OSPFv2中，提到點到點鏈路可以是unnumbered，不編號，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:

某博文中寫道，連接相鄰交換機的兩個接口的鏈路可以不分配IP子網，也就是連接兩個路由器的兩個接口也可以不分配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 .

該博文還寫道，在OSPFv2中，將兩個路由器之間的鏈路配置成不編號接口可以使其變為純粹的拓撲元素，從而簡化網絡配置，路由信息庫近包括真實網絡，存儲空間和SPF也可以更快。此外，這樣還可以節省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，某個接口不編號，某個接口不分配IP地址