📄 ip unnumbered.htm
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<title>Cisco - IP Unnumbered</title>
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<h1>IP Unnumbered</h1>
<hr>
<h2>Introduction</h2>
<p>This document will explain the concept of IP unnumbered, and
will give you several configuration examples for reference. The <b>ip
unnumbered</b> 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. </p>
<h2>IP and IP Unnumbered</h2>
<p>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. </p>
<p>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. </p>
<p><b>Note:</b> The aggregation scheme discussed above assumes a
traditional distance-vector routing protocol like RIP or IGRP. </p>
<p>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. </p>
<p>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). </p>
<p>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. </p>
<h2>Configuration Examples</h2>
<h3>CASE 1</h3>
<p>On either side of the serial link we have the same major net
with different subnets. </p>
<pre>
| ----- | |
| | |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
</pre>
<p>M/S1 -- Major net M subnet 1. M/S2 -- Major net M subnet 2. </p>
<p><b>7000 Configuration:</b> </p>
<pre>
interface Ethernet3/0
ip address 171.68.178.196 255.255.255.192
</pre>
<pre> interface Serial4/3
ip unnumbered Ethernet3/0 </pre>
<pre> router igrp 10
network 171.68.0.0 </pre>
<p><b>2500 Configuration:</b> </p>
<pre>
interface Ethernet 0
ip address 171.68.179.1 255.255.255.192
</pre>
<pre> interface Serial 0
ip unnumbered Ethernet0 </pre>
<pre> router igrp 10
network 171.68.0.0 </pre>
<p><b>7000 Results:</b> </p>
<pre>
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
</pre>
<p><b>2500 Results:</b> </p>
<pre>
S251-1#sh ip route 171.68.0.0</pre>
<pre> 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 </pre>
<p><b>Conclusion:</b> The route information about subnets is
maintained in this scenario. </p>
<h3>CASE 2</h3>
<p>On either side of the serial link we have different major nets
and no subnets. </p>
<pre>
| |
| ---------- ---------- |
| 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
</pre>
<p>M/NS -- Major net M no subnets K/NS -- Major net K no subnets.
</p>
<p><b>7000 Configuration:</b> </p>
<pre>
interface Ethernet3/0
ip address 171.68.178.196 255.255.0.0</pre>
<pre> interface Serial4/3
ip unnumbered Ethernet3/0 </pre>
<pre> router igrp 10
network 171.68.0.0 </pre>
<p><b>2500 Configuration:</b> </p>
<pre>
interface Ethernet 0
ip address 172.68.1.1 255.255.0.0
</pre>
<pre> interface Serial 0
ip unnumbered Ethernet0 </pre>
<pre> router igrp 10
network 172.68.0.0 </pre>
<p><b>7000 Results:</b> </p>
<pre>
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</pre>
<pre> Gateway of last resort is not set </pre>
<pre> 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 </pre>
<pre> 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 </pre>
<p><b>2500 Results:</b> </p>
<pre>
FRCS251-1#sh ip route</pre>
<pre> 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</pre>
<pre> Gateway of last resort is not set </pre>
<pre> 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 </pre>
<pre> FRCS251-1#ping 171.68.178.196
Type escape sequence to abort.
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