Owner's Manual

610 | Chapter 20. IPv4 Unicast Routing
NETGEAR 8800 User Manual
ECMP table entry, so duplicate gateway sets require additional ECMP table entries, which
reduces the total number of gateway sets the ECMP table can support. This approach also
limits the total number of LPM table entries that can use IP route sharing to the total number
of ECMP table entries. The ECMP table is smaller than the LPM table, so IP route sharing is
not available to all LPM table entries on these platforms.
For improved ECMP scaling, each LPM table entry points to a gateway set entry in the ECMP
table:
Each gateway set entry for the platforms listed above is unique and appears only once in the
ECMP table. Multiple LPM table entries can point to the same gateway set entry. This
efficient use of the ECMP table creates more room in the ECMP table for additional gateway
set entries. It also makes IP route sharing available to every entry in the LPM table.
The following command allows you to configure the maximum number of next-hop gateways
for gateway sets in the ECMP table:
configure iproute sharing max-gateways <max_gateways>
Each gateway entry in a gateway set consumes ECMP table space. As the max_gateways
value decreases, the ECMP table supports more gateway sets. If you configure the
max_gateways value to 8, the switch supports route sharing through up to 8 gateways per
subnet, but supports the smallest number of gateway sets. If you do not need to support up to
8 different gateways for any subnet, you can decrease the
max_gateways value to support
more gateway sets.
To determine which gateways might be added to the ECMP table, consider how many local
gateways are connected to the switch and can be used for ECMP, and consider the
max_gateways value. For example, suppose that you have four ECMP gateway candidates
connected to the switch (labeled A, B, C, and D for this example) and the
max_gateways
option is set to
4. For platforms that allow a gateway set entry to support multiple subnets,
this configuration could result in up to 11 gateway sets in the ECMP table: ABCD, ABC, ABD,
ACD, BCD, AB, AC, AD, BC, BD, and CD.
If there are 4 gateways and you set max-gateways to 4, you can use the choose function to
calculate the total number of gateway set possibilities as follows:
(4 choose 4) + (4 choose 3) + (4 choose 2) = 11
To calculate the number of gateway set possibilities for a given number of total gateways and
a specific
max-gateways value, use the choose function in the following formula:
(TGW choose MGW) + (TGW choose MGW-1) + ... + (TGW choose 2) = TGWsets
In the formula above, TGW represents the total local gateways, MGW represents the
max_gateways value, and TGWsets represents the total gateway sets needed to support all
possible shared paths.
To see if your platform supports the total gateway sets needed, do the following:
• Calculate the total ECMP gateway sets possible as described above.
• Compare your result to the IP route sharing (total combinations of gateway sets)
capacities listed in the XCM8800 Release Notes to verify that the switch can support the
desired number of gateway sets.