Netgear XCM8806 Chassis User Manual - Page 608

NETGEAR 8800 User Manual Note: If no IPv4 route is found in the LPM table and IPv4 unicast packets are slow-path forwarded for a given remote host, an IPv4 entry is created for the remote host in either the L3 hash table or LPM table. The hardware does not cache entries for remote IPv6 hosts, so IPv6 routes take precedence over IPv4 routes. IPv4 Host Entry Population Sequence The XCM8800 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 as shown in Table 65. 2. Available space in an L3 Hash table bucket. 3. Available space in the reserved section of the LPM table as shown in Table 65. 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 (Source IP, Group IP, VLAN ID) tuple. The hash table stores entries in groups of 8, 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 NETGEAR 8800, aging is performed independently for each I/O module or stack node based on the ingress traffic for that module or node. Depending on the IPv4 unicast traffic flows, independent IPv4 host caches for each I/O module can provide increased hardware fast-path forwarding. 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, based on whether IP ARP refresh is enabled or disabled, and the value for the configure iparp timeout {vr } command. 608 | Chapter 20. IPv4 Unicast Routing