Discussion

Destination: 192.168.220.0/24, Local: 192.168.220.1,
Broadcast: 192.168.220.3
Addresses, Flags: Primary Preferred Is-Preferred Is-Primary
Destination: 192.168.220.0/24, Local: 192.168.220.2,
Broadcast: 192.168.220.3
It’s helpful to expand the configuration a bit to illustrate how the source addresses
are used. Let’s say we have:
[edit interfaces fe-0/0/0 unit 0 family inet]
address 192.168.220.1/24;
address 192.168.220.2/24 {
primary;
preferred;
}
address 192.168.222.1/24;
address 192.168.222.2/24 {
preferred;
}
IP traffic bound for 192.168.220.10 uses 192.168.220.2 as the source address because
this is the preferred address. Traffic for, say, 100.0.0.1 also uses 192.168.220.2 (if
there is a route to that network pointing out fe-0/0/0) because this is the interface’s
primary address. Traffic for 192.168.222.10 uses 192.168.222.2 as the source address,
again because this is the interface’s preferred address.
Another situation is to have multiple addresses on the loopback interface, which you
might want for any number of reasons. You might want a public address for the
router’s IP address, a private one for management access, and 127.0.0.1 as the traditional
loopback address.
Yet another reason to have multiple address on an interface is to increase the number
of IP addresses on an Ethernet network without renumbering devices that are
already there. While this is often not considered good practice, sometimes it’s the
easiest way to deal with the growing number of addresses. Say you have interface
fe-0/0/0 with IP address 205.134.233.254/24. When you’ve used all the addresses in
that /24 address space, you can either renumber all 250 or so devices on that network,
grow that network (which requires planning ahead; in this case you could just
make the network a /23 if you’re not using 205.134.232.0/24 for anything else), or
just add another network. As an interim measure, you can just add the second
address for the new network address space to the interface and set the desired one to
be the primary and preferred address.
7.11 Using the Management Interface
Problem
You want to enable out-of-band management access to the router.
212 | Chapter 7: Router Interfaces
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