Excluding prefixes from LFA SPF

In the current SR OS implementation, the user can exclude an interface in IS-IS or OSPF, an OSPF area, or an IS-IS level from the LFA SPF.

This feature adds the ability to exclude prefixes from a prefix policy which matches on prefixes or on IS-IS tags.

CLI syntax

config>router>isis>loopfree-alternates exclude 
prefix-policy prefix-policy1 [prefix-policy2…up to 5]
config>router>ospf(3)>loopfree-alternates exclude 
prefix-policy prefix-policy1 [prefix-policy2…up to 5]
config>service>vprn>ospf(3)>loopfree-alternates exclude 
prefix-policy prefix-policy1 [prefix-policy2…up to 5]

The prefix policy is configured as in existing SR OS implementation.

CLI syntax

config
  router
    policy-options
      [no] prefix-list prefix-list1
        prefix 62.225.16.0/24 prefix-length-range 32-
         32
      [no] policy-statements prefix-policy1
        entry 10
        from
          prefix-list "prefix-list1"
        exit
          action accept
        exit
        exit
          default-action reject
exit

If the user enabled the IS-IS prefix prioritization based on tag, it also applies to SPF LFA. If a prefix is excluded from LFA, it is not included in LFA calculation regardless of its priority. However, the prefix tag can be used in the main SPF.

Note: Prefix tags are not defined for OSPF protocol.

The default action of the loopfree-alternates exclude command when not explicitly specified by the user in the prefix policy is a ‟reject”. Consequently, regardless of whether the user did or did not explicitly add the statement ‟default-action reject” to the prefix policy, a prefix that did not match any entry in the policy is accepted into LFA SPF.