Network Working Group D. McPherson
Request for Comments: 3277 TCB
Category: Informational April 2002
Network Working Group D. McPherson
Request for Comments: 3277 TCB
Category: Informational April 2002
Intermediate System to Intermediate System (IS-IS) Transient Blackhole Avoidance
中间系统到中间系统（IS-IS）瞬态黑洞回避
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 (2002). All Rights Reserved.
版权所有（C）互联网协会（2002年）。版权所有。
Abstract
摘要
This document describes a simple, interoperable mechanism that can be employed in Intermediate System to Intermediate System (IS-IS) networks in order to decrease the data loss associated with deterministic blackholing of packets during transient network conditions. The mechanism proposed here requires no IS-IS protocol changes and is completely interoperable with the existing IS-IS specification.
本文档描述了一种简单的、可互操作的机制，该机制可用于中间系统到中间系统（IS-IS）网络中，以减少与瞬态网络条件下的数据包确定性黑洞相关的数据丢失。这里提出的机制不需要更改IS-IS协议，并且完全可以与现有IS-IS规范互操作。
When an IS-IS router that was previously a transit router becomes unavailable as a result of some transient condition such as a reboot, other routers within the routing domain must select an alternative path to reach destinations which have previously transited the failed router. Presumably, the newly selected router(s) comprising the path have been available for some time and, as a result, have complete forwarding information bases (FIBs) which contain a full set of reachability information for both internal and external (e.g., BGP) destination networks.
当先前作为传输路由器的IS-IS路由器由于某些瞬态条件（如重新启动）而变得不可用时，路由域内的其他路由器必须选择替代路径以到达先前传输故障路由器的目的地。据推测，包括路径的新选择的路由器已经可用一段时间，并且因此具有完整的转发信息库（fib），其包含用于内部和外部（例如，BGP）目的地网络的全套可达性信息。
When the previously failed router becomes available again, it is only seconds before the paths that had previously transited the router are again selected as the optimal path by the IGP. As a result, forwarding tables are updated and packets are once again forwarded along the path. Unfortunately, external destination reachability information (e.g., learned via BGP) is not yet available to the router, and as a result, packets bound for destinations not learned via the IGP are unnecessarily discarded.
当先前发生故障的路由器再次可用时，IGP只需几秒钟就可以再次选择先前经过路由器的路径作为最佳路径。结果，转发表被更新，数据包再次沿着路径转发。不幸的是，外部目的地可达性信息（例如，通过BGP学习的）尚不可用于路由器，因此，绑定到未通过IGP学习的目的地的分组被不必要地丢弃。
A simple interoperable mechanism to alleviate the offshoot associated with this deterministic behavior is discussed below.
下面将讨论一种简单的互操作机制，以减轻与此确定性行为相关的分支。
This document describes a simple, interoperable mechanism that can be employed in IS-IS [1, 2] networks in order to avoid transition to a newly available path until other associated routing protocols such as BGP have had sufficient time to converge.
本文档描述了一种简单、可互操作的机制，该机制可用于IS-IS[1，2]网络，以避免过渡到新的可用路径，直到其他相关路由协议（如BGP）有足够的时间收敛。
The benefits of such a mechanism can be realized when considering the following scenario depicted in Figure 1.
在考虑图1所示的以下场景时，可以实现这种机制的好处。
D.1
|
+-------+
| RtrD |
+-------+
/ \
/ \
+-------+ +-------+
| RtrB | | RtrC |
+-------+ +-------+
\ /
\ /
+-------+
| RtrA |
+-------+
|
S.1
D.1
|
+-------+
| RtrD |
+-------+
/ \
/ \
+-------+ +-------+
| RtrB | | RtrC |
+-------+ +-------+
\ /
\ /
+-------+
| RtrA |
+-------+
|
S.1
Figure 1: Example Network Topology
图1：示例网络拓扑
Host S.1 is transmitting data to destination D.1 via a primary path of RtrA->RtrB->RtrD. Routers A, B and C learn of reachability to destination D.1 via BGP from RtrD. RtrA's primary path to D.1 is selected because when calculating the path to BGP NEXT_HOP of RtrD, the sum of the IS-IS link metrics on the RtrA-RtrB-RtrD path is less than the sum of the metrics of the RtrA-RtrC-RtrD path.
主机S.1通过RtrA->RtrB->RtrD的主路径将数据传输到目的地D.1。路由器A、B和C通过BGP从RtrD学习到目的地D.1的可达性。选择RtrA到D.1的主路径是因为在计算到RtrD下一跳BGP的路径时，RtrA RtrB RtrD路径上的is-is链路度量之和小于RtrA RtrC RtrD路径的度量之和。
Assume RtrB becomes unavailable and as a result the RtrC path to RtrD is used. Once RtrA's FIB is updated and it begins forwarding packets to RtrC, everything should behave properly as RtrC has existing forwarding information regarding destination D.1's availability via BGP NEXT_HOP RtrD.
假设RtrB变得不可用，因此使用RtrD的RtrC路径。一旦RtrA的FIB被更新，并且它开始向RtrC转发数据包，一切都应该正常运行，因为RtrC通过BGP下一跳RtrD拥有关于目的地D.1可用性的现有转发信息。
Assume now that RtrB comes back online. In only a few seconds, IS-IS neighbor state has been established with RtrA and RtrD and database synchronization has occurred. RtrA now realizes that the best path to destination D.1 is via RtrB, and therefore updates it FIB appropriately. RtrA begins to forward packets destined to D.1 to RtrB. Though, because RtrB has yet to establish and synchronize its BGP neighbor relationship and routing information with RtrD, RtrB has no knowledge regarding reachability of destination D.1, and therefore discards the packets received from RtrA destined to D.1.
现在假设RtrB重新联机。在短短几秒钟内，通过RtrA和RtrD建立了IS-IS邻居状态，并实现了数据库同步。RtrA现在意识到到达目的地D.1的最佳路径是通过RtrB，因此适当地更新了它。RtrA开始将目的地为D.1的数据包转发给RtrB。尽管如此，由于RtrB尚未建立BGP邻居关系并将其路由信息与RtrD同步，RtrB不知道目的地D.1的可达性，因此丢弃从目的地为D.1的RtrA接收的数据包。
If RtrB were to temporarily set its LSP Overload bit while synchronizing BGP tables with its neighbors, RtrA would continue to use the working RtrA->RtrC->RtrD path, and the LSP should only be used to obtain reachability to locally connected networks (rather than for calculating transit paths through the router, as defined in [1]).
如果RtrB在与其邻居同步BGP表时临时设置其LSP重载位，RtrA将继续使用工作RtrA->RtrC->RtrD路径，并且LSP应仅用于获得对本地连接网络的可达性（而不是用于计算通过路由器的传输路径，如[1]中所定义）。
However, it should be noted that when RtrB goes away, its LSP is still present in the IS-IS databases of all other routers in the routing domain. When RtrB comes back it establishes adjacencies. As soon as its neighbors have an adjacency with RtrB, they will advertise their new adjacency in their new LSP. The result is that all the other routers will receive new LSPs from RtrA and RtrD containing the RtrB adjacency, even though RtrB is still completing its synchronization and therefore has not yet sent its new LSP.
然而，应该注意的是，当RtrB消失时，其LSP仍然存在于路由域中所有其他路由器的is-is数据库中。当RtrB返回时，它会建立邻接关系。一旦它的邻居与RtrB有邻接关系，他们就会在新的LSP中宣传他们的新邻接关系。结果是，所有其他路由器将从RtrA和RtrD接收包含RtrB邻接的新LSP，即使RtrB仍在完成其同步，因此尚未发送其新LSP。
At this time SPF is computed and everyone will include RtrB in their tree since they will use the old version of RtrB LSP (the new one has not yet arrived). Once RtrB has finished establishing all its adjacencies, it will then regenerate its LSP and flood it. Then all other routers within the domain will finally compute SPF with the correct information. Only at that time will the Overload bit be taken into account.
此时将计算SPF，每个人都将在其树中包含RtrB，因为他们将使用旧版本的RtrB LSP（新版本尚未到达）。一旦RtrB完成了其所有邻接的建立，它将重新生成其LSP并将其淹没。然后域内的所有其他路由器将最终使用正确的信息计算SPF。只有在那个时候才会考虑过载位。
As such, it is recommended that each time a router establishes an adjacency, it will update its LSP and flood it immediately, even before beginning database synchronization. This will allow for the Overload bit setting to propagate immediately, and remove the potential for an older version of the reloaded routers LSP to be used.
因此，建议每次路由器建立邻接关系时，它都会更新其LSP并立即使用它，甚至在开始数据库同步之前。这将允许过载位设置立即传播，并消除使用较旧版本的重新加载路由器LSP的可能性。
After synchronization of BGP tables with neighboring routers (or expiry of some other timer or trigger), RtrB would generate a new LSP, clearing the Overload bit, and RtrA could again begin using the optimal path via RtrB.
在BGP表与相邻路由器同步（或其他计时器或触发器到期）后，RtrB将生成新的LSP，清除过载位，RtrA可以通过RtrB再次开始使用最佳路径。
Typically, in service provider networks IBGP connections are done via peerings with 'loopback' addresses. As such, the newly available router must advertise its own loopback (or similar) IP address, as well as associated adjacencies, in order to make the loopbacks accessible to other routers within the routing domain. It is because of this that simply flooding an empty LSP is not sufficient.
通常，服务提供商网络中的IBGP连接是通过具有“环回”地址的对等来完成的。因此，新可用的路由器必须公布其自己的环回（或类似）IP地址以及相关的邻接，以便使路由域内的其他路由器可以访问环回。正因为如此，仅仅泛洪一个空LSP是不够的。
Such a mechanism increases overall network availability and allows network operators to alleviate the deterministic blackholing behavior introduced in this scenario. Similar mechanisms [3] have been defined for OSPF, though only after realizing the usefulness obtained from that of the IS-IS Overload bit technique.
这种机制提高了整体网络可用性，并允许网络运营商缓解此场景中引入的确定性黑洞行为。OSPF也定义了类似的机制[3]，尽管只是在认识到IS-IS过载位技术的有用性之后。
This mechanism has been deployed in several large IS-IS networks for a number of years.
多年来，该机制已部署在多个大型IS-IS网络中。
Triggers for setting the Overload bit as described are left to the implementer. Some potential triggers could perhaps include "N seconds after booting", or "N number of BGP prefixes in the BGP Loc-RIB".
用于设置所述重载位的触发器留给实现者。一些潜在的触发器可能包括“启动后N秒”或“BGP Loc RIB中N个BGP前缀”。
Unlike similar mechanisms employed in [3], if the Overload bit is set in a router's LSP, NO transit paths are calculated through the router. As such, if no alternative paths are available to the destination network, employing such a mechanism may actually have a negative impact on convergence (i.e., the router maintains the only available path to reach downstream routers, but the Overload bit disallows other nodes in the network from calculating paths via the router, and as such, no feasible path exists to the routers).
与[3]中采用的类似机制不同，如果在路由器的LSP中设置过载位，则不会计算通过路由器的传输路径。因此，如果目的地网络没有可用的替代路径，则采用这种机制实际上可能对收敛产生负面影响（即，路由器维护到达下游路由器的唯一可用路径，但过载位不允许网络中的其他节点通过路由器计算路径，因此，路由器不存在可行路径）。
Finally, if all systems within an IS-IS routing domain haven't implemented the Overload bit correctly, forwarding loops may occur.
最后，如果IS-IS路由域中的所有系统没有正确实现过载位，则可能会发生转发循环。
Alternatively, it may be considered more appealing to employ something more akin to [3] for this purpose. With this model, during transient conditions a node advertises excessively high link metrics to serve as an indication, to other nodes in the network that paths transiting the router are "less desirable" than existing paths.
或者，出于此目的，可以认为使用更类似于[3]的东西更具吸引力。使用此模型，在瞬态条件下，节点向网络中的其他节点宣传过高的链路度量，以作为通过路由器的路径比现有路径“不太理想”的指示。
The advantage of a metric-based mechanism over the Overload bit mechanism model proposed here is that transit paths may still be calculated through the router. Another advantage is that a metric-based mechanism does not require that all nodes in the IS-IS domain correctly implement the Overload bit.
基于度量的机制比本文提出的过载位机制模型的优点是，传输路径仍然可以通过路由器计算。另一个优点是，基于度量的机制不需要is-is域中的所有节点都正确实现重载位。
However, as currently deployed, IS-IS provides for only 6 bits of space for link metric allocation, and 10 bits aggregate path metric. Though extensions proposed in [4] remove this limitation, they have not yet been widely deployed. As such, there's currently little flexibility when using link metrics for this purpose. Of course, both methods proposed in this document are backwards-compatible.
然而，正如当前部署的那样，IS-IS仅为链路度量分配提供6位空间，并为聚合路径度量提供10位空间。尽管[4]中提出的扩展消除了这一限制，但它们尚未得到广泛部署。因此，目前在为此目的使用链接度量时几乎没有灵活性。当然，本文中提出的两种方法都是向后兼容的。
The mechanisms specified in this memo introduces no new security issues to IS-IS.
本备忘录中规定的机制不会给IS-IS带来新的安全问题。
The author of this document makes no claim to the originality of the idea. Thanks to Stefano Previdi for valuable feedback on the mechanism discussed in this document.
本文件的作者并不声称该想法具有独创性。感谢Stefano Previdi就本文讨论的机制提供了宝贵的反馈。
[1] ISO, "Intermediate system to Intermediate system routing information exchange protocol for use in conjunction with the Protocol for providing the Connectionless-mode Network Service (ISO 8473)," ISO/IEC 10589:1992.
[1] ISO，“与提供无连接模式网络服务的协议一起使用的中间系统到中间系统路由信息交换协议（ISO 8473）”，ISO/IEC 10589:1992。
[2] Callon, R., "OSI IS-IS for IP and Dual Environment," RFC 1195, December 1990.
[2] Callon，R.，“IP和双环境的OSI IS-IS”，RFC1195，1990年12月。
[3] Retana, A., Nguyen, L., White, R., Zinin, A. and D. McPherson, "OSPF Stub Router Advertisement", RFC 3137, June 2001.
[3] 瑞塔纳，A.，阮，L.，怀特，R.，吉宁，A.和D.麦克弗森，“OSPF存根路由器广告”，RFC3137，2001年6月。
[4] Li, T. and H. Smit, "IS-IS extensions for Traffic Engineering", Work in Progress.
[4] Li，T.和H.Smit，“交通工程的IS-IS扩展”，正在进行中。
Danny McPherson TCB Phone: 303.470.9257 EMail: danny@tcb.net
Danny McPherson TCB电话：303.470.9257电子邮件：danny@tcb.net
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确认
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