RFC2338 - Virtual Router Redundancy Protocol

Network Working Group S. Knight
Request for Comments: 2338 D. Weaver
Category: Standards Track Ascend Communications, Inc.
D. Whipple
Microsoft, Inc.
R. Hinden
D. Mitzel
P. Hunt
Nokia
P. Higginson
M. Shand
Digital Equipment Corp.
A. Lindem
IBM Corporation
April 1998
Virtual Router Redundancy Protocol
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
Abstract
This memo defines the Virtual Router Redundancy Protocol (VRRP).
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 IP address(es) associated with
a virtual router is called the Master, and forwards packets sent to
these IP addresses. The election process provides dynamic fail over
in the forwarding responsibility should the Master become
unavailable. This allows any of the virtual router IP addresses on
the LAN to be used as the default first hop router by end-hosts. 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.
Table of Contents
1. IntrodUCtion...............................................2
2. Required Features..........................................5
3. VRRP Overview..............................................6
4. Sample Configurations......................................8
5. Protocol...................................................9
5.1 VRRP Packet Format....................................10
5.2 IP Field Descriptions.................................10
5.3 VRRP Field Descriptions...............................11
6. Protocol State Machine....................................13
6.1 Parameters............................................13
6.2 Timers................................................15
6.3 State Transition Diagram..............................15
6.4 State Descriptions....................................15
7. Sending and Receiving VRRP Packets........................18
7.1 Receiving VRRP Packets................................18
7.2 Transmitting Packets..................................19
7.3 Virtual MAC Address...................................19
8. Operational Issues........................................20
8.1 ICMP Redirects........................................20
8.2 Host ARP Requests.....................................20
8.3 Proxy ARP.............................................20
9. Operation over FDDI and Token Ring........................21
9.1 Operation over FDDI...................................21
9.2 Operation over Token Ring.............................21
10. Security Considerations...................................23
10.1 No Authentication....................................23
10.2 Simple Text PassWord.................................23
10.3 IP Authentication Header.............................24
11. Acknowledgments...........................................24
12. References................................................24
13. Authors" Addresses........................................25
14. Full Copyright Statement..................................27
1. Introduction
There are a number of methods that an end-host can use to determine
its first hop router towards a particular IP destination. These
include running (or snooping) a dynamic routing protocol such as
Routing Information Protocol [RIP] or OSPF version 2 [OSPF], running
an ICMP router discovery client [DISC] or using a statically
configured default route.
Running a dynamic routing protocol on every end-host may be
infeasible for a number of reasons, including administrative
overhead, processing overhead, security issues, or lack of a protocol
implementation for some platforms. Neighbor or router discovery
protocols may require active participation by all hosts on a network,
leading to large timer values to reduce protocol overhead in the face
of large numbers of hosts. This can result in a significant delay in
the detection of a lost (i.e., dead) neighbor, which may introduce
unacceptably long "black hole" periods.
The use of a statically configured default route is quite popular; it
minimizes configuration and processing overhead on the end-host and
is supported by virtually every IP implementation. This mode of
operation is likely to persist as dynamic host configuration
protocols [DHCP] are deployed, which typically provide configuration
for an end-host IP address and default gateway. However, this
creates a single point of failure. Loss of the default router
results in a catastrophic event, isolating all end-hosts that are
unable to detect any alternate path that may be available.
The Virtual Router Redundancy Protocol (VRRP) is designed to
eliminate the single point of failure inherent in the static default
routed environment. 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 IP
address(es) associated with a virtual router is called the Master,
and forwards packets sent to these IP addresses. The election
process provides dynamic fail-over in the forwarding responsibility
should the Master become unavailable. Any of the virtual router"s IP
addresses on a LAN can then be used as the default first hop router
by end-hosts. 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.
VRRP provides a function similar to a Cisco Systems, Inc. proprietary
protocol named Hot Standby Router Protocol (HSRP) [HSRP] and to a
Digital Equipment Corporation, Inc. proprietary protocol named IP
Standby Protocol [IPSTB].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The IESG/IETF take no position regarding the validity or scope of any
intellectual property right or other rights that might be claimed to
pertain to the implementation or use of the technology, or the extent
to which any license under such rights might or might not be
available. See the IETF IPR web page at http://www.ietf.org/ipr.Html
for additional information.
1.1 Scope
The remainder of this document describes the features, design goals,
and theory of operation of VRRP. The message formats, protocol
processing rules and state machine that guarantee convergence to a
single Virtual Router Master are presented. Finally, operational
issues related to MAC address mapping, handling of ARP requests,
generation of ICMP redirect messages, and security issues are
addressed.
This protocol is intended for use with IPv4 routers only. A separate
specification will be produced if it is decided that similar
functionality is desirable in an IPv6 environment.
1.2 Definitions
VRRP Router A router running the Virtual Router Redundancy
Protocol. It may participate in one or more
virtual routers.
Virtual Router An abstract object managed by VRRP that acts
as a default router for hosts on a shared LAN.
It consists of a Virtual Router Identifier and
a set of associated IP address(es) across a
common LAN. A VRRP Router may backup one or
more virtual routers.
IP Address Owner The VRRP router that has the virtual router"s
IP address(es) as real interface address(es).
This is the router that, when up, will respond
to packets addressed to one of these IP
addresses for ICMP pings, TCP connections,
etc.
Primary IP Address An IP address selected from the set of real
interface addresses. One possible selection
algorithm is to always select the first
address. VRRP advertisements are always sent
using the primary IP address as the source of
the IP packet.
Virtual Router Master The VRRP router that is assuming the
responsibility of forwarding packets sent to
the IP address(es) associated with the virtual
router, and answering ARP requests for these
IP addresses. Note that if the IP address
owner is available, then it will always become
the Master.
Virtual Router Backup The set of VRRP routers available to assume
forwarding responsibility for a virtual router
should the current Master fail.
2.0 Required Features
This section outlines the set of features that were considered
mandatory and that guided the design of VRRP.
2.1 IP Address Backup
Backup of IP addresses is the primary function of the Virtual Router
Redundancy Protocol. While providing election of a Virtual Router
Master and the additional functionality described below, the protocol
should strive to:
- Minimize the duration of black holes.
- Minimize the steady state bandwidth overhead and processing
complexity.
- Function over a wide variety of multiAccess LAN technologies
capable of supporting IP traffic.
- Provide for election of multiple virtual routers on a network for
load balancing
- Support of multiple logical IP subnets on a single LAN segment.
2.2 Preferred Path Indication
A simple model of Master election among a set of redundant routers is
to treat each router with equal preference and claim victory after
converging to any router as Master. However, there are likely to be
many environments where there is a distinct preference (or range of
preferences) among the set of redundant routers. For example, this
preference may be based upon access link cost or speed, router
performance or reliability, or other policy considerations. The
protocol should allow the eXPression of this relative path preference
in an intuitive manner, and guarantee Master convergence to the most
preferential router currently available.
2.3 Minimization of Unnecessary Service Disruptions
Once Master election has been performed then any unnecessary
transitions between Master and Backup routers can result in a
disruption in service. The protocol should ensure after Master
election that no state transition is triggered by any Backup router
of equal or lower preference as long as the Master continues to
function properly.
Some environments may find it beneficial to avoid the state
transition triggered when a router becomes available that is more
preferential than the current Master. It may be useful to support an
override of the immediate convergence to the preferred path.
2.4 Extensible Security
The virtual router functionality is applicable to a wide range of
internetworking environments that may employ different security
policies. The protocol should require minimal configuration and
overhead in the insecure operation, provide for strong authentication
when increased security is required, and allow integration of new
security mechanisms without breaking backwards compatible operation.
2.5 Efficient Operation over Extended LANs
Sending IP packets on a multiaccess LAN requires mapping from an IP
address to a MAC address. The use of the virtual router MAC address
in an extended LAN employing learning bridges can have a significant
effect on the bandwidth overhead of packets sent to the virtual
router. If the virtual router MAC address is never used as the
source address in a link level frame then the station location is
never learned, resulting in flooding of all packets sent to the
virtual router. To improve the efficiency in this environment the
protocol should: 1) use the virtual router MAC as the source in a
packet sent by the Master to trigger station learning; 2) trigger a
message immediately after transitioning to Master to update the
station learning; and 3) trigger periodic messages from the Master to
maintain the station learning cache.
3.0 VRRP Overview
VRRP specifies an election protocol to provide the virtual router
function described earlier. All protocol messaging is performed
using IP multicast datagrams, thus the protocol can operate over a
variety of multiaccess LAN technologies supporting IP multicast.
Each VRRP virtual router has a single well-known MAC address
allocated to it. This document currently only details the mapping to
networks using the IEEE 802 48-bit MAC address. The virtual router
MAC address is used as the source in all periodic VRRP messages sent
by the Master router to enable bridge learning in an extended LAN.
A virtual router is defined by its virtual router identifier (VRID)
and a set of IP addresses. A VRRP router may associate a virtual
router with its real addresses on an interface, and may also be
configured with additional virtual router mappings and priority for
virtual routers it is willing to backup. The mapping between VRID
and addresses must be coordinated among all VRRP routers on a LAN.
However, there is no restriction against reusing a VRID with a
different address mapping on different LANs. The scope of each
virtual router is restricted to a single LAN.
To minimize network traffic, only the Master for each virtual router
sends periodic VRRP Advertisement messages. A Backup router will not
attempt to pre-empt the Master unless it has higher priority. This
eliminates service disruption unless a more preferred path becomes
available. It"s also possible to administratively prohibit all pre-
emption attempts. The only exception is that a VRRP router will
always become Master of any virtual router associated with addresses
it owns. If the Master becomes unavailable then the highest priority
Backup will transition to Master after a short delay, providing a
controlled transition of the virtual router responsibility with
minimal service interruption.
VRRP defines three types of authentication providing simple
deployment in insecure environments, added protection against
misconfiguration, and strong sender authentication in security
conscious environments. Analysis of the protection provided and
vulnerability of each mechanism is deferred to Section 10.0 Security
Considerations. In addition new authentication types and data can be
defined in the future without affecting the format of the fixed
portion of the protocol packet, thus preserving backward compatible
operation.
The VRRP protocol design provides rapid transition from Backup to
Master to minimize service interruption, and incorporates
optimizations that reduce protocol complexity while guaranteeing
controlled Master transition for typical operational scenarios. The
optimizations result in an election protocol with minimal runtime
state requirements, minimal active protocol states, and a single
message type and sender. The typical operational scenarios are
defined to be two redundant routers and/or distinct path preferences
among each router. A side effect when these assumptions are violated
(i.e., more than two redundant paths all with equal preference) is
that duplicate packets may be forwarded for a brief period during
Master election. However, the typical scenario assumptions are
likely to cover the vast majority of deployments, loss of the Master
router is infrequent, and the expected duration in Master election
convergence is quite small ( << 1 second ). Thus the VRRP
optimizations represent significant simplifications in the protocol
design while incurring an insignificant probability of brief network
degradation.
4. Sample Configurations
4.1 Sample Configuration 1
The following figure shows a simple network with two VRRP routers
implementing one virtual router. Note that this example is provided
to help understand the protocol, but is not expected to occur in
actual practice.
+-----+ +-----+
MR1 BR1


VRID=1 +-----+ +-----+
IP A ---------->* *<--------- IP B



------------------+------------+-----+--------+--------+--------+--
^ ^ ^ ^

(IP A) (IP A) (IP A) (IP A)

+--+--+ +--+--+ +--+--+ +--+--+
H1 H2 H3 H4
+-----+ +-----+ +--+--+ +--+--+
Legend:
---+---+---+-- = Ethernet, Token Ring, or FDDI
H = Host computer
MR = Master Router
BR = Backup Router
* = IP Address
(IP) = default router for hosts
The above configuration shows a very simple VRRP scenario. In this
configuration, the end-hosts install a default route to the IP
address of virtual router #1 (IP A) and both routers run VRRP. The
router on the left becomes the Master for virtual router #1 (VRID=1)
and the router on the right is the Backup for virtual router #1. If
the router on the left should fail, the other router will take over
virtual router #1 and its IP addresses, and provide uninterrupted
service for the hosts.
Note that in this example, IP B is not backed up by the router on the
left. IP B is only used by the router on the right as its interface
address. In order to backup IP B, a second virtual router would have
to be configured. This is shown in the next section.
4.2 Sample Configuration 2
The following figure shows a configuration with two virtual routers
with the hosts spitting their traffic between them. This example is
expected to be very common in actual practice.
+-----+ +-----+
MR1 MR2
& &
BR2 BR1
VRID=1 +-----+ +-----+ VRID=2
IP A ---------->* *<---------- IP B



------------------+------------+-----+--------+--------+--------+--
^ ^ ^ ^

(IP A) (IP A) (IP B) (IP B)

+--+--+ +--+--+ +--+--+ +--+--+
H1 H2 H3 H4
+-----+ +-----+ +--+--+ +--+--+
Legend:
---+---+---+-- = Ethernet, Token Ring, or FDDI
H = Host computer
MR = Master Router
BR = Backup Router
* = IP Address
(IP) = default router for hosts
In the above configuration, half of the hosts install a default route
to virtual router #1"s IP address (IP A), and the other half of the
hosts install a default route to virtual router #2"s IP address (IP
B). This has the effect of load balancing the outgoing traffic,
while also providing full redundancy.
5.0 Protocol
The purpose of the VRRP packet is to communicate to all VRRP routers
the priority and the state of the Master router associated with the
Virtual Router ID.
VRRP packets are sent encapsulated in IP packets. They are sent to
the IPv4 multicast address assigned to VRRP.
5.1 VRRP Packet Format
This section defines the format of the VRRP packet and the relevant
fields in the IP header.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version Type Virtual Rtr ID Priority Count IP Addrs
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Auth Type Adver Int Checksum
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP Address (1)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.
.
.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP Address (n)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Authentication Data (1)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Authentication Data (2)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.2 IP Field Descriptions
5.2.1 Source Address
The primary IP address of the interface the packet is being sent
from.
5.2.2 Destination Address
The IP multicast address as assigned by the IANA for VRRP is:
224.0.0.18
This is a link local scope multicast address. Routers MUST NOT
forward a datagram with this destination address regardless of its
TTL.
5.2.3 TTL
The TTL MUST be set to 255. A VRRP router receiving a packet with
the TTL not equal to 255 MUST discard the packet.
5.2.4 Protocol
The IP protocol number assigned by the IANA for VRRP is 112
(decimal).
5.3 VRRP Field Descriptions
5.3.1 Version
The version field specifies the VRRP protocol version of this packet.
This document defines version 2.
5.3.2 Type
The type field specifies the type of this VRRP packet. The only
packet type defined in this version of the protocol is:
1 ADVERTISEMENT
A packet with unknown type MUST be discarded.
5.3.3 Virtual Rtr ID (VRID)
The Virtual Router Identifier (VRID) field identifies the virtual
router this packet is reporting status for.
5.3.4 Priority
The priority field specifies the sending VRRP router"s priority for
the virtual router. Higher values equal higher priority. This field
is an 8 bit unsigned integer field.
The priority value for the VRRP router that owns the IP address(es)
associated with the virtual router MUST be 255 (decimal).
VRRP routers backing up a virtual router MUST use priority values
between 1-254 (decimal). The default priority value for VRRP routers
backing up a virtual router is 100 (decimal).
The priority value zero (0) has special meaning indicating that the
current Master has stopped participating in VRRP. This is used to
trigger Backup routers to quickly transition to Master without having
to wait for the current Master to timeout.
5.3.5 Count IP Addrs
The number of IP addresses contained in this VRRP advertisement.
5.3.6 Authentication Type
The authentication type field identifies the authentication method
being utilized. Authentication type is unique on a per interface
basis. The authentication type field is an 8 bit unsigned integer.
A packet with unknown authentication type or that does not match the
locally configured authentication method MUST be discarded.
The authentication methods currently defined are:
0 - No Authentication
1 - Simple Text Password
2 - IP Authentication Header
5.3.6.1 No Authentication
The use of this authentication type means that VRRP protocol
exchanges are not authenticated. The contents of the Authentication
Data field should be set to zero on transmission and ignored on
reception.
5.3.6.2 Simple Text Password
The use of this authentication type means that VRRP protocol
exchanges are authenticated by a clear text password. The contents
of the Authentication Data field should be set to the locally
configured password on transmission. There is no default password.
The receiver MUST check that the Authentication Data in the packet
matches its configured authentication string. Packets that do not
match MUST be discarded.
Note that there are security implications to using Simple Text
password authentication, and one should see the Security
Consideration section of this document.
5.3.6.3 IP Authentication Header
The use of this authentication type means the VRRP protocol exchanges
are authenticated using the mechanisms defined by the IP
Authentication Header [AUTH] using "The Use of HMAC-MD5-96 within ESP
and AH" [HMAC]. Keys may be either configured manually or via a key
distribution protocol.
If a packet is received that does not pass the authentication check
due to a missing authentication header or incorrect message digest,
then the packet MUST be discarded. The contents of the
Authentication Data field should be set to zero on transmission and
ignored on reception.
5.3.7 Advertisement Interval (Adver Int)
The Advertisement interval indicates the time interval (in seconds)
between ADVERTISEMENTS. The default is 1 second. This field is used
for troubleshooting misconfigured routers.
5.3.8 Checksum
The checksum field is used to detect data corruption in the VRRP
message.
The checksum is the 16-bit one"s complement of the one"s complement
sum of the entire VRRP message starting with the version field. For
computing the checksum, the checksum field is set to zero.
5.3.9 IP Address(es)
One or more IP addresses that are associated with the virtual router.
The number of addresses included is specified in the "Count IP Addrs"
field. These fields are used for troubleshooting misconfigured
routers.
5.3.10 Authentication Data
The authentication string is currently only utilized for simple text
authentication, similar to the simple text authentication found in
the Open Shortest Path First routing protocol [OSPF]. It is up to 8
characters of plain text. If the configured authentication string is
shorter than 8 bytes, the remaining space MUST be zero-filled. Any
VRRP packet received with an authentication string that does not
match the locally configured authentication string MUST be discarded.
The authentication string is unique on a per interface basis.
There is no default value for this field.
6. Protocol State Machine
6.1 Parameters
6.1.1 Parameters per Interface
Authentication_Type Type of authentication being used. Values
are defined in section 5.3.6.
Authentication_Data Authentication data specific to the
Authentication_Type being used.
6.1.2 Parameters per Virtual Router
VRID Virtual Router Identifier. Configured item
in the range 1-255 (decimal). There is no
default.
Priority Priority value to be used by this VRRP
router in Master election for this virtual
router. The value of 255 (decimal) is
reserved for the router that owns the IP
addresses associated with the virtual
router. The value of 0 (zero) is reserved
for Master router to indicate it is
releasing responsibility for the virtual
router. The range 1-254 (decimal) is
available for VRRP routers backing up the
virtual router. The default value is 100
(decimal).
IP_Addresses One or more IP addresses associated with
this virtual router. Configured item. No
default.
Advertisement_Interval Time interval between ADVERTISEMENTS
(seconds). Default is 1 second.
Skew_Time Time to skew Master_Down_Interval in
seconds. Calculated as:
( (256 - Priority) / 256 )
Master_Down_Interval Time interval for Backup to declare Master
down (seconds). Calculated as:
(3 * Advertisement_Interval) + Skew_time
Preempt_Mode Controls whether a higher priority Backup
router preempts a lower priority Master.
Values are True to allow preemption and
False to not prohibit preemption. Default
is True.
Note: Exception is that the router that owns
the IP address(es) associated with the
virtual router always pre-empts independent
of the setting of this flag.
6.2 Timers
Master_Down_Timer Timer that fires when ADVERTISEMENT has not
been heard for Master_Down_Interval.
Adver_Timer Timer that fires to trigger sending of
ADVERTISEMENT based on
Advertisement_Interval.
6.3 State Transition Diagram
+---------------+
+---------> <-------------+
Initialize
+------ ----------+
+---------------+

V V
+---------------+ +---------------+
---------------------->
Master Backup
<----------------------
+---------------+ +---------------+
6.4 State Descriptions
In the state descriptions below, the state names are identified by
{state-name}, and the packets are identified by all upper case
characters.
A VRRP router implements an instance of the state machine for each
virtual router election it is participating in.
6.4.1 Initialize
The purpose of this state is to wait for a Startup event. If a
Startup event is received, then:
- If the Priority = 255 (i.e., the router owns the IP address(es)
associated with the virtual router)
o Send an ADVERTISEMENT
o Broadcast a gratuitous ARP request containing the virtual
router MAC address for each IP address associated with the
virtual router.
o Set the Adver_Timer to Advertisement_Interval
o Transition to the {Master} state
else
o Set the Master_Down_Timer to Master_Down_Interval
o Transition to the {Backup} state
endif
6.4.2 Backup
The purpose of the {Backup} state is to monitor the availability and
state of the Master Router.
While in this state, a VRRP router MUST do the following:
- MUST NOT respond to ARP requests for the IP address(s) associated
with the virtual router.
- MUST discard packets with a destination link layer MAC address
equal to the virtual router MAC address.
- MUST NOT accept packets addressed to the IP address(es) associated
with the virtual router.
- If a Shutdown event is received, then:
o Cancel the Master_Down_Timer
o Transition to the {Initialize} state
endif
- If the Master_Down_Timer fires, then:
o Send an ADVERTISEMENT
o Broadcast a gratuitous ARP request containing the virtual
router MAC address for each IP address associated with the
virtual router
o Set the Adver_Timer to Advertisement_Interval
o Transition to the {Master} state
endif
- If an ADVERTISEMENT is received, then:
If the Priority in the ADVERTISEMENT is Zero, then:
o Set the Master_Down_Timer to Skew_Time
else:
If Preempt_Mode is False, or If the Priority in the
ADVERTISEMENT is greater than or equal to the local
Priority, then:
o Reset the Master_Down_Timer to Master_Down_Interval
else:
o Discard the ADVERTISEMENT
endif
endif
endif
6.4.3 Master
While in the {Master} state the router functions as the forwarding
router for the IP address(es) associated with the virtual router.
While in this state, a VRRP router MUST do the following:
- MUST respond to ARP requests for the IP address(es) associated
with the virtual router.
- MUST forward packets with a destination link layer MAC address
equal to the virtual router MAC address.
- MUST NOT accept packets addressed to the IP address(es) associated
with the virtual router if it is not the IP address owner.
- MUST accept packets addressed to the IP address(es) associated
with the virtual router if it is the IP address owner.
- If a Shutdown event is received, then:
o Cancel the Adver_Timer
o Send an ADVERTISEMENT with Priority = 0
o Transition to the {Initialize} state
endif
- If the Adver_Timer fires, then:
o Send an ADVERTISEMENT
o Reset the Adver_Timer to Advertisement_Interval
endif
- If an ADVERTISEMENT is received, then:
If the Priority in the ADVERTISEMENT is Zero, then:
o Send an ADVERTISEMENT
o Reset the Adver_Timer to Advertisement_Interval
else:
If the Priority in the ADVERTISEMENT is greater than the
local Priority,
or
If the Priority in the ADVERTISEMENT is equal to the local
Priority and the primary IP Address of the sender is greater
than the local primary IP Address, then:
o Cancel Adver_Timer
o Set Master_Down_Timer to Master_Down_Interval
o Transition to the {Backup} state
else:
o Discard ADVERTISEMENT
endif
endif
endif
7. Sending and Receiving VRRP Packets
7.1 Receiving VRRP Packets
Performed the following functions when a VRRP packet is received:
- MUST verify that the IP TTL is 255.
- MUST verify the VRRP version
- MUST verify that the received packet length is greater than or
equal to the VRRP header
- MUST verify the VRRP checksum
- MUST perform authentication specified by Auth Type
If any one of the above checks fails, the receiver MUST discard the
packet, SHOULD log the event and MAY indicate via network management
that an error occurred.
- MUST verify that the VRID is valid on the receiving interface
If the above check fails, the receiver MUST discard the packet.
- MAY verify that the IP address(es) associated with the VRID are
valid
If the above check fails, the receiver SHOULD log the event and MAY
indicate via network management that a misconfiguration was detected.
If the packet was not generated by the address owner (Priority does
not equal 255 (decimal)), the receiver MUST drop the packet,
otherwise continue processing.
- MUST verify that the Adver Interval in the packet is the same as
the locally configured for this virtual router
If the above check fails, the receiver MUST discard the packet,
SHOULD log the event and MAY indicate via network management that a
misconfiguration was detected.
7.2 Transmitting VRRP Packets
The following operations MUST be performed when transmitting a VRRP
packet.
- Fill in the VRRP packet fields with the appropriate virtual
router configuration state
- Compute the VRRP checksum
- Set the source MAC address to Virtual Router MAC Address
- Set the source IP address to interface primary IP address
- Set the IP protocol to VRRP
- Send the VRRP packet to the VRRP IP multicast group
Note: VRRP packets are transmitted with the virtual router MAC
address as the source MAC address to ensure that learning bridges
correctly determine the LAN segment the virtual router is attached
to.
7.3 Virtual Router MAC Address
The virtual router MAC address associated with a virtual router is an
IEEE 802 MAC Address in the following format:
00-00-5E-00-01-{VRID} (in hex in internet standard bit-order)
The first three octets are derived from the IANA"s OUI. The next two
octets (00-01) indicate the address block assigned to the VRRP
protocol. {VRID} is the VRRP Virtual Router Identifier. This
mapping provides for up to 255 VRRP routers on a network.
8. Operational Issues
8.1 ICMP Redirects
ICMP Redirects may be used normally when VRRP is running between a
group of routers. This allows VRRP to be used in environments where
the topology is not symmetric.
The IP source address of an ICMP redirect should be the address the
end host used when making its next hop routing decision. If a VRRP
router is acting as Master for virtual router(s) containing addresses
it does not own, then it must determine which virtual router the
packet was sent to when selecting the redirect source address. One
method to deduce the virtual router used is to examine the
destination MAC address in the packet that triggered the redirect.
It may be useful to disable Redirects for specific cases where VRRP
is being used to load share traffic between a number of routers in a
symmetric topology.
8.2 Host ARP Requests
When a host sends an ARP request for one of the virtual router IP
addresses, the Master virtual router MUST respond to the ARP request
with the virtual MAC address for the virtual router. The Master
virtual router MUST NOT respond with its physical MAC address. This
allows the client to always use the same MAC address regardless of
the current Master router.
When a VRRP router restarts or boots, it SHOULD not send any ARP
messages with its physical MAC address for the IP address it owns, it
should only send ARP messages that include Virtual MAC addresses.
This may entail:
- When configuring an interface, VRRP routers should broadcast a
gratuitous ARP request containing the virtual router MAC address
for each IP address on that interface.
- At system boot, when initializing interfaces for VRRP operation;
delay gratuitous ARP requests and ARP responses until both the IP
address and the virtual router MAC address are configured.
8.3 Proxy ARP
If Proxy ARP is to be used on a VRRP router, then the VRRP router
must advertise the Virtual Router MAC address in the Proxy ARP
message. Doing otherwise could cause hosts to learn the real MAC
address of the VRRP router.
9. Operation over FDDI and Token Ring
9.1 Operation over FDDI
FDDI interfaces remove from the FDDI ring frames that have a source
MAC address matching the device"s hardware address. Under some
conditions, such as router isolations, ring failures, protocol
transitions, etc., VRRP may cause there to be more than one Master
router. If a Master router installs the virtual router MAC address
as the hardware address on a FDDI device, then other Masters"
ADVERTISEMENTS will be removed from the ring during the Master
convergence, and convergence will fail.
To avoid this an implementation SHOULD configure the virtual router
MAC address by adding a unicast MAC filter in the FDDI device, rather
than changing its hardware MAC address. This will prevent a Master
router from removing any ADVERTISEMENTS it did not originate.
9.2 Operation over Token Ring
Token ring has several characteristics which make running VRRP
difficult. These include:
- In order to switch to a new master located on a different bridge
token ring segment from the previous master when using source
route bridges, a mechanism is required to update cached source
route information.
- No general multicast mechanism supported across old and new token
ring adapter implementations. While many newer token ring adapters
support group addresses, token ring functional address support is
the only generally available multicast mechanism. Due to the
limited number of token ring functional addresses these may
collide with other usage of the same token ring functional
addresses.
Due to these difficulties, the preferred mode of operation over token
ring will be to use a token ring functional address for the VRID
virtual MAC address. Token ring functional addresses have the two
high order bits in the first MAC address octet set to B"1". They
range from 03-00-00-00-00-80 to 03-00-02-00-00-00 (canonical format).
However, unlike multicast addresses, there is only one unique
functional address per bit position. The functional addresses
addresses 03-00-00-10-00-00 through 03-00-02-00-00-00 are reserved
by the Token Ring Architecture [TKARCH] for user-defined
applications. However, since there are only 12 user-defined token
ring functional addresses, there may be other non-IP protocols using
the same functional address. Since the Novell IPX [IPX] protocol uses
the 03-00-00-10-00-00 functional address, operation of VRRP over
token ring will avoid use of this functional address. In general,
token ring VRRP users will be responsible for resolution of other
user-defined token ring functional address conflicts.
VRIDs are mapped directly to token ring functional addresses. In
order to decrease the likelihood of functional address conflicts,
allocation will begin with the largest functional address. Most non-
IP protocols use the first or first couple user-defined functional
addresses and it is expected that VRRP users will choose VRIDs
sequentially starting with 1.
VRID Token Ring Functional Address
---- -----------------------------
1 03-00-02-00-00-00
2 03-00-04-00-00-00
3 03-00-08-00-00-00
4 03-00-10-00-00-00
5 03-00-20-00-00-00
6 03-00-40-00-00-00
7 03-00-80-00-00-00
8 03-00-00-01-00-00
9 03-00-00-02-00-00
10 03-00-00-04-00-00
11 03-00-00-08-00-00
Or more succinctly, octets 3 and 4 of the functional address are
equal to (0x4000 >> (VRID - 1)) in non-canonical format.
Since a functional address cannot be used used as a MAC level source
address, the real MAC address is used as the MAC source address in
VRRP advertisements. This is not a problem for bridges since packets
addressed to functional addresses will be sent on the spanning-tree
explorer path [802.1D].
The functional address mode of operation MUST be implemented by
routers supporting VRRP on token ring.
Additionally, routers MAY support unicast mode of operation to take
advantage of newer token ring adapter implementations which support
non-promiscuous reception for multiple unicast MAC addresses and to
avoid both the multicast traffic and usage conflicts associated with
the use of token ring functional addresses. Unicast mode uses the
same mapping of VRIDs to virtual MAC addresses as Ethernet. However,
one important difference exists. ARP request/reply packets contain
the virtual MAC address as the source MAC address. The reason for
this is that some token ring driver implementations keep a cache of
MAC address/source routing information independent of the ARP cache.
Hence, these implementations need have to receive a packet with the
virtual MAC address as the source address in order to transmit to
that MAC address in a source-route bridged network.
Unicast mode on token ring has one limitation which should be
considered. If there are VRID routers on different source-route
bridge segments and there are host implementations which keep their
source-route information in the ARP cache and do not listen to
gratuitous ARPs, these hosts will not update their ARP source-route
information correctly when a switch-over occurs. The only possible
solution is to put all routers with the same VRID on the same source-
bridge segment and use techniques to prevent that bridge segment from
being a single point of failure. These techniques are beyond the
scope this document.
For both the multicast and unicast mode of operation, VRRP
advertisements sent to 224.0.0.18 should be encapsulated as described
in [RFC1469].
10. Security Considerations
VRRP is designed for a range of internetworking environments that may
employ different security policies. The protocol includes several
authentication methods ranging from no authentication, simple clear
text passwords, and strong authentication using IP Authentication
with MD5 HMAC. The details on each approach including possible
attacks and recommended environments follows.
Independent of any authentication type VRRP includes a mechanism
(setting TTL=255, checking on receipt) that protects against VRRP
packets being injected from another remote network. This limits most
vulnerabilities to local attacks.
10.1 No Authentication
The use of this authentication type means that VRRP protocol
exchanges are not authenticated. This type of authentication SHOULD
only be used in environments were there is minimal security risk and
little chance for configuration errors (e.g., two VRRP routers on a
LAN).
10.2 Simple Text Password
The use of this authentication type means that VRRP protocol
exchanges are authenticated by a simple clear text password.
This type of authentication is useful to protect against accidental
misconfiguration of routers on a LAN. It protects against routers
inadvertently backing up another router. A new router must first be
configured with the correct password before it can run VRRP with
another router. This type of authentication does not protect against
hostile attacks where the password can be learned by a node snooping
VRRP packets on the LAN. The Simple Text Authentication combined
with the TTL check makes it difficult for a VRRP packet to be sent
from another LAN to disrupt VRRP operation.
This type of authentication is RECOMMENDED when there is minimal risk
of nodes on a LAN actively disrupting VRRP operation. If this type
of authentication is used the user should be aware that this clear
text password is sent frequently, and therefore should not be the
same as any security significant password.
10.3 IP Authentication Header
The use of this authentication type means the VRRP protocol exchanges
are authenticated using the mechanisms defined by the IP
Authentication Header [AUTH] using "The Use of HMAC-MD5-96 within ESP
and AH", [HMAC]. This provides strong protection against
configuration errors, replay attacks, and packet
corruption/modification.
This type of authentication is RECOMMENDED when there is limited
control over the administration of nodes on a LAN. While this type
of authentication does protect the operation of VRRP, there are other
types of attacks that may be employed on shared media links (e.g.,
generation of bogus ARP replies) which are independent from VRRP and
are not protected.
11. Acknowledgments
The authors would like to thank Glen Zorn, and Michael Lane, Clark
Bremer, Hal Peterson, Tony Li, Barbara Denny, Joel Halpern, Steve
Bellovin, and Thomas Narten for their comments and suggestions.
12. References
[802.1D] International Standard ISO/IEC 10038: 1993, ANSI/IEEE Std
802.1D, 1993 edition.
[AUTH] Kent, S., and R. Atkinson, "IP Authentication Header",
Work in Progress.
[DISC] Deering, S., "ICMP Router Discovery Messages", RFC1256,
September 1991.
[DHCP] Droms, R., "Dynamic Host Configuration Protocol", RFC2131,
March 1997.
[HMAC] Madson, C., and R. Glenn, "The Use of HMAC-MD5-96 within
ESP and AH", Work in Progress.
[HSRP] Li, T., Cole, B., Morton, P., and D. Li, "Cisco Hot Standby
Router Protocol (HSRP)", RFC2281, March 1998.
[IPSTB] Higginson, P., M. Shand, "Development of Router Clusters to
Provide Fast Failover in IP Networks", Digital Technical
Journal, Volume 9 Number 3, Winter 1997.
[IPX] Novell Incorporated., "IPX Router Specification", Version
1.10, October 1992.
[OSPF] Moy, J., "OSPF Version 2", STD 54, RFC2328, April 1998.
[RIP] Hedrick, C., "Routing Information Protocol", RFC1058,
June 1988.
[RFC1469] Pusateri, T., "IP over Token Ring LANs", RFC1469, June
1993.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.
[TKARCH] IBM Token-Ring Network, Architecture Reference, Publication
SC30-3374-02, Third Edition, (September, 1989).
13. Authors" Addresses
Steven Knight Phone: +1 612 943-8990
Ascend Communications EMail: Steven.Knight@ascend.com
High Performance Network Division
10250 Valley View Road, Suite 113
Eden Prairie, MN USA 55344
USA
Douglas Weaver Phone: +1 612 943-8990
Ascend Communications EMail: Doug.Weaver@ascend.com
High Performance Network Division
10250 Valley View Road, Suite 113
Eden Prairie, MN USA 55344
USA
David Whipple Phone: +1 206 703-3876
Microsoft Corporation EMail: dwhipple@microsoft.com
One Microsoft Way
Redmond, WA USA 98052-6399
USA
Robert Hinden Phone: +1 408 990-2004
Nokia EMail: hinden@iprg.nokia.com
232 Java Drive
Sunnyvale, CA 94089
USA
Danny Mitzel Phone: +1 408 990-2037
Nokia EMail: mitzel@iprg.nokia.com
232 Java Drive
Sunnyvale, CA 94089
USA
Peter Hunt Phone: +1 408 990-2093
Nokia EMail: hunt@iprg.nokia.com
232 Java Drive
Sunnyvale, CA 94089
USA
P. Higginson Phone: +44 118 920 6293
Digital Equipment Corp. EMail: higginson@mail.dec.com
Digital Park
Imperial Way
Reading
Berkshire
RG2 0TE
UK
M. Shand Phone: +44 118 920 4424
Digital Equipment Corp. EMail: shand@mail.dec.com
Digital Park
Imperial Way
Reading
Berkshire
RG2 0TE
UK
Acee Lindem Phone: 1-919-254-1805
IBM Corporation E-Mail: acee@raleigh.ibm.com
P.O. Box 12195
Research Triangle Park, NC 27709
USA
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