Prefix Prioritization Overview
Junos OS routes have a predetermined order for route installation. This is no longer sufficient as it is sometimes required to prioritize certain routes or prefixes over others for better convergence or to provide differentiated services. In a network with a large number of routes, it is sometimes important to control the order in which routes get updated due to a change in the network topology. For example, it would be useful to first update OSPF routes that correspond to an IBGP neighbor, and only then update the rest of the OSPF routes. At a system level, Junos OS implements reasonable defaults based on heuristics to determine the order in which routes get updated. However, the default behavior is not always optimal. Prefix prioritization allows the user to control the order in which the routes get updated from LDP or OSPF to rpd, and from rpd to kernel. In Junos OS Release 16.1 and later, you can control the order in which the routes get updated from LDP or OSPF to rpd, and from rpd to kernel.
In Junos OS Release 16.1 and later, the Junos OS policy language is extended to let the user set the relative priority high and low for
prefixes via the existing protocol import policy. Based on the tagged priority, the routes are placed in different priority queues. Routes that are not explicitly
assigned a priority are treated as medium priority. Within the same priority level, routes will continue to be updated in lexicographic order. Prefix prioritization
would need each supporting protocol to prioritize its routes internally. Prefix prioritization ensures that high priority IGP and LDP routes get updated to
the FIB (forwarding table) before medium and low priority routes.
There is an upper limit on how many high priority prefixes are allowed in the kernel. Not more than 10,000 high priority prefixes can coexist in the kernel. If this threshold is crossed in the kernel, then any new high priority prefix addition request will be considered as normal priority. This is a “best effort” prioritization scheme and will not be handled if the kernel is highly loaded.