Network Working Group A. Zinin
Request for Comments: 3509 Alcatel
Category: Informational A. Lindem
Redback Networks
D. Yeung
Procket Networks
April 2003
Network Working Group A. Zinin
Request for Comments: 3509 Alcatel
Category: Informational A. Lindem
Redback Networks
D. Yeung
Procket Networks
April 2003
Alternative Implementations of OSPF Area Border Routers
OSPF区域边界路由器的替代实现
Status of this Memo
本备忘录的状况
This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
本备忘录为互联网社区提供信息。它没有规定任何类型的互联网标准。本备忘录的分发不受限制。
Copyright Notice
版权公告
Copyright (C) The Internet Society (2003). All Rights Reserved.
版权所有（C）互联网协会（2003年）。版权所有。
Abstract
摘要
Open Shortest Path First (OSPF) is a link-state intra-domain routing protocol used for routing in IP networks. Though the definition of the Area Border Router (ABR) in the OSPF specification does not require a router with multiple attached areas to have a backbone connection, it is actually necessary to provide successful routing to the inter-area and external destinations. If this requirement is not met, all traffic destined for the areas not connected to such an ABR or out of the OSPF domain, is dropped. This document describes alternative ABR behaviors implemented in Cisco and IBM routers.
开放最短路径优先（OSPF）是一种用于IP网络路由的链路状态域内路由协议。尽管OSPF规范中区域边界路由器（ABR）的定义不要求具有多个连接区域的路由器具有主干连接，但实际上有必要提供到区域间和外部目的地的成功路由。如果不满足此要求，则所有发送到未连接到此类ABR或OSPF域之外的区域的流量都将被丢弃。本文档描述了在Cisco和IBM路由器中实现的替代ABR行为。
1 Overview
1概述
An OSPF routing domain can be split into several subdomains, called areas, which limit the scope of LSA flooding. According to [Ref1] a router having attachments to multiple areas is called an "area border router" (ABR). The primary function of an ABR is to provide its attached areas with Type-3 and Type-4 LSAs, which are used for describing routes and AS boundary routers (ASBRs) in other areas, as well as to perform actual inter-area routing.
一个OSPF路由域可以分为几个子域，称为区域，这限制了LSA泛洪的范围。根据[参考文献1]将连接到多个区域的路由器称为“区域边界路由器”（ABR）。ABR的主要功能是为其连接区域提供类型3和类型4 LSA，用于描述路由和其他区域中的边界路由器（ASBR），以及执行实际的区域间路由。
In OSPF domains the area topology is restricted so that there must be a backbone area (area 0) and all other areas must have either physical or virtual connections to the backbone. The reason for this star-like topology is that OSPF inter-area routing uses the distance-vector approach and a strict area hierarchy permits avoidance of the "counting to infinity" problem. OSPF prevents inter-area routing loops by implementing a split-horizon mechanism, allowing ABRs to inject into the backbone only Summary-LSAs derived from the intra-area routes, and limiting ABRs' SPF calculation to consider only Summary-LSAs in the backbone area's link-state database.
在OSPF域中，区域拓扑受到限制，因此必须有一个主干区域（区域0），并且所有其他区域必须具有到主干的物理或虚拟连接。这种星形拓扑的原因是OSPF区域间路由使用距离向量方法，严格的区域层次结构允许避免“计数到无穷大”问题。OSPF通过实现分裂层级机制来防止区域间路由环路，从而允许ABRs仅注入来自区域内路由的汇总LSA，并且限制ABRs的SPF计算仅考虑骨干区域链路状态数据库中的汇总LSA。
The last restriction leads to a problem when an ABR has no backbone connection (in OSPF, an ABR does not need to be attached to the backbone). Consider a sample OSPF domain depicted in the Figure 1.
最后一个限制导致ABR没有主干连接时出现问题（在OSPF中，ABR不需要连接到主干）。考虑图1中描述的OSPF域的示例。
. .
. Area 0 .
+--+ +--+
..|R1|.. ..|R2|..
. +--+ .. +--+ .
. .. .
. +--+ .
. Area1 |R3| Area2 .
. +--+ +--+ .
. .. |R4| .
. . . +--+ .
....... .......
. .
. Area 0 .
+--+ +--+
..|R1|.. ..|R2|..
. +--+ .. +--+ .
. .. .
. +--+ .
. Area1 |R3| Area2 .
. +--+ +--+ .
. .. |R4| .
. . . +--+ .
....... .......
Figure 1. ABR dropping transit traffic
图1。丢弃中转业务
In this example R1, R2, and R3 are ABRs. R1 and R2 have backbone connections, while R3 doesn't.
在此示例中，R1、R2和R3是ABR。R1和R2有主干连接，而R3没有。
Following the section 12.4.1 of [Ref1], R3 will identify itself as an
ABR by setting the bit B in its router-LSA. Being an ABR, R3 can
only consider summary-LSAs from the backbone when building the
routing table (according to section 16.2 of [Ref1]), so it will not
have any inter-area routes in its routing table, but only intra-area
routes from both Area 1 and Area 2. Consequently, according to
section 12.4.3 of [Ref1], R3 will originate into Areas 1 and 2 only
summary-LSAs covering destinations in the directly attached areas,
i.e., in Area 2---the summary-LSAs for Area 1, and in Area 1---the
summary-LSAs for Area 2.
Following the section 12.4.1 of [Ref1], R3 will identify itself as an
ABR by setting the bit B in its router-LSA. Being an ABR, R3 can
only consider summary-LSAs from the backbone when building the
routing table (according to section 16.2 of [Ref1]), so it will not
have any inter-area routes in its routing table, but only intra-area
routes from both Area 1 and Area 2. Consequently, according to
section 12.4.3 of [Ref1], R3 will originate into Areas 1 and 2 only
summary-LSAs covering destinations in the directly attached areas,
i.e., in Area 2---the summary-LSAs for Area 1, and in Area 1---the
summary-LSAs for Area 2.
At the same time, router R2, as an ABR connected to the backbone, will inject into Area 2 summary-LSAs describing the destinations in Area 0 (the backbone), Area 1 and other areas reachable through the backbone.
同时，路由器R2作为连接到主干网的ABR，将向区域2注入描述区域0（主干网）、区域1和通过主干网可到达的其他区域中的目的地的摘要LSA。
This results in a situation where internal router R4 calculates its
routes to destinations in the backbone and areas other than Area 1
via R2. The topology of Area 2 itself can be such that the best path
from R4 to R2 is via R3, so all traffic destined for the backbone and
backbone-attached areas goes through R3. Router R3 in turn, having
only intra-area routes for areas 1 and 2, will drop all traffic not
destined for the areas directly attached to it. The same problem can
occur when a backbone-connected ABR loses all of its adjacencies in
the backbone---even if there are other normally functioning ABRs in
the attached areas, all traffic going to the backbone (destined for
it or for other areas) will be dropped.
This results in a situation where internal router R4 calculates its
routes to destinations in the backbone and areas other than Area 1
via R2. The topology of Area 2 itself can be such that the best path
from R4 to R2 is via R3, so all traffic destined for the backbone and
backbone-attached areas goes through R3. Router R3 in turn, having
only intra-area routes for areas 1 and 2, will drop all traffic not
destined for the areas directly attached to it. The same problem can
occur when a backbone-connected ABR loses all of its adjacencies in
the backbone---even if there are other normally functioning ABRs in
the attached areas, all traffic going to the backbone (destined for
it or for other areas) will be dropped.
In a standard OSPF implementation this situation can be remedied by use of Virtual Links (see section 15 of [Ref1] for more details). In this case, router R3 will have a virtual backbone connection, will form an adjacency over it, will receive all LSAs directly from a backbone-attached router (R1 or R2, or both in our example) and will install intra- or inter-area routes.
在标准OSPF实现中，这种情况可以通过使用虚拟链路来补救（更多详细信息，请参见[Ref1]第15节）。在这种情况下，路由器R3将具有虚拟主干连接，将在其上形成邻接，将直接从主干连接的路由器（在我们的示例中为R1或R2，或两者）接收所有LSA，并将安装区域内或区域间路由。
While being an unavoidable technique for repairing a partitioned backbone area, the use of virtual links in the described situation adds extra configuration headaches and system traffic overhead.
虽然修复分区主干区域是不可避免的技术，但在所述情况下使用虚拟链路会增加额外的配置问题和系统流量开销。
Another situation where standard ABR behavior does not provide acceptable results is when it is necessary to provide redundant connectivity to an ASBR via several different OSPF areas. This would allow a provider to aggregate all their customers connecting through a single access point into one area while still offering a redundant connection through another access point in a different area, as shown in Figure 2.
标准ABR行为不能提供可接受结果的另一种情况是，需要通过几个不同的OSPF区域向ASBR提供冗余连接。这将允许提供商将通过单个接入点连接的所有客户聚合到一个区域中，同时仍然通过另一个不同区域中的接入点提供冗余连接，如图2所示。
. .
. Area 0 .
+--+ +--+
..|R1|.. ..|R2|..
. +--+ .. +--+ .
. .. .
. .. .
. Area1 .. Area2 .
. .. .
. .. .
. +--+ .
.......|R3|.......
ASBR+--+
/..\
--+- -+--
CN1 CNx
. .
. Area 0 .
+--+ +--+
..|R1|.. ..|R2|..
. +--+ .. +--+ .
. .. .
. .. .
. Area1 .. Area2 .
. .. .
. .. .
. +--+ .
.......|R3|.......
ASBR+--+
/..\
--+- -+--
CN1 CNx
Customer Networks (CN1--CNx) Advertised as AS External or NSSA External Routes
客户网络（CN1--CNx）广告为外部或NSSA外部路由
Figure 2. Dual Homed Customer Router
图2。双宿客户路由器
This technique is already used in a number of networks including one of a major provider.
这项技术已经在许多网络中使用，包括一家主要提供商。
The next section describes alternative ABR behaviors, implemented in Cisco and IBM routers. The changes are in the ABR definition and inter-area route calculation. Any other parts of standard OSPF are not changed.
下一节描述在Cisco和IBM路由器中实现的替代ABR行为。变化在于ABR定义和区域间路由计算。标准OSPF的任何其他部分均未更改。
These solutions are targeted to the situation when an ABR has no
backbone connection. They imply that a router connected to multiple
areas without a backbone connection is not an ABR and should function
as a router internal to every attached area. This solution emulates
a situation where separate OSPF processes are run for each area and
supply routes to the routing table. It remedies the situation
described in the examples above by not dropping transit traffic.
Note that a router following it does not function as a real border
router---it doesn't originate summary-LSAs. Nevertheless such a
behavior may be desirable in certain situations.
These solutions are targeted to the situation when an ABR has no
backbone connection. They imply that a router connected to multiple
areas without a backbone connection is not an ABR and should function
as a router internal to every attached area. This solution emulates
a situation where separate OSPF processes are run for each area and
supply routes to the routing table. It remedies the situation
described in the examples above by not dropping transit traffic.
Note that a router following it does not function as a real border
router---it doesn't originate summary-LSAs. Nevertheless such a
behavior may be desirable in certain situations.
Note that the proposed solutions do not obviate the need of virtual link configuration in case an area has no physical backbone connection at all. The methods described here improve the behavior of a router connecting two or more backbone-attached areas.
请注意，如果某个区域根本没有物理主干连接，建议的解决方案并不排除虚拟链路配置的需要。这里描述的方法改进了连接两个或多个主干连接区域的路由器的行为。
2 Changes to ABR Behavior
2 ABR行为的变化
The following definitions will be used in this document to describe the new ABR behaviors:
本文件将使用以下定义来描述新的ABR行为：
Configured area: An area is considered configured if the router has at least one interface in any state assigned to that area.
配置区域：如果路由器在任何状态下至少有一个接口分配给该区域，则该区域被视为已配置。
Actively Attached area: An area is considered actively attached if the router has at least one interface in that area in the state other than Down.
主动连接区域：如果路由器在该区域中至少有一个接口处于非关闭状态，则该区域被视为主动连接。
Active Backbone Connection: A router is considered to have an active backbone connection if the backbone area is actively attached and there is at least one fully adjacent neighbor in it.
主动主干连接：如果主干区域是主动连接的，并且其中至少有一个完全相邻的邻居，则认为路由器具有主动主干连接。
Area Border Router (ABR):
区域边界路由器（ABR）：
Cisco Systems Interpretation: A router is considered to be an ABR if it has more than one area Actively Attached and one of them is the backbone area.
Cisco Systems解释：如果路由器有多个主动连接的区域，并且其中一个区域是主干区域，则该路由器被视为ABR。
IBM Interpretation: A router is considered to be an ABR if it has more than one Actively Attached area and the backbone area Configured.
IBM解释：如果路由器具有多个活动连接区域和配置的主干区域，则将其视为ABR。
The following changes are made to the base OSPF, described in [Ref1]:
对基本OSPF进行了以下更改，如参考文献1所述：
1. The algorithm for Type 1 LSA (router-LSA) origination is changed to prevent a multi-area connected router from identifying itself as an ABR by the bit B (as described in section 12.4.1 of [Ref1]) until it considers itself as an ABR according to the definitions given in section 2.1.
1. 改变了1类LSA（路由器LSA）发起的算法，以防止多区域连接路由器通过位B（如参考文献1第12.4.1节所述）将自身识别为ABR，直到其根据第2.1节给出的定义将自身视为ABR为止。
2. The algorithm for the routing table calculation is changed to allow the router to consider the summary-LSAs from all attached areas if it is not an ABR, but has more than one attached area, or it does not have an Active Backbone Connection. Definitions of the terms used in this paragraph are given in section 2.1.
2. 路由表计算的算法被改变，以允许路由器考虑来自所有连接区域的汇总LSA，如果它不是ABR，而是具有多于一个的附加区域，或者它没有活动主干连接。本段中使用的术语定义见第2.1节。
So, the paragraph 1 of section 16.2 of [Ref1] should be interpreted as follows:
因此，[参考文献1]第16.2节第1段应解释为：
"The inter-area routes are calculated by examining summary-LSAs. If the router is an ABR and has an Active Backbone Connection, only backbone summary-LSAs are examined. Otherwise (either the router is not an ABR or it has no Active Backbone Connection), the router should consider summary-LSAs from all Actively Attached areas..."
“区域间路由是通过检查汇总LSA来计算的。如果路由器是ABR并且具有活动主干连接，则只检查骨干概要LSA。否则（路由器不是ABR或它没有活动主干连接），路由器应该考虑来自所有活跃连接区域的汇总LSAs……”
3. For Cisco ABR approach, the algorithm for the summary-LSAs origination is changed to prevent loops of summary-LSAs in situations where the router considers itself an ABR but doesn't have an Active Backbone Connection (and, consequently, examines summaries from all attached areas). The algorithm is changed to allow an ABR to announce only intra-area routes in such a situation.
3. 对于Cisco ABR方法，更改了摘要LSA发起的算法，以防止在路由器认为自己是ABR但没有活动主干连接的情况下（因此，检查所有连接区域的摘要）出现摘要LSA循环。在这种情况下，算法被更改为允许ABR仅宣布区域内路由。
So, the paragraph 2 of subsection 12.4.3 of [Ref1] should be interpreted as follows:
因此，[参考文献1]第12.4.3小节第2段应解释为：
"Summary-LSAs are originated by area border routers. The precise summary routes to advertise into an area are determined by examining the routing table structure (see Section 11) in accordance with the algorithm described below. Note that while only intra-area routes are advertised into the backbone, if the router has an Active Backbone Connection, both intra-area and inter-area routes are advertised into the other areas; otherwise, the router only advertises intra-area routes into non-backbone areas."
“摘要LSA由区域边界路由器发起。通过检查路由表结构（参见第11节），确定要播发到区域的精确摘要路由。”根据下面描述的算法。请注意，虽然只有区域内路由播发到主干网，但如果路由器具有活动主干网连接，则区域内和区域间路由都会播发到其他区域；否则，路由器仅将区域内路由播发到非主干网区域。”
For this policy to be applied we change steps 6 and 7 in the summary origination algorithm to be as follows:
对于要应用的此策略，我们将摘要发起算法中的步骤6和7更改为：
Step 6:
步骤6：
"Else, if the destination of this route is an AS boundary router, a summary-LSA should be originated if and only if the routing table entry describes the preferred path to the AS boundary router (see Step 3 of Section 16.4). If so, a Type 4 summary-LSA is originated for the destination, with Link State ID equal to the AS boundary router's Router ID and metric equal to the routing table entry's cost. If the ABR performing this algorithm does not have an Active Backbone Connection, it can originate Type 4 summary-LSA only if the type of the route to the ASBR is intra-area. Note: Type 4 summary-LSAs should not be generated if Area A has been configured as a stub area."
“否则，如果此路由的目的地是AS边界路由器，则当且仅当路由表条目描述到AS边界路由器的首选路径时，才应生成摘要LSA（参见第16.4节的步骤3）。如果是，则为目的地发起类型4摘要LSA，链路状态ID等于AS边界路由器的路由器ID，度量等于路由表条目的成本。如果执行此算法的ABR没有活动主干连接，则只有当到ASBR的路由类型为内部时，它才能发起类型4摘要LSA-区域。注意：如果区域A已配置为存根区域，则不应生成类型4摘要LSA。“
Step 7:
步骤7：
"Else, the Destination type is network. If this is an inter-area route and the ABR performing this algorithm has an Active Backbone Connection, generate a Type 3 summary-LSA for the destination, with Link State ID equal to the network's address (if necessary, the Link State ID can also have one or more of the network's host bits set; see Appendix E for details) and metric equal to the routing table cost."
否则，目的地类型为网络。如果这是一个区域间路由，并且执行此算法的ABR具有活动主干连接，则为目的地生成一个类型3摘要LSA，链路状态ID等于网络地址（如有必要，链路状态ID还可以设置一个或多个网络主机位；有关详细信息，请参见附录E）和等于路由表成本的度量。”
The changes in the ABR behavior described in this section allow a multi-area connected router to successfully route traffic destined for the backbone and other areas. Note that if the router does not have a backbone area Configured it does not actively attract inter-area traffic, because it does not consider itself an ABR and does not originate summary-LSAs. It still can forward traffic from one attached area to another along intra-area routes in case other routers in corresponding areas have the best inter-area paths over it, as described in section 1.2.
本节中描述的ABR行为的变化允许多区域连接的路由器成功地路由到主干网和其他区域的流量。注意，如果路由器没有配置骨干区域，则它不主动吸引跨区域业务，因为它不认为自己是ABR，并且不源于汇总LSA。如第1.2节所述，如果相应区域中的其他路由器在其上具有最佳区域间路径，它仍然可以沿着区域内路由将流量从一个连接区域转发到另一个连接区域。
By processing all summaries when the backbone is not active, we prevent the ABR, which has just lost its last backbone adjacency, from dropping any packets going through the ABR in question to another ABR and destined towards the backbone or other areas not connected to the ABR directly.
通过在主干网不活动时处理所有摘要，我们可以防止刚刚失去最后一个主干网邻接的ABR丢弃任何通过相关ABR发送到另一个ABR并发送到主干网或其他未直接连接到ABR的区域的数据包。
3 Virtual Link Treatment
3虚拟链接处理
The Cisco ABR approach described in this document requires an ABR to have at least one active interface in the backbone area. This requirement may cause problems with virtual links in those rare situations where the backbone area is purely virtual, as shown in Figure 3, and the state of the VL is determined as in [Ref1].
本文档中描述的Cisco ABR方法要求ABR在主干区域至少有一个活动接口。这种要求可能会导致在主干区域为纯虚拟的罕见情况下出现虚拟链路问题，如图3所示，VL的状态如[Ref1]所示。
....... ........... ......
. . . .
+--+ VL +--+
|R1|***********|R2|
+--+ +--+
Area 1 . . Area 2 . . Area 3
....... ........... ......
....... ........... ......
. . . .
+--+ VL +--+
|R1|***********|R2|
+--+ +--+
Area 1 . . Area 2 . . Area 3
....... ........... ......
Figure 3. Purely Virtual Backbone
图3。纯虚拟主干网
If R1 and R2 treat virtual links as in [Ref1], their virtual links will never go up, because their router-LSAs do not contain the B-bit, which is, in turn, because the routers do not have active interfaces (virtual links) in the backbone and do not consider themselves ABRs.
如果R1和R2在[ReF1]中处理虚拟链路，它们的虚拟链路将永远不会上升，因为它们的路由器LSA不包含B-位，这又是因为路由器在主干中没有活动接口（虚拟链路）并且不考虑它们自身的ARBs。
Note that this problem does not appear if one of the routers has a real interface in the backbone, as it usually is in real networks.
请注意，如果其中一个路由器在主干网中具有真实接口，则不会出现此问题，因为它通常在真实网络中。
Though the situation described is deemed to be rather rare, implementations supporting Cisco ABR behavior may consider changing VL-specific code so that a virtual link is reported up (an InterfaceUp event is generated) when a router with corresponding router-ID is seen via Dijkstra, no matter whether its router-LSA indicates that it is an ABR or not. This means that checking of configured virtual links should be done not in step 4 of the algorithm in 16.1 of [Ref1] when a router routing entry is added, but every time a vertex is added to the SPT in step 3 of the same algorithm.
虽然所描述的情况被认为是相当罕见的，但是支持Cisco ABR行为的实现可以考虑改变VL特定的代码，以便当通过Dijkstra看到具有相应路由器ID的路由器时，报告虚拟链路（生成接口事件），无论其路由器LSA是否指示其为ABR。这意味着，当添加路由器路由条目时，不应在[Ref1]16.1中的算法步骤4中检查配置的虚拟链路，而应在同一算法步骤3中每次向SPT添加顶点时进行检查。
4 Compatibility
4兼容性
The changes of the OSPF ABR operations do not influence any aspects of the router-to-router cooperation and do not create routing loops, and hence are fully compatible with standard OSPF. Proof of compatibility is outside the scope of this document.
OSPF ABR操作的变化不会影响路由器到路由器协作的任何方面，也不会创建路由环路，因此与标准OSPF完全兼容。兼容性证明不在本文档范围内。
5 Deployment Considerations
5部署注意事项
This section discusses the deployments details of the ABR behaviors described in this document. Note that this approach is fully compatible with standard ABR behavior, so ABRs acting as described in [Ref1] and in this document can coexist in an OSPF domain and will function without problems.
本节讨论本文档中描述的ABR行为的详细信息。请注意，该方法与标准ABR行为完全兼容，因此[Ref1]和本文件中所述的ABR可以在OSPF域中共存，并且不会出现问题。
Deployment of ABRs using the alternative methods improves the behavior of a router connected to multiple areas without a backbone attachment, but can lead to unexpected routing asymmetry, as described below.
使用替代方法部署ABR可以改善连接到多个区域但没有主干连接的路由器的行为，但会导致意外的路由不对称，如下所述。
Consider an OSPF domain depicted in Figure 4.
考虑图4中描述的OSPF域。
. Backbone .
. .
. --------------------- .
. |1 1| .
..+--+.............+--+..
..|R1|..... ....|R4|..
. +--+ . . +--+ .
. 1| . . /4 .
. | 8 +--+ 4 / .
. | +-|R3|---+ .
. 1| / +--+\4 .
. +--+ / . . \ 4 +--+ .
. |R2|/8 . . +--|R5| .
. +--+ . . +--+ .
. | . . | .
. --------- . . -------- .
. net N . . net M .
. . . .
. Area 1 . . Area 2 .
........... ..........
. Backbone .
. .
. --------------------- .
. |1 1| .
..+--+.............+--+..
..|R1|..... ....|R4|..
. +--+ . . +--+ .
. 1| . . /4 .
. | 8 +--+ 4 / .
. | +-|R3|---+ .
. 1| / +--+\4 .
. +--+ / . . \ 4 +--+ .
. |R2|/8 . . +--|R5| .
. +--+ . . +--+ .
. | . . | .
. --------- . . -------- .
. net N . . net M .
. . . .
. Area 1 . . Area 2 .
........... ..........
Figure 4. Inter-area routing asymmetry
图4。区域间路由不对称
Assume that R3 uses the approach described in this document. In this case R2 will have inter-area routes to network M via ABR R1 only. R5 in turn will have its inter-area route to network N via R4, but as far as R4 is only reachable via R3, all traffic destined to network N will pass through R3. R3 will have an intra-area route to network N via R2 and will, of course, route it directly to it (because intra-area routes are always preferred over inter-area ones). Traffic going back from network N to network M will pass through R2 and will be routed to R1, as R2 will not have any inter-area routes via R3. So, traffic from N to M will always go through the backbone while traffic from M to N will cross the areas directly via R3 and, in this example, will not use a more optimal path through the backbone.
假设R3使用本文档中描述的方法。在这种情况下，R2将仅通过ABR R1具有到网络M的区域间路由。R5反过来将通过R4有其到网络N的区域间路由，但只要R4只能通过R3到达，所有到网络N的流量都将通过R3。R3将有一个通过R2到网络N的区域内路由，当然，它将直接路由到网络N（因为区域内路由总是优先于区域间路由）。从网络N返回网络M的流量将通过R2，并将路由到R1，因为R2不会通过R3有任何区域间路由。因此，从N到M的流量将始终通过主干网，而从M到N的流量将直接通过R3穿过这些区域，在本例中，不会使用通过主干网的更优路径。
Note that this problem is not caused by the fact that R3 uses the alternative approach. The reason for attracting the attention to it is that R3 is not really functioning as an ABR in case this new behavior is used, i.e., it does not inject summary-LSAs into the attached areas, but inter-area traffic can still go through it.
请注意，这个问题不是由R3使用替代方法造成的。引起注意的原因是，在使用这种新行为的情况下，R3并没有真正起到ABR的作用，也就是说，它没有将摘要LSA注入连接的区域，但区域间的流量仍然可以通过它。
6 Security Considerations
6安全考虑
The alternative ABR behaviors specified in this document do not raise any security issues that are not already covered in [Ref1].
本文件中规定的备选ABR行为不会引发[Ref1]中未涉及的任何安全问题。
7 Acknowledgements
7致谢
Authors would like to thank Alvaro Retana, Russ White, and Liem Nguyen for their review of the document.
作者要感谢Alvaro Retana、Russ White和Liem Nguyen对该文件的审阅。
8 Disclaimer
8免责声明
This document describes OSPF ABR implementations of respective vendors "as is", only for informational purposes, and without any warranties, guarantees or support. These implementations are subject to possible future changes. For the purposes of easier deployment, information about software versions where described behavior was integrated is provided below.
本文件描述了各供应商的OSPF ABR“原样”实施，仅供参考，不提供任何保证、担保或支持。这些实现可能会受到未来更改的影响。为了便于部署，下面提供了有关集成了所述行为的软件版本的信息。
Initial Cisco ABR implementation (slightly different from the one described in this memo, requiring non-backbone areas to be configured, and not necessarily actively attached in the ABR definition) was introduced in Cisco IOS (tm) version 11.1(6). Cisco ABR behavior described in this document was integrated in Cisco IOS (tm) in version 12.1(3)T.
Cisco IOS（tm）版本11.1（6）中引入了Cisco ABR的初始实施（与本备忘录中描述的略有不同，需要配置非主干区域，并且不一定在ABR定义中主动连接）。本文档中描述的Cisco ABR行为已集成到12.1（3）T版的Cisco IOS（tm）中。
The ABR behavior described as IBM ABR approach was implemented by IBM in IBM Nways Multiprotocol Routing Services (MRS) 3.3.
IBM在IBM Nways Multiprotocol Routing Services（MRS）3.3中实现了被称为IBM ABR方法的ABR行为。
Note that the authors do not intend to keep this document in sync with actual implementations.
请注意，作者并不打算使本文档与实际实现保持同步。
10 References
10份参考资料
[Ref1] Moy, J., "OSPF version 2", STD 54, RFC 2328, April 1998.
[参考文献1]莫伊，J.，“OSPF版本2”，STD 54，RFC 23281998年4月。
11 Authors' Addresses
11作者地址
Alex Zinin Alcatel
亚历克斯·齐宁·阿尔卡特
EMail: zinin@psg.com
EMail: zinin@psg.com
Derek M. Yeung Procket Networks 1100 Cadillac Ct Milpitas, CA 95035
Derek M.Yang Procket Networks 1100卡迪拉克Ct加利福尼亚州米尔皮塔斯市95035
Phone: 408-635-7911 EMail: myeung@procket.com
电话：408-635-7911电子邮件：myeung@procket.com
Acee Lindem Redback Networks 102 Carric Bend Court Cary, NC 27519 USA
美国北卡罗来纳州卡里克本德法院102号Acee Lindem Redback Networks 27519
Phone: 919-387-6971 EMail: acee@redback.com
电话：919-387-6971电子邮件：acee@redback.com
12 Full Copyright Statement
12完整版权声明
Copyright (C) The Internet Society (2003). All Rights Reserved.
版权所有（C）互联网协会（2003年）。版权所有。
This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English.
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The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns.
上述授予的有限许可是永久性的，互联网协会或其继承人或受让人不会撤销。
This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
本文件和其中包含的信息是按“原样”提供的，互联网协会和互联网工程任务组否认所有明示或暗示的保证，包括但不限于任何保证，即使用本文中的信息不会侵犯任何权利，或对适销性或特定用途适用性的任何默示保证。
Acknowledgement
确认
Funding for the RFC Editor function is currently provided by the Internet Society.
RFC编辑功能的资金目前由互联网协会提供。