Discussion

7.4 Setting the Router’s Source Address
Problem
All IP traffic that is sourced from the router includes a source address in the IP
header. The address chosen for packets depends on the interface that is used to reach
the destination when the connection is established. You want to configure the router
to used a fixed address.
Solution
Use the following command to always uses lo0 loopback address as the source
address in IP packets:
[edit]
aviva@router1# set system default-address-selection
Discussion
When selecting an address to include in the source address field of IP packets, the
JUNOS software chooses from among the addresses configured on the router. The
first candidate to use is the first non-127.0.0.1 address configured on the lo0 interface.
However, this means that the software usually, but not always, chooses the
loopback address. To ensure that the software always uses the router’s IP address,
use the set system default-address-selection command. Including this command is
considered good practice so that when other systems on the network receive traffic
from the local router, the packets always have the same address in the IP packet’s
source address field. This command forces the router to use one specific address, the
lo0 address, for most of the traffic that originates from the router. The source
address never affects traffic that is forwarded by the router, only packets that are
sourced from the router. If multiple addresses are configured for lo0, the software
chooses the one with the lowest address. However, if you assign the parameter
primary or preferred to a higher IP address, the software uses the higher IP address.
The following command makes one of the loopback addresses the primary one:
[edit]
aviva@RouterA# set interfaces lo0 unit 0 family inet address 192.168.16.1/32 primary
Figure 7-1 illustrates why you should set the router’s source address. In this topology,
if you ping 1.0.2.1 from Mars, Mars normally sends the packet out the so-0/0/0
interface and uses 1.0.2.2 as the source address for that ping packet. Venus receives
the ping and sends a response back to 1.0.2.2 (the source address).
If Mars, Venus, and Earth have lo0 addresses 1.1.0.1/32, 1.1.0.2/32, and 1.1.0.3/32,
and you have the default address selection configured on Mars, what happens is different.
If you ping 1.0.2.1 from Mars, Mars uses 1.1.0.1 (its lo0 address) as the
source address. It still sends the packet out so-0/0/0, and Venus still receives it and
sends a response back to the source address of the ping, 1.1.0.1.
204 | Chapter 7: Router Interfaces
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