These rules are discussed in more detail in the following sections.
This section provides some background on IP unnumbered and some real-life
scenarios of IP unnumbered in action. Figure4-1
represents the network topology used for most of this section.
In Table 4.1, the routing update from RouterC has a bit (1) set in the fourth octet (.1
= 00000001). RouterA compares this update with the mask of the IP address
and the mask used by the unnumbered ethernet0 command. RouterA will
discover that a bit is set outside the range of the mask, which must be interpreted
as a host route.
See the section on mask ambiguity in Chapter 3, “Discontiguous
Networks, Summarization, and
Subnet 0,” for more information on this issue.
As previously explained, ahost
route refers to a route whose associated mask has all 32 bits set
to 1255.255.255.255. For an address and mask such as this, there can
be only one host.
One common cause of host routes is a route advertisement that has bits
set in the range that the receiving router interprets as outside of the subnet
mask. In this case, the router compared the advertisement received from an
adjacent router with the mask of the interface it was received over and found
a mismatch. This is how RIP V1 and IGRP determine the mask to apply to advertisements
Table 4.1 shows an example of an advertisement
that has a bit set to 1 in a region of the address not covered by the subnet
mask in use.
Another common cause of host routes is a routing update explicitly advertised
with a mask of 255.255.255.255. Routing protocols such as OSPF and EIGRP include
the masks associated with the routes in their routing updates.
Host routes are also often manually configured to resolve routing problems
related to configuring dial-on-demand routing (DDR). The following section on DDR goes into more detail on this subject.
Dial-up users may require
host routes to ensure that they have full connectivity to the network. Although
dial-up networking is beyond the scope of this book, this section attempts
to clarify when host routes can be useful. In the scenario in Figure4-2,
hosts A and B use modems to dial into the corporate network.
Figure 4.2. Async dial-up network using IP unnumbered without host routes.
RouterA is using IP
unnumbered ethernet 0 on its Async interfaces. This results in multiple
Async interfaces sharing the same subnet as a local Ethernet. However, the
router needs to be able to keep track of the interfaces as separate subnets
in order to route traffic correctly.
Remember that one of the rules of IP unnumbered is that it must be used
only for point-to-point network connections. Extending the mask already applied
to RouterA's Ethernet 0 interface when the addresses are configured on the
asynchronous interfaces provides this point-to-point connection. Interface
Async1 has an address of 220.127.116.11 255.255.255.255, and interface Async2
has an address of 18.104.22.168 255.255.255.255.
When all of the hosts are connected to RouterA (they have dialed in
to the network), they will have their own unique routes (their host routes)
in RouterA's routing table. If RouterA receives a packet for 22.214.171.124 from
HostA, it does a longest match lookup of its routing table
and determines that the best match is the subnet applied to its Ethernet 0
interface: 126.96.36.199. It forwards the packet out of that interface.
On the other hand, if RouterA receives a packet for 188.8.131.52 from
HostB, the longest match lookup results in a match for the host route applied
to Asynch1: 184.108.40.206. The packet is forwarded out that interface to HostA
(assuming either that the link to HostA is already up or that RouterA is configured
to place a call to HostA when it receives a packet for it).
Without host routes being advertised by RouterA to RouterB for end systems
A and B, RouterB assumes that an IP packet to 220.127.116.11 or 18.104.22.168
is for an end system on the Ethernet network connecting RouterA and RouterB.
The gray box surrounding the (E0) interfaces for routers A and B in Figure4-2
shows where RouterB assumes that addresses 22.214.171.124 and 126.96.36.199 reside.
RouterB ARPs (attempts to determine the layer two MAC address) for 188.8.131.52
or 184.108.40.206 directly instead of forwarding the packet to RouterA
to reach end system A.
RouterA is advertising the host
routes for end systems A and B. RouterB will do a longest match lookup for
packets to either 220.127.116.11 or 18.104.22.168 and discover that the connected
route to 22.214.171.124 is not the longest match. RouterB will determine that
these routes are reachable via its Ethernet 0 address, using RouterA as
the next hop.
Figure 4.3. Async dial-up network using IP unnumbered without host routes.
The gray box surrounding HostA and HostB in Figure4-3
shows where RouterB will assume that addresses 126.96.36.199 and 188.8.131.52
reside now that the correct host routes are being advertised by RouterA.
This section has given you an intentionally simplistic example of DDR.
DDR is an incredibly complex subject that can take up an entire book all by
itself. Consult the documentation for the version of Cisco IOS you are using
for more information on DDR internetworking. The next section explains how to configure IP unnumbered in non-DDR