注：机翻，未校对。一个小众的功能。
OSPF over a Demand Circuit
What is a Demand Circuit? 什么是需求电路？
OSPF can be tuned to better handle demand circuits. But what are demand circuits? Are they still relevant today?
可以调整 OSPF 以更好地处理需求电路。但什么是需求电路？它们在今天仍然适用吗？
This is a type a circuit that’s not always up. One that dials on demand. Think of old ISDN, x.25, and dial-up lines for example.
这是一个 A 型电路，并不总是启动。一种按需拨号的产品。例如，想想旧的 ISDN、x.25 和拨号线路。
In the old days, these may have been expensive, and were only meant to be up when needed. Also, the amount of traffic that was sent over them also needed to be kept to a minumum.
在过去，这些可能很昂贵，并且只打算在需要时使用。此外，通过它们发送的流量也需要保持在最低限度。
OSPF caused problems with demand circuits. The regular hello messages cause the link to stay up. Additionally, the regular LSA refresh (every 30 minutes) caused the circuit to be used more heavily than desired.
OSPF 导致需求电路出现问题。定期的你好消息会导致链接保持运行。此外，定期刷新 LSA（每 30 分钟一次）导致电路的使用比预期的更为频繁。
OSPF With Demand Circuits 带需求电路的 OSPF
Back in 1995 (way back when I started high school!) RFC1793 was released. This included support for OSPF to be aware of demand circuits.
早在 1995 年（早在我开始上高中的时候！RFC1793 被发布。这包括支持 OSPF 了解需求电路。
This feature is enabled per-interface (so it can be enabled selectively per neighbour). When two neighbours start an adjacency, and will negotiate the DC bit. If they both have this bit, then this feature is enabled. If successful, they then set the DNA (DoNotAge) bit in the LSA field.
此功能是按接口启用的（因此可以按邻居选择性地启用此功能）。当两个邻居开始邻接时，将协商直流位。如果它们都有此位，则启用此功能。如果成功，他们将在 LSA 字段中设置 DNA （DoNotAge） 位。
This provides two major changes. Firstly, beyond the initial formation of the neighbour adjacency, hellos are suppressed. They’re not sent at all. This prevents OSPF from keeping a circuit up unnecessarily.
这提供了两个主要变化。首先，除了邻接关系的最初形成之外，hello 被抑制了。他们根本没有被发送。这样可以防止 OSPF 不必要地保持电路正常运行。
This does have a side effect. One neighbour won’t know (from an OSPF standpoint) when its neighbour goes down. The adjacency will remain up, even if the underlying circuit is down.
这确实有副作用。一个邻居不会知道（从OSPF的角度来看）它的邻居何时 down 。即使底层电路关闭，邻接关系也将保持 on 。
Note: Hello suppression applies to point-to-point and point-to-multipoint network types only. Hellos are not suppressed on other network types.
注意：Hello 抑制仅适用于点对点和点对多点网络类型。在其他网络类型上不会禁止显示 hello。
The second change is that the periodic LSA refresh is also suppressed. This is because of the DNA bit I mentioned earlier. This means that the LSDB entries for this neighbour will not age out.
第二个变化是，定期 LSA 刷新也被抑制。这是因为我之前提到的 DNA 位。这意味着此邻居的 LSDB 条目不会过期。
There will still be LSA refreshes, but now they will occur on demand, when there are changes in the network.
仍会进行 LSA 刷新，但现在，当网络发生变化时，它们将按需发生。
Is it Still Relevant Today? 它今天仍然适用吗？
We generally don’t use circuits like dial-up, and x.25, and ISDN is nearly extinct too. So, is this important at all?
我们通常不使用拨号和 x.25 等电路，ISDN 也几乎灭绝了。那么，这重要吗？
Well, there are still some technologies that are on demand. Think of site-to-site VPNs for example. In particular, think of technologies like Cisco’s DMVPN (phase 2 or 3), or Juniper’s Auto Discovery VPN. These dynamically build tunnels between spoke routers on demand.
好吧，仍然有一些技术是按需的。例如，以站点到站点 VPN 为例。特别是，想想像思科的 DMVPN（第 2 阶段或第 3 阶段）或瞻博网络的自动发现 VPN 这样的技术。它们按需在分支路由器之间动态构建隧道。
In cases like this, you may decide to this feature in OSPF. That is, of course, if the network type is point-to-point or point-to-multipoint (in an NBMA or broadcast network, OSPF will still send hellos).
在这种情况下，您可以决定在 OSPF 中使用此功能。也就是说，当然，如果网络类型是点对点或点对多点（在NBMA或广播网络中，OSPF仍将发送hellos）。
Configuration 配置
Cisco
interface gi0/0
ip ospf demand-circuit
Juniper
set protocols ospf area 0 interface ge-0/0/0 demand-circuit
protocols {ospf {area 0.0.0.0 {interface ge-0/0/0 {demand-circuit;}}}
}
References 引用
Cisco – OSPF Demand Circuit Feature
Cisco – OSPF 需求电路功能
Juniper – Configuring OSPF Interfaces (Example: Configuring OSPF Demand Circuits)
Juniper – 配置 OSPF 接口（示例：配置 OSPF 需求电路）
IETF – RFC 1793: Extending OSPF to Support Demand Circuits
IETF – RFC 1793：扩展 OSPF 以支持需求电路
via：
-
OSPF over a Demand Circuit - Network Direction
https://networkdirection.net/articles/routingandswitching/ospf-over-a-demand-circuit/
- OSPF DC按需链路技术剖析_按需链路ospf-CSDN博客
https://blog.csdn.net/jjyy201301/article/details/89343752
