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4 Resilient Next-hop Groups
7 Resilient groups are a type of next-hop group that is aimed at minimizing
9 weights of constituent next hops.
12 the legacy multipath next-hop group, which uses the hash-threshold
15 To select a next hop, hash-threshold algorithm first assigns a range of
16 hashes to each next hop in the group, and then selects the next hop by
17 comparing the SKB hash with the individual ranges. When a next hop is
19 reassignment of parts of hash space from one next hop to another. RFC 2992
28               Before and after deletion of next hop 3
31 Note how next hop 2 gave up part of the hash space in favor of next hop 1,
33 previous and the new distribution, some traffic flows change the next hop
46 To mitigate the above-mentioned flow redirection, resilient next-hop groups
48 constituent next hops: a hash table. The selection algorithm uses SKB hash
49 to choose a hash table bucket, then reads the next hop that this bucket
53 algorithm, the range of hashes associated with a next hop must be
55 the individual next hops is arbitrary. Therefore when a next hop is deleted
56 the buckets that held it are simply reassigned to other next hops::
66 	    Before and after deletion of next hop 3
69 When weights of next hops in a group are altered, it may be possible to
71 traffic, and use those to satisfy the new next-hop distribution demands,
74 the next-hop group change.
79 In a nutshell, the algorithm works as follows. Each next hop deserves a
82 this number a "wants count" of a next hop. In case of an event that might
83 cause bucket allocation change, the wants counts for individual next hops
86 Next hops that have fewer buckets than their wants count, are called
88 overweight (and therefore no underweight) next hops in the group, it is
97 After assigning wants counts to next hops, an "upkeep" algorithm runs. For
100 1) that have no assigned next hop, or
101 2) whose next hop has been removed, or
102 3) that are idle and their next hop is overweight,
104 upkeep changes the next hop that the bucket references to one of the
105 underweight next hops. If, after considering all buckets in this manner,
106 there are still underweight next hops, another upkeep run is scheduled to a
110 of all next hops. Another attribute of a resilient group is the "unbalanced
119 4) whose next hop is overweight, and the amount of time that the table has
128 among next hops is still the one in SW. Drivers are notified of updates to
129 next hop groups in the following three ways:
144 flag in the notification. Those are used for the cases where e.g. the next
184 Next Hop Get
187 Requests to get resilient next-hop groups use the ``RTM_GETNEXTHOP``
188 message in exactly the same way as other next hop get requests. The
205   ``NHA_ID``          ID of the next-hop group that the bucket belongs to.
223   ``NHA_ID``          If specified, limits the dump to just the next-hop group
226                       next hops that use the device with this ifindex.
228                       next hops that use a device in the VRF with this ifindex.
236                            that contain the next hop with this ID.
249 The last command creates a resilient next-hop group. It will have 8 buckets
255 Changing next-hop weights leads to change in bucket allocation::
272 were migrated after the next-hop replace command to satisfy the new demand
273 that next hop 1 be given 6 buckets instead of 4.
280 For example, the following will mark bucket 23 in next-hop group 10 as
286 next attempt to migrate a bucket should fail::