | | ownership scenarios, illustrating |
| | | deployment demand in cloud |
| | | environments [JUNIPER-CNR-VRRP] |
| | | [JUNIPER-CNR-EKS] |
+----------------+------------+------------------------------------+
| MikroTik | No public | Public documentation reviewed by |
| RouterOS | unicast | the authors remains aligned with |
| | support | multicast VRRP behavior |
| | found | [MIKROTIK-VRRP] |
+----------------+------------+------------------------------------+
| FRRouting | No public | Public documentation reviewed by |
| | unicast | the authors describes VRRPv2 and |
| | support | VRRPv3 behavior with the standard |
| | found | RFC Virtual MAC and multicast |
| | | advertisements; reviewed source |
| | | code likewise remains aligned with |
| | | the classic multicast model and |
| | | does not show a unicast mode |
| | | [FRR-VRRP] [FRR-SRC] |
+----------------+------------+------------------------------------+
Table 1: Examples of publicly documented unicast VRRP support
The reviewed material also suggests that the MAC-related differences
are not about changing the VRRP Virtual Router MAC into a multicast
MAC. Instead, the main implementation differences are whether a
given product preserves the classic VRRP virtual-gateway model at
all, and, if it does, whether unicast advertisements are sent on the
same logical interface that owns the virtual MAC or on the base
interface beneath it.
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The existence of multiple deployed implementations supports the case
for standardization. At the same time, the documented differences
across implementations show why this document intentionally
standardizes only a narrow unicast mode that remains close to
[RFC9568].
11. Security Considerations
The security considerations of [RFC9568] continue to apply. In
particular, VRRP still provides no confidentiality and does not
prevent a hostile node from attempting to act as an Active Router.
Unicast mode introduces explicit peer configuration and requires that
a receiver validate the source address against the configured Unicast
Peer List. This provides an additional filtering mechanism beyond
the receive-side IPv4 TTL or IPv6 Hop Limit check.
Because this document preserves the requirement that the IPv4 TTL or
IPv6 Hop Limit be 255 on transmitted and accepted packets, the
protection against packets arriving from a remote network described
in [RFC5082] continues to apply.
A future specification for multi-hop unicast operation would need a
different security model and is outside the scope of this document.
12. IANA Considerations
This document has no IANA actions.
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC9568] Lindem, A. and A. Dogra, "Virtual Router Redundancy
Protocol (VRRP) Version 3 for IPv4 and IPv6", RFC 9568,
DOI 10.17487/RFC9568, April 2024,
<https://www.rfc-editor.org/info/rfc9568>.
13.2. Informative References
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[CISCO-XR-VRRP]
Cisco Systems, "Implementing VRRP on Cisco IOS XR",
Includes a section on Unicast VRRP,
<https://www.cisco.com/c/en/us/td/docs/routers/asr9000/
software/asr9k-r7-9/ip-addresses/configuration/guide/b-ip-
addresses-cg-asr9000-79x/implementing-vrrp.html>.
[FRR-SRC] FRRouting Project, "FRRouting vrrpd Source", Reviewed for
Virtual Router MAC construction and advertisement
transmission behavior,
<https://github.com/FRRouting/frr/blob/master/vrrpd/
vrrp.c>.
[FRR-VRRP] FRRouting Project, "FRRouting VRRP",
<https://docs.frrouting.org/en/stable-7.5/vrrp.html>.
[HUAWEI-UC-VRRP]
Huawei, "VRRP in Unicast Mode", Huawei HedEx
documentation, EDOC1100363264.
[HUAWEI-UC-VRRP-PORT]
Huawei, "unicast-vrrp port Command Reference", Huawei
HedEx documentation, EDOC1100277644.
[JUNIPER-CNR-EKS]
Juniper Networks, "Cloud-Native Router Deployment on
Amazon EKS",
<https://www.juniper.net/documentation/us/en/software/
cloud-native-router25.4/cloud-native-router-deployment-
guide/topics/concept/system-resource-requirements-
eks.html>.
[JUNIPER-CNR-VRRP]
Juniper Networks, "Cloud-Native Router VRRP Overview",
<https://www.juniper.net/documentation/us/en/software/
cloud-native-router23.4/cloud-native-router-
user/topics/concept/l3-vrrp.html>.
[KEEPALIVED]
Keepalived Project, "keepalived.conf(5) Manpage",
<https://www.keepalived.org/manpage.html>.
[KEEPALIVED-SRC]
Keepalived Project, "Keepalived VRRP VMAC Source",
Reviewed together with vrrp_parser.c for VMAC construction
and base-interface transmit behavior in unicast mode,
<https://github.com/acassen/keepalived/blob/master/
keepalived/vrrp/vrrp_vmac.c>.
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[MIKROTIK-VRRP]
MikroTik, "RouterOS VRRP",
<https://help.mikrotik.com/docs/spaces/ROS/pages/81362945/
VRRP>.
[RFC5082] Gill, V., Heasley, J., and D. Meyer, "The Generalized TTL
Security Mechanism (GTSM)", RFC 5082,
DOI 10.17487/RFC5082, October 2007,
<https://www.rfc-editor.org/info/rfc5082>.
[VYOS-HA] VyOS Project, "VyOS High Availability",
<https://docs.vyos.io/en/1.4/configuration/
highavailability/>.
Authors' Addresses
Aditya Dogra
Cisco Systems
Sarjapur Outer Ring Road
Bangalore 560103
Karnataka
India
Email: addogra@cisco.com
Abin Abraham
Cisco Systems
Sarjapur Outer Ring Road
Bangalore 560103
Karnataka
India
Email: abiabrah@cisco.com
Seshan Krishnamurthy
Cisco Systems
Sarjapur Outer Ring Road
Bangalore 560103
Karnataka
India
Email: seshakri@cisco.com
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