This document will explain the concept of IP unnumbered, and will give you several configuration examples for reference. The ip unnumbered configuration command allows you to enable IP processing on a serial interface without assigning it an explicit IP address. This is a good way to conserve network and address space.
In a Cisco router, every IP network interface not residing on a common "wire" must belong to a unique subnet. This is the way IP works. Based on network information contained in an IP routing table, any uniquely identified IP network not directly attached to the router can be reached by forwarding a packet to a "next hop" address on a network that is directly attached. When the final destination network is on an interface directly attached to the destination, the packet can be simply delivered to the end host.
The IP routing table is made up of routes. The routes are either "subnet" routes or "major network" routes. Each route has one or more feasible "next hop" addresses which are directly attached network addresses. Subnet routes are only passed around within the subnetted major network. At each end of the major net, the subnets are aggregated into a single major network route and advertised to other major networks. This helps keeps the size of the routing tables small.
Note: The aggregation scheme discussed above assumes a traditional distance-vector routing protocol like RIP or IGRP.
So let's consider a class B network subnetted with eight bits. Every interface in the network including the serial lines will require a subnet. Since each serial line has only two nodes, this wastes 252 addresses on each serial line. Here's where IP unnumbered comes in handy. For any point-to-point serial link or point-to-point sub- interface, IP unnumbered lets you borrow the address of some LAN interface to use as a source address for routing updates and packets from that interface. No network is wasted, and precious address space is conserved.
So how is the routing table updated? Normally, a router receiving an update will use the source address of the routing update as the next hop since we are normally guaranteed to be directly connected to networks sending the update (using the 1 interface, 1 network model).
However, since the remote serial line borrowed the address from a remote LAN, we are almost guaranteed that the source address will not be directly connected to the router receiving the update. So, instead of simply entering a next hop address based on the source address of the update, routes learned on the IP unnumbered interface name are entered into the routing table as interface routes, which means that the invalid next hop address is bypassed in favor of the interface name from which we received the update. Because of this, IP unnumbered only makes sense for point-to-point links.
On either side of the serial link we have the same major net with different subnets.
| ----- | |
| | |171.68.178.193 | ---------- ---------- |
|---| |---------------| M/S1| |S4/3 | |M/S2 |
| ----- | | 7000 |----- | 2500 | |
| |------| | / S0| |-------|
| | E3/0| 10.2(1)| -----| 9.14(4)|E0 |
| | | | | | |
| | ---------- ---------- |
| | |
171.68.178.196 171.68.179.1
255.255.255.192 255.255.255.192
M/S1 -- Major net M subnet 1. M/S2 -- Major net M subnet 2.
7000 Configuration:
interface Ethernet3/0 ip address 171.68.178.196 255.255.255.192
interface Serial4/3 ip unnumbered Ethernet3/0
router igrp 10 network 171.68.0.0
2500 Configuration:
interface Ethernet 0 ip address 171.68.179.1 255.255.255.192
interface Serial 0 ip unnumbered Ethernet0
router igrp 10 network 171.68.0.0
7000 Results:
FRCS7-2# sh ip route 171.68.0.0
Routing entry for 171.68.0.0 (mask 255.255.255.192), 50 known subnets
Attached (1 connections)
Redistributing via igrp 10
I 171.68.174.64 [100/1310] via 171.68.178.193, 00:01:02, Ethernet3/0
I 171.68.170.64 [100/1310] via 171.68.178.193, 00:01:02, Ethernet3/0
I 171.68.231.0 [100/1330] via 171.68.178.193, 00:01:02, Ethernet3/0
I 171.68.179.0 [100/1310] via 171.68.179.1, 00:01:02, Serial4/3
I 171.68.230.0 [100/1330] via 171.68.178.193, 00:01:02, Ethernet3/0
.....
FRCS7-2#ping 171.68.179.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 171.68.179.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/3/4 ms
2500 Results:
S251-1#sh ip route 171.68.0.0
Routing entry for 171.68.0.0 (mask 255.255.255.192), 50 known subnets Attached (1 connections) Known via "connected", distance 0, metric 0 (connected) Tag 0 Redistributing via igrp 10 ..... I 171.68.174.64 [100/8786] via 171.68.178.196, 0:00:29, Serial0 I 171.68.170.64 [100/8786] via 171.68.178.196, 0:00:29, Serial0 I 171.68.231.0 [100/8806] via 171.68.178.196, 0:00:29, Serial0 I 171.68.230.0 [100/8806] via 171.68.178.196, 0:00:29, Serial0 I 171.68.178.192 [100/8806] via 171.68.178.196, 0:00:29, Serial0 ..... FRCS251-1#ping 171.68.178.196 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 171.68.178.196, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 1/3/4 ms
Conclusion: The route information about subnets is maintained in this scenario.
On either side of the serial link we have different major nets and no subnets.
| |
| ---------- ---------- |
| M/NS| |S4/3 | |K/NS |
| | 7000 |----- | 2500 | |
|------| | / S0| |-------|
| E3/0| 10.2(1)| -----| 9.14(4)|E0 |
| | | | | |
| ---------- ---------- |
| |
171.68.178.196 172.68.1.1
255.255.0.0 255.255.0.0
M/NS -- Major net M no subnets K/NS -- Major net K no subnets.
7000 Configuration:
interface Ethernet3/0
ip address 171.68.178.196 255.255.0.0
interface Serial4/3 ip unnumbered Ethernet3/0
router igrp 10 network 171.68.0.0
2500 Configuration:
interface Ethernet 0 ip address 172.68.1.1 255.255.0.0
interface Serial 0 ip unnumbered Ethernet0
router igrp 10 network 172.68.0.0
7000 Results:
FRCS7-2#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate
default
Gateway of last resort is not set
I 172.68.0.0 [100/8576] via 172.68.1.1, 00:00:02, Serial4/3 C 171.68.0.0 is directly connected, Ethernet3/0
FRCS7-2#ping 172.68.1.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.68.1.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 1/3/4 ms
2500 Results:
FRCS251-1#sh ip route
Codes: I - IGRP derived, R - RIP derived, H - HELLO derived, O - OSPF derived C - connected, S - static, E - EGP derived, B - BGP derived i - IS-IS derived * - candidate default route, IA - OSPF inter area route E1 - OSPF external type 1 route, E2 - OSPF external type 2 route L1 - IS-IS level-1 route, L2 - IS-IS level-2 route
Gateway of last resort is not set
C 172.68.0.0 is directly connected, Ethernet0 I 171.68.0.0 [100/8576] via 171.68.178.196, 0:00:01, Serial0
FRCS251-1#ping 171.68.178.196 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 171.68.178.196, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 1/3/4 ms
Conclusion: The major net information is maintained across the unnumbered point-to- point link.
On one side of the serial link we have a major net with a subnet, and on the other side a major net with no subnet.
| |
| ---------- ---------- |
| M/S1| |S4/3 | |K/NS |
| | 7000 |----- | 2500 | |
|------| | / S0| |-------|
| E3/0| 10.2(1)| -----| 9.14(4)|E0 |
| | | | | |
| ---------- ---------- |
| |
171.68.178.196 172.68.1.1
255.255.255.192 255.255.0.0
M/S1 -- Major net M subnet 1. K/NS -- Major net K with no subnet.
7000 Configuration:
interface Ethernet3/0 ip address 171.68.178.196 255.255.255.192
interface Serial4/3 ip unnumbered Ethernet3/0
router igrp 10 network 171.68.0.0
2500 Configuration:
interface Ethernet 0 ip address 172.68.1.1 255.255.0.0
interface Serial 0 ip unnumbered Ethernet0
router igrp 10 network 172.68.0.0
7000 Results:
FRCS7-2# sh ip route 172.68.0.0
Routing entry for 172.68.0.0 (mask 255.255.0.0)
Known via "igrp 10", distance 100, metric 8576
Redistributing via igrp 10
Advertised by igrp 10
Last update from 172.68.1.1 on Serial4/3, 00:00:58 ago
Routing Descriptor Blocks:
* 172.68.1.1, from 172.68.1.1, 00:00:58 ago, via Serial4/3
Route metric is 8576, traffic share count is 1
Total delay is 21000 microseconds, minimum bandwidth is 1544 Kbit
Reliability 128/255, minimum MTU 1500 bytes
Loading 1/255, Hops 0
S7-2#ping 172.68.1.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.68.1.1, timeout is 2 seconds: ..... Success rate is 0 percent (0/5)
2500 Results:
FRCS251-1#sh ip route 171.68.0.0
Routing entry for 171.68.0.0 (mask 255.255.255.255), 49 known subnets Known via "igrp 10", distance 100, metric 8786 Tag 0 Redistributing via igrp 10 Last update from 171.68.178.196 on Serial0, 00:00:36 seconds ago
I 171.68.174.64 [100/8786] via 171.68.178.196, 0:00:36, Serial0 I 171.68.170.64 [100/8786] via 171.68.178.196, 0:00:36, Serial0 I 171.68.231.0 [100/8806] via 171.68.178.196, 0:00:36, Serial0 I 171.68.230.0 [100/8806] via 171.68.178.196, 0:00:36, Serial0 I 171.68.187.64 [100/8696] via 171.68.178.196, 0:00:36, Serial0 I 171.68.159.64 [100/9696] via 171.68.178.196, 0:00:36, Serial0 I 171.68.158.64 [100/9696] via 171.68.178.196, 0:00:36, Serial0
As we see here, the 7000 has sent updates containing its subnets but the 2500 does not expect to see subnets in the updates it receives. As a result, the 2500 marks the updates as being learned with a subnet mask of 255.255.255.255, which makes the subnets look like host routes. Since there is no route for 171.68.178.196, the following ping won't work:
S251-1#ping 171.68.178.196 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 171.68.178.196, timeout is 2 seconds: ..... Success rate is 0 percent (0/5)
Solution: The 7000 knows how to reach the network 172.68.0.0, but the 2500 has lost that information by treating the subnet updates as host routes. A static route in the 2500 like ip route 171.68.0.0 s 0 will do the trick.
FRCS251-1#sh ip route 171.68.0.0
Routing entry for 171.68.0.0 (mask 255.255.0.0), 50 known subnets Attached (1 connections) Variably subnetted with 2 masks Known via "igrp 10", distance 100, metric 8786 (connected) Tag 0 Redistributing via igrp 10 Last update from 171.68.178.196 on Serial0, 0:00:26 seconds ago
S 171.68.0.0 is directly connected, Serial0 I 171.68.174.64 255.255.255.255 [100/8786] via 171.68.178.196, 0:00:26, Serial0 I 171.68.170.64 255.255.255.255 [100/8786] via 171.68.178.196, 0:00:26, Serial0
FRCS251-1#ping 171.68.178.196 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 171.68.178.196, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
Conclusion: The subnet information gets lost since it is treated as a host route. Add the static route to fix this.
On both sides of the serial link we have two different major nets with respective subnets.
| |
| ---------- ---------- |
| M/S1| |S4/3 | |K/S1 |
| | 7000 |----- | 2500 | |
|------| | / S0| |-------|
| E3/0| 10.2(1)| -----| 9.14(4)|E0 |
| | | | | |
| ---------- ---------- |
| |
171.68.178.196 172.68.1.1
255.255.255.192 255.255.255.192
M/S1 -- Major net M subnet 1. K/S1 -- Major net K subnet 1.
7000 Configuration:
interface Ethernet3/0 ip address 171.68.178.196 255.255.255.192
interface Serial4/3 ip unnumbered Ethernet3/0
router igrp 10 network 171.68.0.0
2500 Configuration:
interface Ethernet 0 ip address 172.68.1.1 255.255.255.192
interface Serial 0 ip unnumbered Ethernet0
router igrp 10 network 172.68.0.0
7000 Results:
FRCS7-2#sh ip route 172.68.0.0
Routing entry for 172.68.0.0 (mask 255.255.255.255),
Redistributing via igrp 10
Advertised by igrp 10
I 172.68.254.0 255.255.255.255 [100/8576] via 172.68.1.1, 00:01:06, Serial4/3 I 172.68.1.0 255.255.255.255 [100/8576] via 172.68.1.1, 00:01:06, Serial4/3
FRCS7-2#ping 172.68.1.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.68.1.1, timeout is 2 seconds: ..... Success rate is 0 percent (0/5)
2500 Results:
FRCS251-1#sh ip route 171.68.0.0
Routing entry for 171.68.0.0 (mask 255.255.255.255), 50 known subnets
Known via "igrp 10", distance 100, metric 8786
Tag 0
Redistributing via igrp 10
Last update from 171.68.178.196 on Serial0, 0:01:17 seconds ago
I 171.68.178.192 [100/8786] via 171.68.178.196, 0:01:17, Serial0 I 171.68.174.64 [100/8786] via 171.68.178.196, 0:01:17, Serial0 I 171.68.170.64 [100/8786] via 171.68.178.196, 0:01:17, Serial0 I 171.68.231.0 [100/8806] via 171.68.178.196, 0:01:17, Serial0 I 171.68.230.0 [100/8806] via 171.68.178.196, 0:01:17, Serial0 .... FRCS251-1# ping 171.68.178.196 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 171.68.178.196, timeout is 2 seconds: ..... Success rate is 0 percent (0/5)
Solution: This is an extension of Case 3. To solve this we apply static routes to both ends of the serial link:
7000: ip route 172.68.0.0 s 4/3
2500: ip route 171.68.0.0 s 0
7000 Results:
FRCS7-2#ping 172.68.1.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.68.1.1, timeout is 2 secs: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
2500 Results:
FRCS251-1#ping 171.68.178.196 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 171.68.178.196, timeout is 2 secs: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 1/3/4 ms
Conclusion: Like Case 3, the subnet information gets lost since it is treated like a host route. Adding the static routes fixes this.
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