The ExtremeXOS software populates the hardware tables with IPv4 host entries by searching for available space in the following sequence:
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 (Virtual LAN) 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. Depending on the IPv4 unicast traffic flows, independent IPv4 host caches for each stack node can provide increased hardware fast-path forwarding compared with ExtremeXOS releases prior to 12.1. 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.
Note
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.
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