Jeff Doyle关于OSPF水平分割的回信及翻译

Jeff Doyle关于OSPF水平分割的回信及翻译

翻译：Cedric CCIE #25467

Hi!
 
Split horizon is used on distance vector algorithms (rip, eigrp), to prevent a route from being advertised back out an interface in which it was received.  OSPF is not a distance vector algorithm.  A loop free topology is built, and using shortest path, route tables are built within areas.  The DV type behavior is between areas, with type 3 summary lsas.  In some cases, such as with   To prevent ospf type 3 LSAs from being advertised, you could use a filter list, area area-id filter-list prefix prefix-name in|out , and a prefix list.  The "in" keyword filters type 3 lsa from area 0 into the area-id specified.  "out" filters type 3 lsas from the area specified into other areas.   You could summarize the LSAs, which would allow the ABR to advertise a summarized version of the route into the area specified.
 
or, maybe they are wondering more about configuring ospf on nbma networks.  They could be thinking about configuring subinterfaces, which were developed to get around split horizon issues on an NBMA network using a DV algorithm.  Maybe they are thinking about getting all the link state information into the database within an area, and are thinking of the command IP OSPF NETWORK point-to-multipoint to configure OSPF over a nbma network.  
 
I'd probably want to clarify the interview question, by asking something like: "So, your question is how to enable type 3 LSAs to be advertised back into an area in which the route exists in the topology table?", which shows the interviewer that you know about how OSPF works, you could then tell them what you know.  Maybe by asking the question, you'll better understand what the interviewer is trying to ask.
 
Good luck!
 

 

 

 

hi:

水平分割是用于距离矢量算法（RIP,EIGRP），目的是防止：一条路由信息从某个接口再通告出去（这条路由信息是从这个接口学习到的）。 OSPF不是距离矢量！（OSPF）构建了一个无环的拓扑，并使用最短路径优先，在区域内构建路由表（我理解这里所说的路由表是LSU或者LSA，链路状态更新或链路状态通告）。OSPF也有距离矢量特性：表现在区域间，而且指的是类型3的LSA（链路状态通告）。在某些特定情况下，诸如：为防止OSPF类型3的LSA从学到的区域再通告出去，你可以使用“过滤列表”，例：area area-id filter-list prefix prefix-name in|out(配置命令)，或者前缀匹配列表。“in”关键词过滤从区域0到某个你特殊指定区域的类型3的LSA。“out”关键词过滤你特殊指定的区域去往另外区域的类型3的LSA。（这里“特殊指定的区域”指你挂过滤列表的区域）。或者你可以汇总LSA，让ABR（区域边界路由器）通告一个汇总版本的路由去那些你觉得可能有水平分割风险的区域。

 

或者，思考OSPF水平分割的人正疑惑配置OSPF NBMA（非广播多路访问）网络时水平分割的问题。他们可能正思考着配置子接口时距离矢量协议由于水平分割所产生的问题。可能他们正考虑如何收集所有链路状态信息放入某个区域的链路状态数据库，并思考着在一个NBMA网络使用IP OSPF NETWORK point-to-multipoint来配置OSPF。

 

我想要弄清楚那些向我提“OSPF水平分割”的问题，他们提的“怎样让类型3的LSA再被通告回某个存在这条路由的区域”，这些问题显示出他们对于OSPF工作原理是有一定理解的，那你可以告诉他们你理解的（前面所说：区域间的水平分割）。可能当你被问到这个问题的时候，你会更好的理解提问者想问的问题（水平分割，OSPF水平分割）

 

 

Good luck!

 

谨以此文向TCP/IP 卷一、卷二的作者 Jeff Doyle CCIE #1919致敬！

也希望这篇文章能与热爱技术、追求技术的人共勉！

欢迎技术讨论：新浪微博：t.sina.com.cn/mycedric    QQ:332390494

 

以下提供PDF下载