INTERNET-DRAFT S. Knight
October 23, 1997 D. Weaver
Ascend Communications, Inc.
D. Whipple
Microsoft, Inc.
R. Hinden
D. Mitzel
P. Hunt
Ipsilon Networks, Inc.
P. Higginson
M. Shand
Digital Equipment Corp.
Virtual Router Redundancy Protocol
<draft-ietf-vrrp-spec-03.txt>
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
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This internet draft expires on April 23, 1998.
Abstract
This memo defines the Virtual Router Redundancy Protocol (VRRP).
VRRP specifies an election protocol that dynamically allows a set of
routers running VRRP to backup each other on a LAN. The VRRP router
controlling one or more IP addresses is called the Master router, and
forwards packets sent to these IP addresses. The election process
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provides dynamic fail over in the forwarding responsibility should
the Master become unavailable. This allows any of the VRRP routers
IP addresses on the LAN to be used as the default first hop router by
end-hosts. The advantage gained from using the 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...............................................3
2. Required Features..........................................5
3. VRRP Overview..............................................6
4. Sample Configurations......................................8
5. Protocol..................................................10
5.1 VRRP Packet Format...................................10
5.2 IP Field Descriptions................................10
5.3 VRRP Field Descriptions..............................11
6. Protocol State Machine....................................14
6.1 Parameters.............................................14
6.2 Timers.................................................14
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...................................18
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.............................................21
9. Operation over FDDI and Token Ring........................21
10. Security Considerations...................................22
10.1 No Authentication.....................................22
10.2 Simple Text Password..................................22
10.3 IP Authentication Header..............................22
11. Acknowledgments...........................................23
12. References................................................24
13. Authors' Addresses........................................24
14. Changes from Previous Drafts..............................26
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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 allows a set of routers to backup each other. The VRRP
router controlling one or more IP addresses is called the Master
router, 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 IP
addresses on a virtual router can then be used as the default first
hop router by end-hosts. The advantage gained from using the 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.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC 2119].
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 Master router 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
Virtual Router One of a set of routers running VRRP on a LAN.
IP Address Owner The virtual router than has the 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.
Master Router The virtual router that is assuming the
responsibility of forwarding packets sent to the
IP addresses associated with a virtual router
and answering ARP requests for these IP
addresses. The Master Router may or may not be
the owner. Note that if the IP address owner is
available, then it will always be the master
router.
Backup Router The set of virtual routers available to assume
forwarding responsibility for a virtual router
should the current master router fail.
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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 Master router 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 or in 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
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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 messages sent by
the Master router to enable bridge learning in an extended LAN.
A virtual router is identified by its virtual router identifier. A
VRRP router has a set of addresses that it owns and one or more other
virtual routers it is responsible for backing up. On an interface
running VRRP, each VRRP router must be configured with a virtual
router identifier for the addresses it owns, and the other virtual
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router identifiers and associated IP addresses that it is responsible
for backing up. In addition, each VRRP router is assigned a priority
to indicate it's preference in Master election for each virtual
router. Multiple virtual routers can be elected on a network and a
single router can backup one or more virtual routers.
To minimize network traffic, only the Master router sends periodic
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 to this is that the owner will always become master
when it is up. 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.
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4. Sample Configurations
4.1 Sample Configuration 1
The following figure shows a simple network with two virtual routers.
VRID=1 VRID=2
+-----+ +-----+
| MR1 | | MR2 |
| & | | & |
| BR2 | | BR1 |
+-----+ +-----+
IP A ---------->* *<---------- IP B
| |
| |
| |
------------------+------------+-----+--------+--------+--------+--
^ ^ ^ ^
| | | |
(IP A) (IP A) (IP A) (IP A)
| | | |
+--+--+ +--+--+ +--+--+ +--+--+
| H1 | | H2 | | H3 | | H4 |
+-----+ +-----+ +--+--+ +--+--+
Legend:
---+---+---+-- = 802 network, Ethernet or FDDI
H = Host computer
MR = Master Router
BR = Backup Router
* = IP Address
(IP) = default router for hosts
The above configuration shows a simple VRRP scenario. In this
configuration, the end-hosts install a default route to the IP
address of one of the virtual routers (IP A) and the routers run
VRRP. The router on the left (VRID=1) becomes the Master router for
the IP addresses it owns (IP A) and the router on the right (VRID=2)
becomes the Master router for the IP addresses it owns (IP B). Each
router also backs up the other router. If the router on the left
(VRID=1) should fail, the other router will take over its IP
addresses and provide uninterrupted service for the hosts.
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4.2 Sample Configuration 2
The following figure shows a configuration with two virtual routers
with the hosts slitting their traffic between them.
VRID=1 VRID=2
+-----+ +-----+
| MR1 | | MR2 |
| & | | & |
| BR2 | | BR1 |
+-----+ +-----+
IP A ---------->* *<---------- IP B
| |
| |
| |
------------------+------------+-----+--------+--------+--------+--
^ ^ ^ ^
| | | |
(IP A) (IP A) (IP B) (IP B)
| | | |
+--+--+ +--+--+ +--+--+ +--+--+
| H1 | | H2 | | H3 | | H4 |
+-----+ +-----+ +--+--+ +--+--+
Legend:
---+---+---+-- = 802 network, Ethernet 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.
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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
an 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 VRRP IP multicast address assigned by the IANA. It is defined to
be:
224.0.0.(TBD IANA assignment)
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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 VRRP IP protocol number assigned by the IANA. It is defined to
be (TBD).
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 router's priority for the virtual
router. Higher values equal higher priority. This field is an 8 bit
unsigned field.
The priority value for the router that owns the IP address(es)
associated with the virtual router MUST be 255 (decimal).
VRRP routers backing up another virtual router MAY use priority
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values between 1-254 (decimal). The default priority value for
routers backing up another 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 number. 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.
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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 with an authentication string that does not match its
configured authentication string SHOULD be discarded. The
authentication string is unique on a per interface basis.
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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
Virtual Router Identifier. Configured item in the range 1-255
(decimal). There is no default.
Priority Priority value to be used 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
has stopped running VRRP. 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 )
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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 preempt and False to not
preempt. 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.
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6.4.1 Initialize
{Initialize} is the state a virtual router takes when it is inactive
with respect to the virtual router. 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 Send 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, an virtual router MUST do the following:
- MUST NOT respond to ARP requests for the IP address(s) associated
with this VRID.
- MUST discard packets with a destination link layer MAC address
equal to the virtual router MAC address for this VRID.
- MUST NOT accept packets addressed to the IP address(es) associated
with this VRID.
- If a Shutdown event is received, then:
o Cancel the Master_Down_Timer
o Transition to the {Initialize} state
endif
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- If the Master_Down_Timer fires, then:
o Send an ADVERTISEMENT
o Send a gratuitous ARP request containing their 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 virtual router MUST do the following:
- MUST respond to ARP requests for the IP address(es) associated
with the VRID with the virtual router MAC address.
- 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
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for the virtual router if it is not the IP address owner.
- MUST accept packets addressed to the IP address(es) 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
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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 that the received packet length is greater than or
equal to the VRRP header
- MUST verify the VRRP checksum
- MUST verify the VRRP version
- 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 checks 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 an error occurred. If the
Priority does not equal 255 (decimal), the receiver MUST drop the
packet. If the Priority equals 255 (decimal) 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 an
error occurred.
7.2 Transmitting Packets
The following operations MUST be performed prior to 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
- Send the VRRP packet to the VRRP IP multicast group
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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-XX-XX-{VRID} (in hex in internet standard bit-order)
The first three octets are derived from the IANA's OUI. The next two
octets (to be assigned by the IANA) indicate the address block
assigned to the VRRP protocol. {VRID} is the VRRP 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.
When acting as a Master for a VRID it is not the owner, the virtual
router MUST send ICMP Redirects using the IP address associated with
the VRID as the source of the ICMP Redirect. This entails looking at
the destination MAC address in the packet that is being redirected
and selecting the appropriate IP address.
It may be useful to disable Redirects for specific cases where is
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 routers IP
addresses, the Master router MUST respond to the ARP request with the
virtual MAC address for the virtual router. The virtual router MUST
NOT respond with it's physical MAC address. This allows the client
to always use the same MAC address regardless of the current Master
router. The request MUST be handled as a standard ARP reply.
When a virtual router restarts or boots, it SHOULD not send any ARP
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messages with it's physical MAC addresses for the IP addresses it
owns. They should only send ARP messages that include Virtual MAC
addresses. This may entail:
- When configuring their interfaces, virtual routers should send a
gratuitous ARP request containing their virtual MAC address for
each IP address they own on that interface.
- At system boot, when initializing any of its IP addresses for
which VRRP is configured, delay gratuitous ARP requests and ARP
responses for that interface until both the IP address and the
virtual MAC address are configured.
8.3 Proxy ARP
If Proxy ARP is to be used on a router running VRRP, 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 routers.
9. Operation over FDDI and Token Ring
9.1 Operation over FDDI
FDDI interfaces strip 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 stripped off 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 stripping any ADVERTISEMENTS it did not originate.
9.2 Operation over Token Ring
Token Ring has several characteristics which make running VRRP
problematic. This includes:
- No general multicast mechanism. Required use of "functional
addresses" as a substitute, which may collide with other usage of
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the same "functional addresses".
- Token Ring interfaces may have a limited ability to receive on
multiple MAC addresses.
- In order to switch to a new master located on a different physical
ring from the previous master when using source route bridges, a
mechanism is required to update cached source route information.
Due the these issues and the limited knowledge about the detailed
operation of Token Ring by the authors, this version of VRRP does not
work over Token Ring networks. This may be remedied in new version
of this document, or in a separate document.
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
link).
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 link. 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
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VRRP packets on the link. The Simple Text Authentication combined
with the TTL check makes it difficult for a VRRP packet to be sent
from another link to disrupt VRRP operation.
This type of authentication is RECOMMENDED when there is minimal risk
of nodes on the link actively disrupting VRRP operation.
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 the link. 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 Acee Lindem for their comments and suggestions.
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12. References
[AUTH] Atkinson, R., "IP Authentication Header", RFC-1826, August
1995.
[DISC] Deering, S., "ICMP Router Discovery Messages", RFC-1256,
September 1991.
[DHCP] Droms, R., "Dynamic Host Configuration Protocol", RFC-1541,
October 1993.
[HMAC] Madson, C., R. Glenn, "The Use of HMAC-MD5-96 within ESP
and AH", Internet Draft, <draft-ietf-ipsec-auth-hmac-
md5-96-00.txt> , July 1997.
[HSRP] Li, T., B. Cole, P. Morton, D. Li, "Hot Standby Router
Protocol (HSRP)", Internet Draft, <draft-li-hsrp-00.txt>,
June 1997.
[OSPF] Moy, J., "OSPF version 2", RFC-1583, July 1997.
[RIP] Hedrick, C., "Routing Information Protocol" , RFC-1058,
June 1988.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC-2119, BCP14, March 1997.
13. Author's 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
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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
Ipsilon Networks, Inc. EMail: hinden@ipsilon.com
232 Java Drive
Sunnyvale, CA 94089
USA
Danny Mitzel Phone: +1 408 990-2037
Ipsilon Networks, Inc. EMail: mitzel@ipsilon.com
232 Java Drive
Sunnyvale, CA 94089
USA
Peter Hunt Phone: +1 408 990-2093
Ipsilon Networks, Inc. EMail: hunt@ipsilon.com
232 Java Drive
Sunnyvale, CA 94089
USA
P. Higginson Phone: +44 118 920 6293
REO2-F/E9 EMail: higginson@mail.dec.com
Digital Equipment Corp.
Digital Park
Imperial Way
Reading
Berkshire
RG2 0TE
UK
M. Shand Phone: +44 118 920 4424
REO2-F/D9 EMail: shand@mail.dec.com
Digital Equipment Corp.
Digital Park
Imperial Way
Reading
Berkshire
RG2 0TE
UK
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14. Changes from Previous Drafts
Changes from <draft-ietf-vrrp-spec-02.txt>
- Updated text and references to point to "The Use of HMAC-MD5-96
within ESP and AH" that is the correct reference for the use of
IPSEC AH with MD5.
Changes from <draft-ietf-vrrp-spec-01.txt>
Major change to use real IP addresses instead of virtual IP
addresses. Changes include:
- Updated version number to 2.
- Modified packet header
- New terminology (removed cluster, virtual IP address, etc., added
VRID, associated IP address(es), etc.).
- Special case of priority = 255 for router owning VRID and
associated IP address(es).
- Reworked examples.
- Rewrote introductory and overview sections.
- Added rules for redirects and ARP.
- Added sending gratuitous ARP request when transitioning to Master.
Changes from <draft-ietf-vrrp-spec-00.txt>
- Added Preempt_Mode to allow user control over preemption
independent of configured priorities.
- Rewrote authentication section and expanded security
considerations.
- Expanded State Description section and removed State Table which
become redundant and impossible to edit.
- Changed authentication to be on a per interface basis (not per
cluster).
- Clarified text on disabling ICMP Redirects.
- Added text on FDDI and Token Ring issues.
- Added HSRP acknowledgment.
- Rewrote Introduction, Required Features, and VRRP Overview
sections.
- Many small text clarifications.
Changes from <draft-hinden-vrrp-00.txt>
- Changed default behavior to stay with current master when
priorities are equal. This behavior can be changed by configuring
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explicit priorities.
- Changed Master state behavior to not send Advertisements when
receiving Advertisement with lower priority. Change reduces worst
case election message overhead to "n", where "n" is number of
configured equal priority VRRP routers.
- Added Skew_Time parameter and changed receiving advertisement with
zero priority behavior to cause resulting advertisement sent to be
skewed by priority.
- Changed sending behavior to send VRRP packets with VMAC as source
MAC and added text describing why this is important for bridged
environments.
- Changed definition of VMAC to be in IANA assigned unicast MAC
block.
- Added Advertisement Interval to VRRP header.
- Added text regarding ICMP Redirects, Proxy ARP, and network
management issues.
- Various small text clarifications.
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