rfc9568v2.txt   rfc9568.txt 
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access link cost or speed, router performance or reliability, or access link cost or speed, router performance or reliability, or
other policy considerations. The protocol should allow the other policy considerations. The protocol should allow the
expression of this relative path preference in an intuitive manner expression of this relative path preference in an intuitive manner
and guarantee Active Router convergence to the most preferred Virtual and guarantee Active Router convergence to the most preferred Virtual
Router currently available. Router currently available.
2.3. Minimization of Unnecessary Service Disruptions 2.3. Minimization of Unnecessary Service Disruptions
Once Active Router election has been performed, any unnecessary Once Active Router election has been performed, any unnecessary
transition between Active and Backup Routers can result in a transition between Active and Backup Routers can result in a
disruption in service. The protocol should ensure that, after Active disruption of service. The protocol should ensure that, after Active
Router election, no state transition is triggered by any Backup Router election, no state transition is triggered by any Backup
Router of equal or lower preference as long as the Active Router Router of equal or lower preference as long as the Active Router
continues to function properly. continues to function properly.
Some environments may find it beneficial to avoid the state Some environments may find it beneficial to avoid the state
transition triggered when a router that is preferred over the current transition triggered when a router that is preferred over the current
Active Router becomes available. It may be useful to support an Active Router becomes available. It may be useful to support an
override of the immediate restoration to the preferred path. override of the immediate restoration to the preferred path.
2.4. Efficient Operation over Extended LANs 2.4. Efficient Operation over Extended LANs
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One possible problematic scenario that may occur when using a small One possible problematic scenario that may occur when using a small
Advertisement_Interval (refer to Section 6.1) is when a VRRP Router Advertisement_Interval (refer to Section 6.1) is when a VRRP Router
is generating more packets than it can transmit, and a queue builds is generating more packets than it can transmit, and a queue builds
up on the VRRP Router. When this occurs, it is possible that packets up on the VRRP Router. When this occurs, it is possible that packets
being transmitted onto the VRRP-protected LAN could see a larger being transmitted onto the VRRP-protected LAN could see a larger
queueing delay than the smallest Advertisement_Interval. In this queueing delay than the smallest Advertisement_Interval. In this
case, the Active_Down_Interval (refer to Section 6.1) may be small case, the Active_Down_Interval (refer to Section 6.1) may be small
enough that normal queuing delays might cause a Backup Router to enough that normal queuing delays might cause a Backup Router to
conclude that the Active Router is down and, hence, promote itself to conclude that the Active Router is down and, hence, promote itself to
Active Router. Very shortly afterwards, the delayed VRRP packets Active Router. Very shortly afterwards, the delayed VRRP packets
from the original Active Router cause a switch back to the Backup from the original Active Router cause the VRRP Router to switch back
Router. Furthermore, this process can repeat many times per second, to Backup Router. Furthermore, this process can repeat many times
causing a significant disruption of traffic. To mitigate this per second, causing a significant disruption of traffic. To mitigate
problem, giving VRRP packets priority on egress interface queues this problem, giving VRRP packets priority on egress interface queues
should be considered. If the Active Router observes that this is should be considered. If the Active Router observes that this is
occurring, it SHOULD log the problem (subject to rate-limiting). occurring, it SHOULD log the problem (subject to rate-limiting).
3. VRRP Overview 3. VRRP Overview
VRRP specifies an election protocol to provide the Virtual Router VRRP specifies an election protocol to provide the Virtual Router
function described earlier. All protocol messaging is performed function described earlier. All protocol messaging is performed
using either IPv4 or IPv6 multicast datagrams. Thus, the protocol using either IPv4 or IPv6 multicast datagrams. Thus, the protocol
can operate over a variety of multiaccess LAN technologies supporting can operate over a variety of multiaccess LAN technologies supporting
IPvX multicast. Each link of a VRRP Virtual Router has a single IPvX multicast. Each link of a VRRP Virtual Router has a single
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Figure 1: Sample VRRP Network 1 Figure 1: Sample VRRP Network 1
In the IPv4 case, i.e., IPvX is IPv4 everywhere in the figure, each In the IPv4 case, i.e., IPvX is IPv4 everywhere in the figure, each
router is permanently assigned an IPv4 address on the LAN interface router is permanently assigned an IPv4 address on the LAN interface
(Router-1 is assigned IPv4 A and Router-2 is assigned IPv4 B), and (Router-1 is assigned IPv4 A and Router-2 is assigned IPv4 B), and
each host installs a default route (learned through DHCPv4 or via a each host installs a default route (learned through DHCPv4 or via a
configured static route) through one of the routers (in this example, configured static route) through one of the routers (in this example,
they all use Router-1's IPv4 A). they all use Router-1's IPv4 A).
In the IPv6 case, i.e., IPvX is IPv6 everywhere in the figure, each In the IPv6 case, i.e., IPvX is IPv6 everywhere in the figure, each
router has its own Link-Local IPv6 address on the LAN interface and a router has its own link-local IPv6 address on the LAN interface and a
link-local IPv6 address per VRID that is shared with the other link-local IPv6 address per VRID that is shared with the other
routers that serve the same VRID. Each host learns a default route routers that serve the same VRID. Each host learns a default route
from Router Advertisements through one of the routers (in this from Router Advertisements through one of the routers (in this
example, they all use Router-1's IPv6 Link-Local A). example, they all use Router-1's IPv6 Link-Local A).
In an IPv4 VRRP environment, each router supports reception and In an IPv4 VRRP environment, each router supports reception and
transmission for the exact same IPv4 address. Router-1 is said to be transmission for the exact same IPv4 address. Router-1 is said to be
the IPv4 address owner of IPv4 A, and Router-2 is the IPv4 address the IPv4 address owner of IPv4 A, and Router-2 is the IPv4 address
owner of IPv4 B. A Virtual Router is then defined by associating a owner of IPv4 B. A Virtual Router is then defined by associating a
unique identifier (the VRID) with the address owned by Router-1. unique identifier (the VRID) with the address owned by Router-1.
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Note: VRRP packets are transmitted with the Virtual Router MAC Note: VRRP packets are transmitted with the Virtual Router MAC
address as the source MAC address to ensure that learning bridges address as the source MAC address to ensure that learning bridges
correctly determine the LAN segment to which the Virtual Router is correctly determine the LAN segment to which the Virtual Router is
attached. attached.
7.3. Virtual Router MAC Address 7.3. Virtual Router MAC Address
The Virtual Router MAC address associated with a Virtual Router is an The Virtual Router MAC address associated with a Virtual Router is an
IEEE 802 MAC address [RFC9542] in the following format: IEEE 802 MAC address [RFC9542] in the following format:
IPv4 case: 00-00-5E-00-01-{VRID} (in hex, in network bit order) IPv4 case: 00-00-5E-00-01-{VRID} (in hex, in network byte order)
The first three octets are derived from the IANA's Organizationally The first three octets are derived from the IANA's Organizationally
Unique Identifier (OUI). The next two octets (00-01) indicate the Unique Identifier (OUI). The next two octets (00-01) indicate the
address block assigned to the VRRP protocol for the IPv4 protocol. address block assigned to the VRRP protocol for the IPv4 protocol.
{VRID} is the Virtual Router Identifier. This mapping provides for {VRID} is the Virtual Router Identifier. This mapping provides for
up to 255 IPv4 VRRP Routers on a LAN. up to 255 IPv4 VRRP Routers on a LAN.
IPv6 case: 00-00-5E-00-02-{VRID} (in hex, in network bit order) IPv6 case: 00-00-5E-00-02-{VRID} (in hex, in network byte order)
The first three octets are derived from the IANA's OUI. The next two The first three octets are derived from the IANA's OUI. The next two
octets (00-02) indicate the address block assigned to the VRRP octets (00-02) indicate the address block assigned to the VRRP
protocol for the IPv6 protocol. {VRID} is the Virtual Router protocol for the IPv6 protocol. {VRID} is the Virtual Router
Identifier. This mapping provides for up to 255 IPv6 VRRP Routers on Identifier. This mapping provides for up to 255 IPv6 VRRP Routers on
a LAN. a LAN.
7.4. IPv6 Interface Identifiers 7.4. IPv6 Interface Identifiers
[RFC8064] specifies that [RFC7217] be used as the default scheme for [RFC8064] specifies that [RFC7217] be used as the default scheme for
generating a stable address in IPv6 Stateless Address generating a stable address in IPv6 Stateless Address
Autoconfiguration (SLAAC) [RFC4862]. The Virtual Router MAC MUST NOT Autoconfiguration (SLAAC) [RFC4862]. The Virtual Router MAC MUST NOT
be used for the Net_Iface parameter used in the Interface Identifier be used for the Net_Iface parameter used in the Interface Identifier
(IID) derivation algorithms in [RFC7217] and [RFC8981]. (IID) derivation algorithms in [RFC7217] and [RFC8981].
Similarly, the Virtual Router MAC MUST NOT be used for the Net_Iface
parameter used for the Interface Identifier (IID) derivation
algorithms in [RFC7217] and [RFC8981].
This VRRP specification describes how to advertise and resolve the This VRRP specification describes how to advertise and resolve the
VRRP Router's IPv6 link-local address and other associated IPv6 VRRP Router's IPv6 link-local address and other associated IPv6
addresses into the Virtual Router MAC address. addresses into the Virtual Router MAC address.
8. Operational Issues 8. Operational Issues
8.1. IPv4 8.1. IPv4
8.1.1. ICMP Redirects 8.1.1. ICMP Redirects
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detected, the condition SHOULD be logged (subject to rate-limiting). detected, the condition SHOULD be logged (subject to rate-limiting).
If no VRRP Router has this priority, and preemption is disabled, then If no VRRP Router has this priority, and preemption is disabled, then
no preemption will occur. no preemption will occur.
In order to avoid two or more Backup Routers simultaneously becoming In order to avoid two or more Backup Routers simultaneously becoming
Active Routers after the previous Active Router fails or is shut Active Routers after the previous Active Router fails or is shut
down, all Virtual Routers SHOULD be configured with different down, all Virtual Routers SHOULD be configured with different
priorities and with sufficient differences in the priorities so that priorities and with sufficient differences in the priorities so that
lower priority Backup Routers do not transition to the Active state lower priority Backup Routers do not transition to the Active state
before receiving an advertisement from the highest priority Backup before receiving an advertisement from the highest priority Backup
Router following it transitioning to the Active Router. If multiple Router when it transitions to the Active Router. If multiple VRRP
VRRP Routers advertising the same priority are detected, this Routers advertising the same priority are detected, this condition
condition MAY be logged as a warning (subject to rate-limiting). MAY be logged as a warning (subject to rate-limiting).
Since the Skew_Time is reduced as the priority is increased, faster Since the Skew_Time is reduced as the priority is increased, faster
convergence can be obtained by using a higher priority for the convergence can be obtained by using a higher priority for the
preferred Backup Router. However, with multiple Backup Routers, the preferred Backup Router. However, with multiple Backup Routers, the
priorities should have sufficient differences, as previously priorities should have sufficient differences, as previously
recommended. recommended.
8.4. VRRPv3 and VRRPv2 Interoperation 8.4. VRRPv3 and VRRPv2 Interoperation
8.4.1. Assumptions 8.4.1. Assumptions
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