OSPFv2中，提到点到点链路可以是unnumbered，不编号，不分配IP地址

12.4.1.1.  Describing point-to-point interfaces
                For point-to-point interfaces, one or more link
                descriptions are added to the router-LSA as follows:
                o   If the neighboring router is fully adjacent, add a
                    Type 1 link (point-to-point). The Link ID should be
                    set to the Router ID of the neighboring router. For
                    numbered point-to-point networks, the Link Data
                    should specify the IP interface address. For
                    unnumbered point-to-point networks, the Link Data
                    field should specify the interface‘s MIB-II [Ref8]
                    ifIndex value. The cost should be set to the output
                    cost of the point-to-point interface.
                o   In addition, as long as the state of the interface
                    is "Point-to-Point" (and regardless of the
                    neighboring router state), a Type 3 link (stub
                    network) should be added. There are two forms that
                    this stub link can take:

 某博文中写道，连接相邻交换机的两个接口的链路可以不分配IP子网，也就是连接两个路由器的两个接口也可以不分配IP地址。

 Can we get around the requirement of configuring IPv4 subnets on links that connect two routed interfaces on adjacent switches? Of course – we’ve been using unnumbered interfaces on point-to-point links for ages. It’s just that the routing protocol programmers haven’t realized the days of thick coax cable are gone; in this century most people use Ethernet on point-to-point links. There’s even a 6-year-old informational RFC describing this idea .

 该博文还写道，在OSPFv2中，将两个路由器之间的链路配置成不编号接口可以使其变为纯粹的拓扑元素，从而简化网络配置，路由信息库近包括真实网络，存储空间和SPF也可以更快。此外，这样还可以节省IP地址。

And now for the claims

The Cumulus documentation claims:

In OSPFv2, configuring unnumbered interfaces reduces the links between routers into pure topological elements, and thus dramatically simplifies network configuration and reconfiguration. In addition, routing database contains only the real networks, hence memory footprint is reduced and SPF is faster.

Let’s walk through all of these claims:

Configuring unnumbered interfaces reduces the links between routers into pure topological elements

Translated into engineering terms: the Type-1 (router) LSA no longer contains the stub networks for inter-router subnets. You can do something similar on Cisco IOS with OSPF prefix suppression .

… and thus dramatically simplifies network configuration and reconfiguration.

Marketese for “ we don’t check IP subnets in OSPF hello packets ”.

In addition, routing database contains only the real networks,

I don’t know what they call the routing database . OSPF database contains exactly the same number of LSAs, the routing table does contain smaller number of routes (but see also prefix suppression).

… hence memory footprint is reduced and SPF is faster.

Memory footprint is reduced. SPF speedup is probably measured in per mils – after all, the router considers the stub networks attached to Type-1 router LSAs only in the second (distance vector) part of the SPF algorithm, which has linear complexity.

IP unnumbered interface，某个接口不编号，某个接口不分配IP地址