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:
Only physical interfaces can be configured as unnumbered interfaces. Unnumbered interfaces are not supported for virtual Ethenet (VE) or switched virtual interface (SVI) interfaces.
The donor interface can be any other Layer 3 interface (Ethernet/VE/loopback).
Unnumbered interface support is provided for IPv4 address family. Consequently, only an IPv4 address can be borrowed for an unnumbered interface.
Once the interface is configured as an unnumbered interface, it is treated as a point-to-point interface and there can be only one remote neighbor attached to the interface.
Because no network subnet is associated with the unnumbered interface, ARP is not supported on an unnumbered interface.
Unnumbered interfaces are not supported in the management VRF, because routing protocols (OSPF/BGP) are not enabled in the management VRF.
The following diagram illustrates the overall concept of unnumbered interfaces and donor interfaces.
IP unnumbered and donor interfaces
Interfaces Ethernet 0/1 and Ethernet 0/2 on R1 and R2 are unnumbered interfaces.
Interfaces Ethernet 0/1 on R1 and R2 borrow IPv4 addresses from a loopback interface.
Interfaces Ethernet 0/2 on R1 and R2 borrow IPv4 addresses from physical interface Ethernet 0/3.
The end points of the unnumbered interface may or may not be in the same subnet.
The following functionalities are required to support Layer 3 over unnumbered interfaces:
Neighbor discovery
Host routes to reach neighbors over unnumbered interfaces
Routes using unnumbered interfaces as next-hops
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.
