An unnumbered interface will use (or borrow) the address of the specified interface as the source address whenever it generates a packet. The address of the specified interface is also used to determine the routing processes that are sending routing updates over the unnumbered interface.
Ex: An RIP router that is configured when the network 10.0.0.0 router subcommand indicates that all physical interfaces with an address in network 10.0.0.0 and all unnumbered interfaces that specify an interface that has an address in network 10.0.0.0 will participate in the RIP.
Below list the restrictions of unnumbered interfaces:
i) Serial interfaces with HDLC, PPP, LAPB, Frame Relay, SLIP, and tunnel interfaces ca be unnumbered. X.25 and SMDS interfaces cannot be unnumbered.
ii) The ping command cannot be used to determine whether an unnumbered interface is up, as the interface has no address. SNMP can be used to monitor the status of the interface.
iii) IP security features are not supported on unnumbered interfaces.
iv) IP unnumbered is supported on point-to-point interfaces only. The "Point-to-point (non-multi-access) interfaces only" error message will be received when configuring this feature on a multi-access (eg: Ethernet) or loopback interface.
Note: The following information is inaccurate and has been proven using real network setups!
Routers normally insert the source address of a routing update as the next hop in the routing table, and the next hop is often an address on a directly connected network. However, this is no longer the case when the IP unnumbered interfaces of a serial link are associated with interfaces reside in different subnets or major networks. Routes learned through an IP unnumbered interface have the interface as the next hop instead of the source address of the routing update. The purpose is to avoid the invalid next hop address problem in which a routing update sourced from a next hop of a non-directly connected network is received.
Ex: RT1 and RT2 are connected with an unnumbered serial link. RT1 has all its interfaces and unnumbered interface in network 172.16.0.0, while RT2 has all its interfaces and unnumbered interface in network 172.17.0.0. If OSPF is configured to run on the unnumbered serial link, it must be configured to summarize the subnet information as OSPF won’t send it across the link.
Below shows the steps for configuring an unnumbered serial interface:
Router(config)#int lo0 Router(config-if)#ip add 10.10.10.1 255.255.255.0 Router(config-if)#int s1/0 Router(config-if)#ip unnumbered lo0 Router(config-if)#^Z Router#sh ip int brief Interface IP-Address OK? Method Status Protocol Loopback0 10.10.10.1 YES manual up up Serial1/0 10.10.10.1 YES NVRAM up up Router#
Same Major Network, Different Subnets IP Unnumbered Network
Below shows the routing tables of RT1 and RT2 for the configuration above:
RT1#sh ip route 172.16.0.0/24 is subnetted, 3 subnets R 172.16.0.0 [120/1] via 172.16.2.1, 00:00:10, Serial1/0 C 172.16.1.0 is directly connected, FastEthernet0/0 R 172.16.2.0 [120/1] via 172.16.2.1, 00:00:10, Serial1/0 RT1# RT1#ping 172.16.2.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.16.2.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 16/47/92 ms RT1#ping 172.16.2.0 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.16.2.0, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 12/36/64 ms ---------------------------------------------------------------------- RT2#sh ip route 172.16.0.0/24 is subnetted, 3 subnets R 172.16.0.0 [120/1] via 172.16.1.1, 00:00:15, Serial1/0 R 172.16.1.0 [120/1] via 172.16.1.1, 00:00:15, Serial1/0 C 172.16.2.0 is directly connected, FastEthernet0/0 RT2# RT2#ping 172.16.1.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.16.1.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 16/33/88 ms RT2#ping 172.16.1.0 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.16.1.0, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 8/26/44 ms
Different Major Networks, No Subnet IP Unnumbered Network
Below shows the routing tables of RT1 and RT2 for the configuration above:
RT1#sh ip route R 172.17.0.0/16 [120/1] via 172.17.1.1, 00:00:11, Serial1/0 C 172.16.0.0/16 is directly connected, FastEthernet0/0 RT1# RT1#ping 172.17.1.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.17.1.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 16/28/48 ms RT1#ping 172.17.1.0 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.17.1.0, timeout is 2 seconds: ..... Success rate is 0 percent (0/5) RT1# ---------------------------------------------------------------------- RT2#sh ip route C 172.17.0.0/16 is directly connected, FastEthernet0/0 R 172.16.0.0/16 [120/1] via 172.16.1.1, 00:00:11, Serial1/0 RT2# RT2#ping 172.16.1.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.16.1.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 8/41/88 ms RT2#ping 172.16.1.0 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.16.1.0, timeout is 2 seconds: ..... Success rate is 0 percent (0/5) RT2#
Different Major Networks, With and Without Subnets IP Unnumbered Network
Below shows the routing tables of RT1 and RT2 for the configuration above:
RT1#sh ip route R 172.17.0.0/16 [120/1] via 172.17.1.1, 00:00:01, Serial1/0 172.16.0.0/24 is subnetted, 1 subnets C 172.16.1.0 is directly connected, FastEthernet0/0 RT1# RT1#ping 172.17.1.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.17.1.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 16/41/68 ms RT1#ping 172.17.1.0 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.17.1.0, timeout is 2 seconds: ..... Success rate is 0 percent (0/5) ---------------------------------------------------------------------- RT2#sh ip route C 172.17.0.0/16 is directly connected, FastEthernet0/0 172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks R 172.16.0.0/16 [120/1] via 172.16.1.1, 00:00:17, Serial1/0 R 172.16.1.0/32 [120/1] via 172.16.1.1, 00:00:17, Serial1/0 RT2# RT2#ping 172.16.1.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.16.1.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 12/41/64 ms RT2#ping 172.16.1.0 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.16.1.0, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 28/30/36 ms
Different Major Networks, Different Subnets IP Unnumbered Network
Below shows the routing tables of RT1 and RT2 for the configuration above:
RT1#sh ip route 172.17.0.0/16 is variably subnetted, 2 subnets, 2 masks R 172.17.1.0/32 [120/1] via 172.17.1.1, 00:00:06, Serial1/0 R 172.17.0.0/16 [120/1] via 172.17.1.1, 00:00:06, Serial1/0 172.16.0.0/24 is subnetted, 1 subnets C 172.16.1.0 is directly connected, FastEthernet0/0 RT1#ping 172.17.1.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.17.1.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 4/45/104 ms RT1#ping 172.17.1.0 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.17.1.0, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 16/36/60 ms ---------------------------------------------------------------------- RT2#sh ip route 172.17.0.0/24 is subnetted, 1 subnets C 172.17.1.0 is directly connected, FastEthernet0/0 172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks R 172.16.0.0/16 [120/1] via 172.16.1.1, 00:00:03, Serial1/0 R 172.16.1.0/32 [120/1] via 172.16.1.1, 00:00:03, Serial1/0 RT2#ping 172.16.1.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.16.1.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 12/35/48 ms RT2#ping 172.16.1.0 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.16.1.0, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 8/42/64 ms
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