注：机翻，未校对。作者原文标题与内容有点不符，但关于 LSU 部分可以看看。

OSPF LSA (Link State Advertisement)

OSPF LSA（链路状态通告）

Each OSPF LSA has its own aging timer that its link state age field carries.
每个 OSPF LSA 都有自己的老化计时器，其链路状态年龄字段带有该计时器。

Default of 30 minutes expressed in seconds i.e. 1800 seconds is the Link State Age. What this means is that router that originated the LSA resends the LSA now with a higher sequence number in a Link State update to verify the link is still active.
默认值为 30 分钟，以秒为单位，即 1800 秒是链路状态期限。这意味着，发起 LSA 的路由器现在在链路状态更新中以更高的序列号重新发送 LSA，以验证链路是否仍处于活动状态。

LSA also has a max age timer field with default value of 60 minutes. If a LSA isn’t before this period the Max age timer will expire and the LSA would be discarded in that case.
LSA 还有一个最大年龄计时器字段，其默认值为 60 分钟。如果 LSA 未在此期限之前，则“最长期限”计时器将过期，在这种情况下，LSA 将被丢弃。

A link state update can contain more than one LSA. LSA validation method saves on bandwidth compared to distance vector method which sends the entire routing table at short periodic intervals.
链接状态更新可以包含多个 LSA。与距离向量方法相比，LSA验证方法节省了带宽，距离向量方法以短周期间隔发送整个路由表。

Let us understand what a router does when it receives an LSU (Link State Update) using the below flowchart:
让我们使用下面的流程图了解路由器在接收 LSU（链路状态更新）时会做什么：

• If the LSA entry doesn’t exist the router adds the entry to LSDB, acknowledges the receipt of LSA with LSAck, floods the new information to other routers, runs SPF and updates its routing table.
  如果 LSA 条目不存在，路由器将该条目添加到 LSDB，使用 LSAck 确认收到 LSA，将新信息泛洪到其他路由器，运行 SPF 并更新其路由表。

• If the entry already exists with the same sequence number the router must ignore the LSA.
  如果已存在具有相同序列号的条目，则路由器必须忽略 LSA。

• If the LSA already exists but contains newer information i.e. it has higher sequence number the router adds the entry to LSDB, acknowledges the receipt of LSA with LSAck, floods the new information to other routers, runs SPF and updates its routing table.
  如果 LSA 已存在但包含较新的信息，即它具有更高的序列号，则路由器将条目添加到 LSDB，使用 LSAck 确认接收 LSA，将新信息泛洪到其他路由器，运行 SPF 并更新其路由表。

• If the entry already exists but the LSA includes older information it sends an LSU to sender with its newer information.
  如果条目已存在，但 LSA 包含较旧的信息，则它会将 LSU 与其较新的信息一起发送给发件人。

via：

• Understanding OSPF LSA Data-Structure - IP With Ease Rashmi Bhardwaj

  https://ipwithease.com/understanding-ospf-lsa-data-structure/