Cisco：深入理解 OSPF NSSA “P-bit”

注：机翻，未校对。

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OSPF Forward Address Filtering

OSPF 转发地址过滤

In this lesson we’ll take a closer look at the OSPF NSSA “P-bit”. When we redistribute something into an OSPF NSSA area then these prefixes are flooded within the NSSA area as LSA type 7. Once these LSAs make it to an ABR, they are translated into LSA type 5 and advertised to other areas.
在本课中，我们将更深入地了解 OSPF NSSA “P-bit”。当我们将某些内容重新分发到 OSPF NSSA 区域中时，这些前缀将作为 LSA 类型 7 泛洪在 NSSA 区域内。一旦这些 LSA 进入 ABR，它们就会被翻译成 LSA 类型 5 并在其他地区发布广告。

The P-bit (P stands for propagate) can be found in the options field of an LSA type 7 and it tells the ABR if the LSA type 7 should be translated into a LSA type 5 or not. Only LSAs with the P-bit will be translated and automatically this bit will be set for all prefixes that are redistributed.
P 位（P 代表传播）可以在 LSA 类型 7 的选项字段中找到，它告诉 ABR 是否应该将 LSA 类型 7 转换为 LSA 类型 5。只有具有 P 位的 LSA 才会被转换，并且将自动为所有重新分发的前缀设置此位。

Let’s take a look at this P-bit in action. First I’ll show you where you can find it and afterwards we’ll look at some examples how you prevent the translation from LSA type 7 to 5. I’ll use the following topology:
让我们来看看这个 P-bit 的实际效果。首先，我将向您展示在哪里可以找到它，然后我们将查看一些示例，如何防止从 LSA 类型 7 到 5 的翻译。我将使用以下拓扑：

R1 is in