Network Working Group T. Bates
Request for Comments: 2260 Cisco Systems
Category: Informational Y. Rekhter
Cisco Systems
January 1998
Network Working Group T. Bates
Request for Comments: 2260 Cisco Systems
Category: Informational Y. Rekhter
Cisco Systems
January 1998
Scalable Support for Multi-homed Multi-provider Connectivity
可扩展支持多宿多提供商连接
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 (1998). All Rights Reserved.
版权所有（C）互联网协会（1998年）。版权所有。
This document describes addressing and routing strategies for multi-homed enterprises attached to multiple Internet Service Providers (ISPs) that are intended to reduce the routing overhead due to these enterprises in the global Internet routing system.
本文档描述了连接到多个Internet服务提供商（ISP）的多宿企业的寻址和路由策略，旨在减少这些企业在全球Internet路由系统中的路由开销。
An enterprise may acquire its Internet connectivity from more than one Internet Service Provider (ISP) for some of the following reasons. Maintaining connectivity via more than one ISP could be viewed as a way to make connectivity to the Internet more reliable. This way when connectivity through one of the ISPs fails, connectivity via the other ISP(s) would enable the enterprise to preserve its connectivity to the Internet. In addition to providing more reliable connectivity, maintaining connectivity via more than one ISP could also allow the enterprise to distribute load among multiple connections. For enterprises that span wide geographical area this could also enable better (more optimal) routing.
由于以下原因，企业可以从多个互联网服务提供商（ISP）获得其互联网连接。通过多个ISP保持连接可以被视为使互联网连接更加可靠的一种方式。这样，当通过其中一个ISP的连接失败时，通过其他ISP的连接将使企业能够保持其与Internet的连接。除了提供更可靠的连接外，通过多个ISP保持连接还可以允许企业在多个连接之间分配负载。对于跨越广泛地理区域的企业，这也可以实现更好（更优化）的路由。
The above considerations, combined with the decreasing prices for the Internet connectivity, motivate more and more enterprises to become multi-homed to multiple ISPs. At the same time, the routing overhead that such enterprises impose on the Internet routing system becomes more and more significant. Scaling the Internet, and being able to support a growing number of such enterprises demands mechanism(s) to contain this overhead. This document assumes that an approach where routers in the "default-free" zone of the Internet would be required
上述考虑，再加上互联网连接价格的下降，促使越来越多的企业成为多家ISP的多址运营商。同时，这些企业对互联网路由系统的路由开销也变得越来越大。为了扩展互联网，并能够支持越来越多的此类企业，需要一种机制来控制这种开销。本文档假设需要一种方法，其中路由器位于互联网的“默认自由”区域
to maintain a route for every multi-homed enterprise that is connected to multiple ISPs does not provide an adequate scaling. Moreover, given the nature of the Internet, this document assumes that any approach to handle routing for such enterprises should minimize the amount of coordination among ISPs, and especially the ISPs that are not directly connected to these enterprises.
为每个连接到多个ISP的多宿企业维护路由并不能提供足够的扩展。此外，鉴于互联网的性质，本文件假定，为此类企业处理路由的任何方法都应尽量减少ISP之间的协调，尤其是与这些企业没有直接连接的ISP之间的协调。
There is a difference of opinions on whether the driving factors behind multi-homing to multiple ISPs could be adequately addressed by multi-homing just to a single ISP, which would in turn eliminate the negative impact of multi-homing on the Internet routing system. Discussion of this topic is beyond the scope of this document.
对于多归属于多个ISP背后的驱动因素是否可以通过仅针对单个ISP的多归属来充分解决，从而消除多归属对Internet路由系统的负面影响，存在着不同的意见。本主题的讨论超出了本文件的范围。
The focus of this document is on the routing and addressing strategies that could reduce the routing overhead due to multi-homed enterprises connected to multiple ISPs in the Internet routing system.
本文档的重点是路由和寻址策略，这些策略可以减少由于在Internet路由系统中多家企业连接到多个ISP而产生的路由开销。
The strategies described in this document are equally applicable to both IPv4 and IPv6.
本文档中描述的策略同样适用于IPv4和IPv6。
A multi-homed enterprise connected to a set of ISPs would be allocated a block of addresses (address prefix) by each of these ISPs (an enterprise connected to N ISPs would get N different blocks). The address allocation from the ISPs to the enterprise would be based on the "address-lending" policy [RFC2008]. The allocated addresses then would be used for address assignment within the enterprise.
连接到一组ISP的多宿企业将由每个ISP分配一个地址块（地址前缀）（连接到N个ISP的企业将获得N个不同的块）。ISP向企业分配的地址将基于“地址借用”政策[RFC2008]。然后，分配的地址将用于企业内的地址分配。
One possible address assignment plan that the enterprise could employ is to use the topological proximity of a node (host) to a particular ISP (to the interconnect between the enterprise and the ISP) as a criteria for selecting which of the address prefixes to use for address assignment to the node. A particular node (host) may be assigned address(es) out of a single prefix, or may have addresses from different prefixes.
企业可以采用的一种可能的地址分配计划是使用节点（主机）到特定ISP（到企业和ISP之间的互连）的拓扑接近度作为选择用于向节点分配地址的地址前缀的标准。特定节点（主机）可以从单个前缀中分配地址，或者可以具有来自不同前缀的地址。
The issue of routing information exchange between an enterprise and its ISPs is decomposed into the following components:
企业与其ISP之间的路由信息交换问题分解为以下几个部分：
a) reachability information that an enterprise border router advertises to a border router within an ISP
a) 企业边界路由器向ISP内的边界路由器播发的可达性信息
b) reachability information that a border router within an ISP advertises to an enterprise border router
b) ISP中的边界路由器向企业边界路由器播发的可达性信息
The primary focus of this document is on (a); (b) is covered only as needed by this document.
本文件的主要重点是（a）；（b） 仅在本文件需要时涵盖。
When an enterprise border router connected to a particular ISP determines that the connectivity between the enterprise and the Internet is up through all of its ISPs, the router advertises (to the border router of that ISP) reachability to only the address prefix that the ISP allocated to the enterprise. This way in a steady state routes injected by the enterprise into its ISPs are aggregated by these ISPs, and are not propagated into the "default-free" zone of the Internet.
当连接到特定ISP的企业边界路由器确定企业与Internet之间的连接已通过其所有ISP建立时，该路由器（向该ISP的边界路由器）仅向ISP分配给该企业的地址前缀播发可达性。这样，在稳定状态下，企业注入其ISP的路由由这些ISP聚合，并且不会传播到Internet的“无默认”区域。
When an enterprise border router connected to a particular ISP detemrines that the connectivity between the enterprise and the Internet through one or more of its other ISPs is down, the router starts advertising reachability to the address prefixes that was allocated by these ISPs to the enterprise. This would result in injecting additional routing information into the "default-free" zone of the Internet. However, one could observe that the probability of all multi-homed enterprises in the Internet concurrently losing connectivity to the Internet through one or more of their ISPs is fairly small. Thus on average the number of additional routes in the "default-free" zone of the Internet due to multi-homed enterprises is expected to be a small fraction of the total number of such enterprises.
当连接到特定ISP的企业边界路由器检测到企业通过其一个或多个其他ISP与Internet之间的连接已断开时，路由器将开始公布这些ISP分配给企业的地址前缀的可达性。这将导致将额外的路由信息注入互联网的“无默认”区域。然而，人们可以观察到，互联网上所有多宿企业通过一个或多个ISP同时失去与互联网的连接的可能性相当小。因此，由于多宿企业的存在，互联网“无默认”区域内的额外路由的平均数量预计只占此类企业总数的一小部分。
The approach described above is predicated on the assumption that an enterprise border router has a mechanism(s) by which it could determine (a) whether the connectivity to the Internet through some other border router of that enterprise is up or down, and (b) the address prefix that was allocated to the enterprise by the ISP connected to the other border router. One such possible mechanism could be provided by BGP [RFC1771]. In this case border routers within the enterprise would have an IBGP peering with each other. Whenever one border router determines that the intersection between the set of reachable destinations it receives via its EBGP (from its directly connected ISP) peerings and the set of reachable destinations it receives from another border router (in the same enterprise) via IBGP is empty, the border router would start advertising to its external peer reachability to the address prefix that was allocated to the enterprise by the ISP connected to the other border router. The other border router would advertise (via IBGP) the address prefix that was allocated to the enterprise by the ISP connected to that router. This approach is known as "auto route injection".
上述方法基于这样一种假设，即企业边界路由器具有一种机制，通过该机制可以确定（a）通过该企业的某个其他边界路由器与互联网的连接是向上还是向下，以及（b）连接到其他边界路由器的ISP分配给企业的地址前缀。BGP[RFC1771]可以提供这样一种可能的机制。在这种情况下，企业内的边界路由器将具有相互对等的IBGP。当一个边界路由器确定它通过其EBGP（从其直接连接的ISP）对等接收的可到达目的地集和它通过IBGP从另一个边界路由器（在同一企业中）接收的可到达目的地集之间的交集为空时，边界路由器将开始向连接到另一个边界路由器的ISP分配给企业的地址前缀的外部对等可访问性播发。另一个边界路由器将公布（通过IBGP）连接到该路由器的ISP分配给企业的地址前缀。这种方法称为“自动路线注入”。
As an illustration consider an enterprise connected to two ISPs, ISP-A and ISP-B. Denote the enterprise border router that connects the enterprise to ISP-A as BR-A; denote the enterprise border router that connects the enterprise to ISP-B as BR-B. Denote the address prefix that ISP-A allocated to the enterprise as Pref-A; denote the address prefix that ISP-B allocated to the enterprise as Pref-B. When the set of routes BR-A receives from ISP-A (via EBGP) has a non-empty intersection with the set of routes BR-A receives from BR-B (via IBGP), BR-A advertises to ISP-A only the reachability to Pref-A. When the intersection becomes empty, BR-A would advertise to ISP-A reachability to both Pref-A and Pref-B. This would continue for as long as the intersection remains empty. Once the intersection becomes non-empty, BR-A would stop advertising reachability to Pref-B to ISP-A (but would still continue to advertise reachability to Pref-A to ISP-A). Figure 1 below describes this method graphically.
作为一个例子，考虑连接到两个ISP的企业，ISP A和ISP B表示将企业连接到ISP A作为B-A的企业边界路由器；将企业与ISP-B连接的企业边界路由器表示为BR-B。将ISP-A分配给企业的地址前缀表示为Pref-A；表示ISP-B作为Pref-B分配给企业的地址前缀。当从ISP-A（通过EBGP）接收的路由集BR-A与从BR-B（通过IBGP）接收的路由集BR-A具有非空交叉点时，BR-A仅向ISP-A播发到Pref-A的可达性。当交叉点变为空时，BR-A将向ISP-A播发Pref-A和Pref-B的可达性。只要交叉口保持为空，这种情况就会持续。一旦交叉口变为非空，BR-A将停止向ISP-A播发Pref-B的可达性（但仍将继续向ISP-A播发Pref-A的可达性）。下面的图1以图形方式描述了此方法。
+-------+ +-------+ +-------+ +-------+
( ) ( ) ( ) ( )
( ISP-A ) ( ISP-B ) ( ISP-A ) ( ISP-B )
( ) ( ) ( ) ( )
+-------+ +-------+ +-------+ +-------+
| /\ | /\ | /\ |
| || | || | Pref-A (connection
| Pref-A | Pref-B | Pref-B broken)
| || | || | || |
+-----+ +-----+ +-----+ +-----+
| BR-A|------|BR-B | | BR-A|------|BR-B |
+-----+ IBGP +-----+ +-----+ IBGP +-----+
+-------+ +-------+ +-------+ +-------+
( ) ( ) ( ) ( )
( ISP-A ) ( ISP-B ) ( ISP-A ) ( ISP-B )
( ) ( ) ( ) ( )
+-------+ +-------+ +-------+ +-------+
| /\ | /\ | /\ |
| || | || | Pref-A (connection
| Pref-A | Pref-B | Pref-B broken)
| || | || | || |
+-----+ +-----+ +-----+ +-----+
| BR-A|------|BR-B | | BR-A|------|BR-B |
+-----+ IBGP +-----+ +-----+ IBGP +-----+
non-empty intersection empty intersection
非空交叉口空交叉口
Figure 1: Reachability information advertised
图1：公布的可达性信息
Although strictly an implementation detail, calculating the intersection could potentially be a costly operation for a large set of routes. An alternate solution to this is to make use of a selected single (or more) address prefix received from an ISP (the ISP's backbone route for example) and configure the enterprise border router to perform auto route injection if the selected prefix is not present via IBGP. Let's suppose ISP-B has a well known address prefix, ISP-Pref-B for its backbone. ISP-B advertises this to BR-B and BR-B in turn advertises this via IBGP to BR-A. If BR-A sees a withdraw for ISP-Pref-B it advertises Pref-B to ISP-A.
尽管严格来说，交叉口的计算是一个实施细节，但对于大量路线而言，计算交叉口可能是一项成本高昂的操作。另一种解决方案是使用从ISP（例如ISP的主干路由）接收的选定单个（或多个）地址前缀，并配置企业边界路由器，以便在所选前缀不通过IBGP时执行自动路由注入。假设ISP-B有一个众所周知的地址前缀ISP-Pref-B作为主干。ISP-B向BR-B播发此消息，BR-B通过IBGP向BR-A播发此消息。如果BR-A看到ISP-Pref-B退出，则向ISP-A播发Pref-B。
The approach described in this section may produce less than the full Internet-wide connectivity in the presence of ISPs that filter out routes based on the length of their address prefixes. One could observe however, that this would be a problem regardless of how the enterprise would set up its routing and addressing.
如果ISP根据地址前缀的长度过滤掉路由，则本节中描述的方法可能会产生低于完整互联网范围的连接。然而，可以观察到，无论企业如何设置其路由和寻址，这都将是一个问题。
The approach described in the previous section allows to significantly reduce the routing overhead in the "default-free" zone of the Internet due to multi-homed enterprises. The approach described in this section allows to completely eliminate this overhead.
上一节中描述的方法允许显著减少由于多宿企业而导致的Internet“无默认”区域中的路由开销。本节中描述的方法允许完全消除这种开销。
An enterprise border router would maintain EBGP peering not just with the directly connected border router of an ISP, but with the border router(s) in one or more ISPs that have their border routers directly connected to the other border routers within the enterprise. We refer to such peering as "non-direct" EBGP.
企业边界路由器将不仅与ISP直接连接的边界路由器保持EBGP对等，还与一个或多个ISP中的边界路由器保持EBGP对等，这些ISP的边界路由器直接连接到企业内的其他边界路由器。我们将这种对等称为“非直接”EBGP。
An ISP that maintains both direct and non-direct EBGP peering with a particular enterprise would advertise the same set of routes over both of these peerings. An enterprise border router that maintains either direct or non-direct peering with an ISP advertises to that ISP reachability to the address prefix that was allocated by that ISP to the enterprise. Within the ISP routes received over direct peering should be preferred over routes received over non-direct peering. Likewise, within the enterprise routes received over direct peering should be preferred over routes received over non-direct peering.
与特定企业保持直接和非直接EBGP对等的ISP将在这两个对等上公布相同的路由集。与ISP保持直接或非直接对等的企业边界路由器向该ISP播发地址前缀的可达性，该地址前缀由该ISP分配给企业。在ISP中，通过直接对等接收的路由应优先于通过非直接对等接收的路由。同样，在企业内部，通过直接对等接收的路由应优先于通过非直接对等接收的路由。
Forwarding along a route received over non-direct peering should be accomplished via encapsulation [RFC1773].
应通过封装[RFC1773]沿通过非直接对等接收的路由进行转发。
As an illustration consider an enterprise connected to two ISPs, ISP-A and ISP-B. Denote the enterprise border router that connects the enterprise to ISP-A as E-BR-A, and the ISP-A border router that is connected to E-BR-A as ISP-BR-A; denote the enterprise border router that connects the enterprise to ISP-B as E-BR-B, and the ISP-B border router that is connected to E-BR-B as ISP-BR-B. Denote the address prefix that ISP-A allocated to the enterprise as Pref-A; denote the address prefix that ISP-B allocated to the enterprise as Pref-B. E-BR-A maintains direct EBGP peering with ISP-BR-A and advertises reachability to Pref-A over that peering. E-BR-A also maintain a non-direct EBGP peering with ISP-BR-B and advertises reachability to Pref-B over that peering. E-BR-B maintains direct EBGP peering with ISP-BR-B, and advertises reachability to Pref-B over that peering. E-BR-B also maintains a non-direct EBGP peering
作为一个例子，考虑连接到两个ISP的企业，ISP A和ISP B表示将企业连接到ISP A作为E-BR-A的企业边界路由器，以及作为ISB-B-A连接到E-BR-A的ISP A边界路由器；将企业连接到ISP-B的企业边界路由器表示为E-BR-B，将连接到E-BR-B的ISP-B边界路由器表示为ISP-BR-B。将ISP-A分配给企业的地址前缀表示为Pref-A；表示ISP-B作为Pref-B分配给企业的地址前缀。E-BR-A与ISP-BR-A保持直接EBGP对等，并通过该对等向Pref-A播发可达性。E-BR-A还与ISP-BR-B保持非直接EBGP对等，并通过该对等向Pref-B宣传可达性。E-BR-B与ISP-BR-B保持直接EBGP对等，并通过该对等向Pref-B通告可达性。E-BR-B还保持非直接EBGP对等
with ISP-BR-A, and advertises reachability to Pref-A over that peering.
使用ISP-BR-A，并在该对等网络上宣传Pref-A的可达性。
When connectivity between the enterprise and both of its ISPs (ISP-A and ISP-B is up, traffic destined to hosts whose addresses were assigned out of Pref-A would flow through ISP-A to ISP-BR-A to E-BR-A, and then into the enterprise. Likewise, traffic destined to hosts whose addresses were assigned out of Pref-B would flow through ISP-B to ISP-BR-B to E-BR-B, and then into the enterprise. Now consider what would happen when connectivity between ISP-BR-B and E-BR-B goes down. In this case traffic to hosts whose addresses were assigned out of Pref-A would be handled as before. But traffic to hosts whose addresses were assigned out of Pref-B would flow through ISP-B to ISP-BR-B, ISP-BR-B would encapsulate this traffic and send it to E-BR-A, where the traffic will get decapsulated and then be sent into the enterprise. Figure 2 below describes this approach graphically.
当企业与其两个ISP之间的连接（ISP A和ISP B是上行的，流量是由PREF-A分配给主机的流量将通过ISP A到ISP-B-A流到E-BR-A，然后流入企业，同样地，命中到由PREF-B分配地址的主机的流量将通过ISP B流到ISP B-B到E-BR B，然后进入企业。当ISP-BR-B和E-BR-B之间的连接中断时，可能会发生这种情况。在这种情况下，发送到地址由Pref-A分配的主机的流量将像以前一样处理。但发送到地址由Pref-B分配的主机的流量将通过ISP-B流向ISP-BR-B，ISP-BR-B将封装此流量并将其发送到E-BR-A，在E-BR-A中，traffic将被解封，然后被发送到企业中。
+---------+ +---------+
( ) ( )
( ISP-A ) ( ISP-B )
( ) ( )
+---------+ +---------+
| |
+--------+ +--------+
|ISP-BR-A| |ISP-BR-B|
+--------+ +--------+
| /+/ |
/\ | Pref-B /+/ |
|| | /+/ \./
Pref-A| /+/ non- /.\
|| | /+/ direct |
| /+/ EBGP |
+------+ +-------+
|E-BR-A|-----------|E-BR-B |
+------+ IBGP +-------+
+---------+ +---------+
( ) ( )
( ISP-A ) ( ISP-B )
( ) ( )
+---------+ +---------+
| |
+--------+ +--------+
|ISP-BR-A| |ISP-BR-B|
+--------+ +--------+
| /+/ |
/\ | Pref-B /+/ |
|| | /+/ \./
Pref-A| /+/ non- /.\
|| | /+/ direct |
| /+/ EBGP |
+------+ +-------+
|E-BR-A|-----------|E-BR-B |
+------+ IBGP +-------+
Figure 2: Reachability information advertised via non-direct EBGP
图2：通过非直接EBGP发布的可达性信息
Observe that with this scheme there is no additional routing information due to multi-homed enterprises that has to be carried in the "default-free" zone of the Internet. In addition this scheme doesn't degrade in the presence of ISPs that filter out routes based on the length of their address prefixes.
请注意，使用此方案，由于多宿企业必须在Internet的“默认自由”区域中携带，因此没有额外的路由信息。此外，当ISP根据地址前缀的长度过滤掉路由时，该方案不会降级。
Note that the set of routers within an ISP that maintain non-direct peering with the border routers within an enterprise doesn't have to be restricted to the ISP's border routers that have direct peering
请注意，ISP中与企业中的边界路由器保持非直接对等的路由器集不必限于具有直接对等的ISP边界路由器
with the enterprise's border routers. The non-direct peering could be maintained with any router within the ISP. Doing this could improve the overall robustness in the presence of failures within the ISP.
与企业的边界路由器。非直接对等可以通过ISP内的任何路由器进行维护。这样做可以提高ISP内部出现故障时的整体健壮性。
One could observe that while the approach described in Section 5.2 allows to completely eliminate the routing overhead due to multi-homed enterprises in the "default-free" zone of the Internet, it may result in a suboptimal routing in the presence of link failures. The sub-optimality could be reduced by combining the approach described in Section 5.2 with a slightly modified version of the approach described in Section 5.1. The modification consists of constraining the scope of propagation of additional routes that are advertised by an enterprise border router when the router detects problems with the Internet connectivity through its other border routers. A way to constrain the scope is by using the BGP Community attribute [RFC1997].
人们可以观察到，虽然第5.2节中描述的方法允许完全消除由于互联网“无默认”区域中的多宿企业而产生的路由开销，但它可能会在存在链路故障的情况下导致次优路由。通过将第5.2节所述方法与第5.1节所述方法的稍微修改版本相结合，可以降低次优性。修改包括限制企业边界路由器在通过其其他边界路由器检测到互联网连接问题时播发的其他路由的传播范围。限制范围的一种方法是使用BGP社区属性[RFC1997]。
The approach described in this document assumes that in a steady state an enterprise border router would advertise to a directly connected ISP border router only the reachability to the address prefix that this ISP allocated to the enterprise. As a result, traffic originated by other enterprises connected to that ISP and destined to the parts of the enterprise numbered out of other address prefixes would not enter the enterprise at this border router, resulting in potentially suboptimal paths. To improve the situation the border router may (in steady state) advertise reachability not only to the address prefix that was allocated by the ISP that the router is directly connected to, but to the address prefixes allocated by some other ISPs (directly connected to some other border routers within the enterprise). Distribution of such advertisements should be carefully constrained, or otherwise this may result in significant additional routing information that would need to be maintained in the "default-free" part of the Internet. A way to constrain the distribution of such advertisements is by using the BGP Community attribute [RFC1997].
本文档中描述的方法假设在稳定状态下，企业边界路由器只向直接连接的ISP边界路由器通告该ISP分配给企业的地址前缀的可达性。因此，由连接到该ISP的其他企业发起并发送到企业中其他地址前缀不足的部分的流量将不会通过该边界路由器进入企业，从而导致潜在的次优路径。为了改善这种情况，边界路由器可能（在稳定状态下）不仅向路由器直接连接到的ISP分配的地址前缀，而且向一些其他ISP分配的地址前缀（直接连接到企业内的一些其他边界路由器）通告可达性。应谨慎限制此类广告的分发，否则这可能会导致大量额外的路由信息，需要在互联网的“无默认”部分进行维护。限制此类广告分发的一种方法是使用BGP社区属性[RFC1997]。
CIDR [RFC1518] proposes several possible address allocation strategies for multi-homed enterprises that are connected to multiple ISPs. The following briefly reviews the alternatives being used today, and compares them with the approaches described above.
CIDR[RFC1518]为连接到多个ISP的多宿企业提出了几种可能的地址分配策略。以下简要回顾了目前使用的替代方案，并将其与上述方法进行了比较。
One possible solution suggested in [RFC1518] is for each multi-homed enterprise to obtain its IP address space independently from the ISPs to which it is attached. This allows each multi-homed enterprise to base its IP assignments on a single prefix, and to thereby summarize the set of all IP addresses reachable within that enterprise via a single prefix. The disadvantage of this approach is that since the IP address for that enterprise has no relationship to the addresses of any particular ISPs, the reachability information advertised by the enterprise is not aggregatable with any, but default route. results in the routing overhead in the "default-free" zone of the Internet of O(N), where N is the total number of multi-homed enterprises across the whole Internet that are connected to multiple ISPs.
[RFC1518]中建议的一种可能的解决方案是，每个多宿企业独立于其所连接的ISP获取其IP地址空间。这允许每个多宿企业将其IP分配基于单个前缀，从而汇总通过单个前缀可在该企业内访问的所有IP地址集。这种方法的缺点是，由于该企业的IP地址与任何特定ISP的地址没有关系，因此该企业发布的可达性信息不能通过任何默认路由聚合。导致Internet的“无默认”区域的路由开销为O（N），其中N是整个Internet上连接到多个ISP的多址企业的总数。
As a result, this approach can't be viewed as a viable alternative for all, but the enterprises that provide high enough degree of addressing information aggregation. Since by definition the number of such enterprises is likely to be fairly small, this approach isn't viable for most of the multi-homed enterprises connected to multiple ISPs.
因此，这种方法不能被视为所有企业都可行的替代方法，但对于提供足够高程度的寻址信息聚合的企业来说，它是可行的。由于根据定义，此类企业的数量可能相当少，因此这种方法对于大多数连接到多个ISP的多宿企业来说是不可行的。
Another possible solution suggested in [RFC1518] is to assign each multi-homed enterprise a single address prefix, based on one of its connections to one of its ISPs. Other ISPs to which the multi-homed enterprise is attached maintain a routing table entry for the organization, but are extremely selective in terms of which other ISPs are told of this route and would need to perform "proxy" aggregation. Most of the complexity associated with this approach is due to the need to perform "proxy" aggregation, which in turn requires t addiional inter-ISP coordination and more complex router configuration.
[RFC1518]中建议的另一种可能的解决方案是，根据每个多址企业与其中一个ISP的连接，为其分配一个地址前缀。多宿企业所连接的其他ISP维护组织的路由表条目，但在告知其他ISP此路由并需要执行“代理”聚合方面，这些ISP具有极高的选择性。与此方法相关的大多数复杂性是由于需要执行“代理”聚合，这反过来又需要额外的ISP间协调和更复杂的路由器配置。
The approach described in this document assumes that addresses that an enterprise would use are allocated based on the "address lending" policy. Consequently, whenever an enterprise changes its ISP, the enterprise would need to renumber part of its network that was numbered out of the address block that the ISP allocated to the enterprise. However, these issues are not specific to multihoming and should be considered accepted practice in todays internet. The approach described in this document effectively eliminates any distinction between single-home and multi-homed enterprise with respect to the impact of changing ISPs on renumbering.
本文档中描述的方法假设企业将使用的地址是根据“地址借出”策略分配的。因此，每当企业更改其ISP时，企业都需要重新编号ISP分配给企业的地址块之外编号的部分网络。然而，这些问题并不特定于多主机，应该被认为是当今互联网上公认的做法。本文件中描述的方法有效地消除了单一家庭企业和多家庭企业之间在ISP变更对重新编号的影响方面的任何区别。
The approach described in this document also requires careful address assignment within an enterprise, as address assignment impacts traffic distribution among multiple connections between an enterprise and its ISPs.
本文档中描述的方法还需要在企业内仔细分配地址，因为地址分配会影响企业与其ISP之间的多个连接之间的流量分布。
Both the issue of address assignment and renumbering could be addressed by the appropriate use of network address translation (NAT). The use of NAT for multi-homed enterprises is the beyond the scope of this document.
地址分配和重新编号问题都可以通过适当使用网络地址转换（NAT）来解决。对于多宿企业使用NAT超出了本文档的范围。
Use of auto route injection (as described in Section 5.1) increases the number of routers in the default-free zone of the Internet that could be affected by changes in the connectivity of multi-homed enterprises, as compared to the use of provider-independed addresses (as described in Section 6.1). Specifically, with auto route injection when a multi-homed enterprise loses its connectivity through one of its ISPs, the auto injected route has to be propagated to all the routers in the default-free zone of the Internet. In contrast, when an enterprise uses provider-independent addresses, only some (but not all) of the routers in the default-free zone would see changes in routing when the enterprise loses its connectivity through one of its ISPs.
与使用独立于提供商的地址（如第6.1节所述）相比，使用自动路由注入（如第5.1节所述）增加了互联网默认自由区中可能受多宿企业连接变化影响的路由器数量。具体而言，当多宿企业通过其ISP之一失去连接时，自动注入路由必须传播到Internet默认自由区中的所有路由器。相反，当企业使用独立于提供商的地址时，当企业通过其ISP之一失去连接时，默认自由区中只有部分（但不是全部）路由器的路由会发生变化。
To supress excessive routing load due to link flapping the auto injected route has to be advertised until the connectivity via the other connection (that was previously down and that triggered auto route injection) becomes stable.
为了抑制由于链路抖动而导致的过度路由负载，必须通告自动注入路由，直到通过另一个连接（先前已关闭并触发自动路由注入）的连接变得稳定。
Use of the non-direct EBGP approach (as described in Section 5.2) allows to eliminate route flapping due to multi-homed enterprises in the default-free zone of the Internet. That is the non-direct EBGP approach has better properties with respect to routing stability than the use of provider-independent addresses (as described in Section 6.1).
使用非直接EBGP方法（如第5.2节所述）可以消除由于互联网默认自由区内多家企业造成的路由抖动。也就是说，非直接EBGP方法在路由稳定性方面比使用独立于提供程序的地址（如第6.1节所述）具有更好的特性。
The approach described in this document could be applicable to a small to medium size ISP that is connected to several upstream ISPs. The ISP would acquire blocks of addresses (address prefixes) from its upstream ISPs, and would use these addresses for allocations to its customers. Either auto route injection, or the non-direct EBGP approach, or a combination of both could be used by the ISP when peering with its upstream ISPs. Doing this would provide routability for the customers of such ISP, without advertsely affecting the overall scalability of the Internet routing system.
本文件中描述的方法可适用于连接到多个上游ISP的中小型ISP。ISP将从其上游ISP获取地址块（地址前缀），并使用这些地址分配给其客户。ISP在与其上游ISP进行对等时，可以使用自动路由注入或非直接EBGP方法，或两者的组合。这样做将为此类ISP的客户提供可路由性，而不会影响互联网路由系统的整体可扩展性。
Since the non-direct EBGP approach (as described in Section 5.2) requires EBGP sessions between routers that are more than one IP hop from each other, routers that maintain these sessions should use an appropriate authentication mechanism(s) for BGP peer authentication.
由于非直接EBGP方法（如第5.2节所述）需要在相互之间有多个IP跃点的路由器之间进行EBGP会话，因此维护这些会话的路由器应使用适当的身份验证机制进行BGP对等身份验证。
Security issues related to the IBGP peering, as well as the EBGP peering between routers that are one IP hop from each other are outside the scope of this document.
与IBGP对等以及路由器之间的EBGP对等相关的安全问题（相互之间只有一个IP跃点）超出了本文档的范围。
The authors of this document do not make any claims on the originality of the ideas described in this document. Anyone who thought about these ideas before should be given all due credit.
本文件的作者不对本文件中所述想法的独创性提出任何主张。任何以前考虑过这些想法的人都应该得到应有的赞扬。
[RFC1518] Rekhter, Y., and T. Li, "An Architecture for IP Address Allocation with CIDR", RFC 1518, September 1993.
[RFC1518]Rekhter，Y.和T.Li，“使用CIDR的IP地址分配架构”，RFC 1518，1993年9月。
[RFC1771] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4)", RFC 1771, March 1995.
[RFC1771]Rekhter，Y.和T.Li，“边境网关协议4（BGP-4）”，RFC 17711995年3月。
[RFC1773] Hanks, S., Li, T., Farinacci, T., and P. Traina, "Generic Routing Encapsulation over IPv4 networks", RFC 1773, October 1994.
[RFC1773]Hanks，S.，Li，T.，Farinaci，T.，和P.Traina，“IPv4网络上的通用路由封装”，RFC 1773，1994年10月。
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot G.J., and E. Lear, "Address Allocation for Private Internets", RFC 1918, February 1996.
[RFC1918]Rekhter，Y.，Moskowitz，B.，Karrenberg，D.，de Groot G.J.，和E.Lear，“私人互联网地址分配”，RFC 1918，1996年2月。
[RFC1997] Chandra, R., Traina, P., and T. Li, "BGP Communities Attribute", RFC 1997, August 1996.
[RFC1997]Chandra，R.，Traina，P.，和T.Li，“BGP社区属性”，RFC 1997，1996年8月。
[RFC2008] Rekhter, Y., and T. Li, "Implications of Various Address Allocation Policies for Internet Routing", BCP 7, RFC 2008, October 1996.
[RFC2008]Rekhter，Y.和T.Li，“各种地址分配策略对互联网路由的影响”，BCP 7，RFC 2008，1996年10月。
Tony Bates Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134
Tony Bates Cisco Systems，Inc.加利福尼亚州圣何塞西塔斯曼大道170号，邮编95134
EMail: tbates@cisco.com
EMail: tbates@cisco.com
Yakov Rekhter Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134 EMail: yakov@cisco.com
Yakov Rekhter Cisco Systems，Inc.加利福尼亚州圣何塞市西塔斯曼大道170号，邮编95134电子邮件：yakov@cisco.com
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