📄 rfc2338.txt
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Network Working Group S. KnightRequest for Comments: 2338 D. WeaverCategory: 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 ProtocolStatus 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.Knight, et. al. Standards Track [Page 1] RFC 2338 VRRP April 1998Table 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..................................271. 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 discoveryKnight, et. al. Standards Track [Page 2] RFC 2338 VRRP April 1998 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 [RFC 2119]. 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.Knight, et. al. Standards Track [Page 3] RFC 2338 VRRP April 19981.1 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.Knight, et. al. Standards Track [Page 4] RFC 2338 VRRP April 1998 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.Knight, et. al. Standards Track [Page 5] RFC 2338 VRRP April 1998 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.Knight, et. al. Standards Track [Page 6] RFC 2338 VRRP April 1998 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⌨️ 快捷键说明
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