Administrator Guide

Layer 3 Routing Commands 1565
User Guidelines
IP unnumbered interfaces are supported in the default VRF only.
The interface should be configured as able to borrow an IP address, i.e. a
routing interface with no IP address.
The loopback interface is the numbered interface providing the borrowed
address. The providing loopback interface cannot be unnumbered. The
loopback interface is identified by its loopback interface number.
It is a misconfiguration for two routers, R1 and R2, to be connected by a link
where R1’s interface is unnumbered and R2’s interface is numbered. If a
static route is configured on R2 using R1’s IP address as next hop, the static
route will never be installed in the routing table because the next hop is not
in a local subnet. If a static route is configured on R1 using R2’s IP address as
next hop, the static route will be installed in the routing table. R1 will ARP for
the next hop address. R2 will ignore the ARP Request because the source IP
address is not in a local subnet.
It is a misconfiguration to enable OSPF on both ends of an unnumbered
interface without setting the OSPF network type to point-to-point. Each
router will reject its neighbor’s HELLOs because the source IP address is not
in a local subnet. Adjacencies never progress beyond the INIT state.
If three or more routers are connected to the same physical Ethernet and all
are configured to treat the Ethernet as a point-to-point link, adjacencies may
not form. The OSPF database description packets intended for a specific
neighbor will be processed by all neighbors, causing errors that reset
adjacencies.
Command History
Introduced in version 6.2.0.1 firmware.
Example
console(config-if-vlan1)#ip unnumbered 10.130.14.55
ip unnumbered gratuitous-arp accept
This command enables installation of a static interface route to the
unnumbered peer upon receiving a gratuitous ARP.
2CSNXXX_SWUM204.book Page 1565 Monday, January 25, 2016 1:25 PM