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Configuring Priority for Route Prefixes in RPD Infrastructure

Prefix prioritization helps users to prioritize certain routes or prefixes for better convergence and 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 changes in the network topology. 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 provides the user the ability to control the order in which the routes get updated from LDP or OSPF to rpd, and rpd to kernel. The Junos OS policy language is extended to let the user set relative priority (high and low) for prefixes through the existing import policy in protocols. Based on the tagged priority, the routes are placed in different priority queues. In the event of a topology change, high priority prefixes are updated in the routing table first, followed by low priority prefixes. Within the same priority level, routes will continue to be updated in lexicographic order. Routes that are not explicitly assigned a priority are treated as medium priority.

Before you begin to configure prefix prioritization in rpd for protocols such as OSPF, LDP, and BGP:

  • Configure the router interfaces.

  • Configure MPLS.

  • Configure the OSPF, BGP, and LDP protocols.

To configure the priority high for the OSPF protocol:

  1. Configure the policy term.

    For example:

  2. Configure the policy term to accept routes from OSPF.
  3. Specify the desired route as a match condition for which you want to set priority high.

    For example:

  4. Specify that the route is to be accepted and set priority high for the route if the previous conditions are matched.
  5. Verify the configuration.

LDP inherits from OSPF.

To configure priority high for LDP:

  1. Configure the policy term that imports from OSPF.

    For example:

  2. Configure the term to accept routes and priority from OSPF.

    For example:

  3. Verify the configuration.

To configure the priority high for the BGP protocol:

  1. Configure the policy term.

    For example:

  2. Specify the desired route as a match condition.

    For example:

  3. Specify that the route is to be accepted and set the priority high for the route if the previous conditions are matched.

  4. Verify the configuration.

Note:

For BGP, you can also configure priority based on the route-distinguisher (RD) value in case of L3VPN. For example, you can configure priority for BGP with route-distinguisher 51.51.51.51:111.

To configure priority for BGP based on the route-distinguisher (RD) value:

  1. Configure the policy term.

    For example:

  2. Specify the desired route as a match condition.

    For example:

  3. Specify that the route is to be accepted and set the priority high for the route if the previous conditions are matched.

  4. Verify the configuration.

Note:

Low priority prefixes are installed only after the high priority prefixes in the routing table. You can also configure priority low similarly to priority high for the routes you want to set to low priority.

Note:

Priority is applied only when routes are pushed from RIB to FIB. Therefore, you cannot modify the priority of routes that are already installed. Changing the priority of routes already installed does not make sense. If you try changing the priority of routes already installed, there is a show output difference:

As the route is already installed in FIB, LDP does not show the priority as High.

Restarting the routing daemon to remove the routes and adding it again reflects the proper priority from both the OSPF and LDP protocol perspective.

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