Unnumbered Ethernet VLAN interfaces
From CT3
IOS releases 12.3T, 12.4, 12.2SXE, 12.2SB and 12.2SR support unnumbered Ethernet VLAN interfaces. This feature can help you save IP address space as you’re replacing point-to-point WAN links between the routers with Carrier Ethernet links between layer-3 switches (traditionally you have to spend a /30 subnet on each point-to-point layer-3 VLAN link).
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Configuration
The unnumbered Ethernet interface configuration is simple: you’re using the well-known ip unnumbered interface configuration command which can be applied on a VLAN subinterface like the following configuration example:
Sample unnumbered VLAN interface configuration
interface Loopback0 ip address 10.2.0.1 255.255.255.255 ip router isis ! interface FastEthernet0/1 no ip address ! interface FastEthernet0/1.1 encapsulation dot1Q 100 ip unnumbered Loopback0
Limitations
The feature has several limitations:
- You can use the ip unnumbered command only on Ethernet subinterfaces, not on the main interface. To use it on a non-VLAN link, you have to create a subinterface with dot1q native encapsulation:
Unnumbered non-VLAN interface
interface FastEthernet0/1 no ip address duplex auto speed auto ! interface FastEthernet0/1.1 encapsulation dot1Q 1 native ip unnumbered Loopback0
- OSPF cannot be run as the routing protocol over the unnumbered Ethernet interfaces, it does not recognize the “unnumbered” interface status, as illustrated in the following example:
OSPF refuses to start on an unnumbered VLAN interface
R1#conf t Enter configuration commands, one per line. End with CNTL/Z. R1(config)#interface fa0/1.1 R1(config-subif)#ip ospf 1 area 0 R1(config-subif)# *Jun 21 10:42:34.189: %OSPF-4-NO_IPADDRESS_ON_INT: No IP address for interface FastEthernet0/1.1 R1(config-subif)#
All other interior routing protocols (EIGRP, RIP and IS-IS) can be used over unnumbered VLAN interfaces, the following printout contains a sample IS-IS configuration:
Running IS-IS over an unnumbered VLAN interface
interface Loopback0 ip address 10.2.0.1 255.255.255.255 ip router isis ! interface FastEthernet0/1 duplex auto speed auto ! interface FastEthernet0/1.1 encapsulation dot1Q 1 native ip unnumbered Loopback0 ip router isis isis network point-to-point ! router isis net 49.0001.0000.0000.0001.00
References
Sample configurations
These printouts contain the configurations of routers used in the test. The routers were running IOS release 12.4(22)T.
Configuration of R1
upgrade fpd auto version 12.4 service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption ! hostname R1 ! boot-start-marker boot-end-marker ! logging message-counter syslog logging buffered 4096 ! no aaa new-model ip source-route ip cef ! no ip domain lookup no ipv6 cef ! multilink bundle-name authenticated ! archive log config hidekeys ! interface Loopback0 ip address 10.2.0.1 255.255.255.255 ip router isis ! interface FastEthernet0/0 ip address 10.2.1.1 255.255.255.0 duplex half speed auto ! interface FastEthernet0/1 no ip address duplex auto speed auto ! interface FastEthernet0/1.1 encapsulation dot1Q 1 native ip unnumbered Loopback0 ip router isis isis network point-to-point isis csnp-interval 10 ! router isis net 49.0001.0000.0000.0001.00 ! ip forward-protocol nd ip http server no ip http secure-server ! logging alarm informational ! control-plane ! gatekeeper shutdown ! line con 0 exec-timeout 0 0 privilege level 15 logging synchronous transport preferred none stopbits 1 line aux 0 stopbits 1 line vty 0 4 exec-timeout 0 0 privilege level 15 logging synchronous no login transport preferred none ! ntp logging end
Configuration of R2
upgrade fpd auto version 12.4 service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption ! hostname R2 ! boot-start-marker boot-end-marker ! logging message-counter syslog logging buffered 4096 ! no aaa new-model ip source-route ip cef ! no ip domain lookup no ipv6 cef ! multilink bundle-name authenticated ! archive log config hidekeys ! interface Loopback0 ip address 10.2.0.2 255.255.255.255 ip router isis ! ! interface FastEthernet0/0 ip address 10.2.2.1 255.255.255.0 duplex half speed auto ! interface FastEthernet0/1 no ip address duplex auto speed auto ! interface FastEthernet0/1.1 encapsulation dot1Q 1 native ip unnumbered Loopback0 ip router isis isis network point-to-point isis csnp-interval 10 ! router isis net 49.0001.0000.0000.0002.00 ! ip forward-protocol nd ip http server no ip http secure-server ! logging alarm informational ! control-plane ! gatekeeper shutdown ! ! line con 0 exec-timeout 0 0 privilege level 15 logging synchronous transport preferred none stopbits 1 line aux 0 stopbits 1 line vty 0 4 exec-timeout 0 0 privilege level 15 logging synchronous no login transport preferred none ! ntp logging end