IP unnumbered interface

In a typical Layer 3 Clos network, routers are directly connected and require the assignment of IP addresses to each pair of routers, typically from the same subnet.

With large numbers of routers and redundant Layer 3 interfaces, a significant number of IP addresses are consumed just to configure the network itself. Using /31 masks reduces the consumption of addresses, but two IP addresses are still consumed per interface. Using unnumbered interfaces greatly reduces the number of IP addresses consumed in the configuration of the network.

This feature borrows an IP address from another Layer 3 interface already configured on the router. This address is used as a source address in the Layer 3 packets that are sent out the unnumbered interface. The interface from which the IP address is borrowed is called the donor interface. The following points highlight some of the key aspects of the use and functionality of this feature:
The following diagram illustrates the overall concept of unnumbered interfaces and donor interfaces.
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IP unnumbered and donor interfaces
The following functionalities are required to support Layer 3 over unnumbered interfaces:
Note

Note

For a simple three-stage IP Fabric, IP unnumbered interfaces can be used. For a five-stage IP Fabric, numbered interfaces are highly recommended, although IP unnumbered interfaces can be used in five-stage IP Fabric deployments if third-party devices are not included in the design. Extreme does not recommend using a mix of unnumbered and numbered interfaces within an IP Fabric.