Jeff Doyle關於OSPF水平分割的回信及翻譯
翻譯：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!
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:
水平分割是用於距離矢量算法（RIP,EIGRP），目的是防止：一條路由信息從某個接口再通告出去（這條路由信息是從這個接口學習到的）。OSPF不是距離矢量！（OSPF）構建了一個無環的拓撲，並使用最短路徑優先，在區域內構建路由表（我理解這裏所說的路由表是LSU或者LSA，鏈路狀態更新或鏈路狀態通告）。OSPF也有距離矢量特性：表現在區域間，而且指的是類型3的LSA（鏈路狀態通告）。在某些特定情況下，諸如：爲防止OSPF類型3的LSA從學到的區域再通告出去，你可以使用“過濾列表”，例：area area-id filter-list prefix prefix-name in|out(配置命令)，或者前綴匹配列表。“in”關鍵詞過濾從區域0到某個你特殊指定區域的類型3的LSA。“out”關鍵詞過濾你特殊指定的區域去往另外區域的類型3的LSA。（這裏“特殊指定的區域”指你掛過濾列表的區域）。或者你可以彙總LSA，讓ABR（區域邊界路由器）通告一個彙總版本的路由去那些你覺得可能有水平分割風險的區域。
或者，思考OSPF水平分割的人正疑惑配置OSPF NBMA（非廣播多路訪問）網絡時水平分割的問題。他們可能正思考着配置子接口時距離矢量協議由於水平分割所產生的問題。可能他們正考慮如何收集所有鏈路狀態信息放入某個區域的鏈路狀態數據庫，並思考着在一個NBMA網絡使用IP OSPF NETWORK point-to-multipoint來配置OSPF。
我想要弄清楚那些向我提“OSPF水平分割”的問題，他們提的“怎樣讓類型3的LSA再被通告回某個存在這條路由的區域”，這些問題顯示出他們對於OSPF工作原理是有一定理解的，那你可以告訴他們你理解的（前面所說：區域間的水平分割）。可能當你被問到這個問題的時候，你會更好的理解提問者想問的問題（水平分割，OSPF水平分割）
Good luck!
謹以此文向TCP/IP 卷一、卷二的作者 Jeff Doyle CCIE #1919致敬！
也希望這篇文章能與熱愛技術、追求技術的人共勉！
歡迎技術討論：新浪微博：t.sina.com.cn/mycedric QQ:332390494
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