Configuring Next-Hop Address Learning on MX Series and PTX Series Routers for Destinations Accessible Over Multiple Paths

Starting in Junos OS Release 16.1, you can enable learning of next-hop addresses to correctly report the next hop address, output SNMP, destination IP address, and destination IP mask values in the flow records when a destination is reachable through multiple paths. By default, this behavior of learning the next-hop addresses is disabled for inline active flow monitoring.

Starting in Junos OS Release 20.3R1, if you enable learning of next-hop addresses, certain supported devices report the packet loss priority (PLP) and the first two characters of the configured forwarding class name in the IPv4 and IPv6 IPFIX flow records. The collector uses this information to derive the DSCP bits that the packet would contain when exiting the router. The first two letters of a configured forwarding class name must be unique. For tunnel termination, 0xFF is exported in the PLP field and NULL (0) is exported in the forwarding class name field. The mapping between the PLP exported in the record and the loss priority names is as follows:

• 0x00: Low

• 0x01: Medium-low

• 0x02: Medium-high

• 0x03: High

• 0xFF: Unknown

You do not need to enable learning of next-hop addresses on Junos OS Evolved to report the packet loss priority and forwarding class information.

When learning next-hop addresses is disabled, data is reported as follows:

• If the destination address of the sampled IPv4 flow is reachable through multiple paths, the IPv4 next hop address and the output SNMP address are reported in the flow records as the same as the gateway address and SNMP index of the first path seen in the forwarding table.

• If the destination address of the sampled IPv6 flow is reachable through multiple paths, the IPv4 next hop address and the output SNMP address are reported as 0 in the flow records.

• If the Incoming Interface (IIF) and Outgoing Interface (OIF) are not in the same VRF, then the destination IP address, destination IP mask, IPv4 next hop address, and the ouput SNMP address are reported as 0 in the flow records.

• The packet loss priority and forwarding class information is not reported for devices running Junos OS.

When you enable learning of next-hop addresses, the output SNMP, destination IP address, destination IP mask values, packet loss priority, and the first two characters of the configured forwarding class name in the flow records are reported correctly when a destination is reachable through multiple paths. To enable next-hop learning, include the nexthop-learning enable statement at the [edit services flow-monitoring (version-ipfix | version9) template template-name] hierarchy level.