Technical Documentation

Configuring the JUNOS Software to Select a Fixed Source Address for Locally Generated TCP/IP Packets

By default, the source address included in locally generated Transmission Control Protocol/IP (TCP/IP) packets, such as FTP traffic, and in User Datagram Protocol (UDP) and IP packets, such as Network Time Protocol (NTP) requests, is chosen as the local address for the interface on which the traffic is transmitted. This means that the local address chosen for packets to a particular destination might change from connection to connection based on the interface that the routing protocol has chosen to reach the destination when the connection is established. If multiple equal-cost next hops are present for a destination, locally generated packets use the lo0 address as a source.

To configure the software to select a fixed address to use as the source for locally generated IP packets, include the default-address-selection statement at the [edit system] hierarchy level:

If you include the default-address-selection statement in the configuration, the JUNOS Software chooses the system default address as the source for most locally generated IP packets. The default address is usually an address configured on the lo0 loopback interface. For example, if you specified that SSH and telnet use a particular address, but you also have default-address selection configured, the system default address is used.

For IP packets sent by IP routing protocols—including Open Shortest Path First (OSPF), Routing Information Protocol (RIP), Resource Reservation Protocol (RSVP), and the multicast protocols, but not including Intermediate System-to-Intermediate System (IS-IS)—the local address selection is often constrained by the protocol specification so that the protocol operates correctly. When this constraint exists in the routing protocol, the packet’s source address is unaffected by the presence of the default-address-selection statement in the configuration. For protocols in which the local address is unconstrained by the protocol specification, for example, internal Border Gateway Protocol (IBGP) and multihop external BGP (EBGP), if you do not configure a specific local address when configuring the protocol, the local address is chosen using the same method as other locally generated IP packets.

Published: 2010-04-26