IPv4 Host Entry Population Sequence

The ExtremeXOS software populates the hardware tables with IPv4 host entries by searching for available space in the following sequence:

  1. Unreserved space in the LPM tables.
  2. Available space in an L3 Hash table bucket.
  3. Available space in the reserved section of the LPM table.
  4. Space used by the oldest host entries in the LPM and L3 Hash tables.

The L3 Hash table is named for the hash function, which stores host and multicast entries based on an algorithm applied to the host IP address or multicast tuple (Source IP, Group IP, VLAN ID). The hash table stores entries in groups of 8 or 16 (depending on the hardware), and these groups are called buckets. When a bucket is full, any additional host or multicast addresses that map or hash to that bucket cannot be added. Another benefit of the extended IPv4 host cache feature is that you can reduce these conflicts (or “hash table collisions”), by making room for IPv4 hosts in the LPM table and reducing demand for the L3 Hash table.

A hardware-based aging mechanism is used to remove any remote IPv4 host entries that have not had IPv4 unicast packets forwarded to them in the previous hour. (Note that remote IPv4 hosts only need to be cached when all IPv4 routes do not fit within the number of routes reserved.) Aging helps to preserve resources for the hosts that are needed most. In a SummitStack, aging is performed independently for each stack node based on the ingress traffic for that node. Even with aging, it is still possible that the Next Hop table, LPM table, or L3 Hash bucket do not have space to accept a new host. In those cases, a least-recently used algorithm is used to remove the oldest host to make space for the new host in hardware.

Local IPv4 host entries are only subject to hardware-based aging if there has been a large amount of least-recently used replacement, indicating severe contention for HW table resources. Otherwise, local IPv4 host entries are retained just as in ExtremeXOS releases prior to 12.1, based on whether IP ARP refresh is enabled or disabled, and the value for the configure iparp timeout command.



Gateway entries are entries that represent routers or tunnel endpoints used to reach remote hosts. Gateway entries are not aged and are not replaced by IPv6 hosts or multicast entries in the L3 Hash table or by any entries requiring space in the Next Hop table. The software can move gateway entries from the LPM table to the L3 Hash table to make room for new reserved routes.