Web Management Guide-R01

Chapter 17 | General IP Routing
Equal-cost Multipath Routing
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Equal-cost Multipath Routing
Use the IP > Routing > Routing Table (Configure ECMP Number) page to configure
the maximum number of equal-cost paths that can transmit traffic to the same
destination. The Equal-cost Multipath routing algorithm is a technique that
supports load sharing over multiple equal-cost paths for data passing to the same
destination. Whenever multiple paths with equal path cost to the same destination
are found in the routing table, the ECMP algorithm first checks if the cost is lower
than that of any other entries in the routing table. If the cost is the lowest in the
table, the switch will use up to eight of the paths with equal lowest cost to balance
the traffic forwarded to the destination. ECMP uses equal-cost multipaths manually
configured in the static routing table. Normal unicast routing simply selects the
path to the destination that has the lowest cost. Multipath routing still selects the
path with the lowest cost, but can forward traffic over multiple paths if they all have
the same lowest cost. ECMP is enabled by default on the switch. If there is only one
lowest cost path toward the destination, this path will be used to forward all traffic.
If there is more than one lowest-cost path configured in the static routing table (see
“Configuring Static Routes” on page 624), then up to 8 paths with the same lowest
cost can be used to forward traffic to the destination.
Command Usage
◆ Each path toward the same destination with equal-cost takes up one entry in
the routing table to record routing information. In other words, a route with 8
paths will take up 8 entries.
◆ The routing table can only have up to 8 equal-cost multipaths for static routing
for a common destination. However, the system supports up to 256 total ECMP
entries in ASIC for fast switching, with any additional entries handled by
software routing.
◆ When there are multiple paths toward the same destination with equal-cost,
the system chooses one of these paths to forward each packet toward the
destination by applying a load-splitting algorithm.
A hash value is calculated based upon the source and destination IP fields of
each packet as an indirect index to one of the multiple paths. Because the hash
algorithm is calculated based upon the packet header information which can
identify specific traffic flows, this technique minimizes the number of times a
path is changed for individual flows. In general, path changes for individual
flows will only occur when a path is added or removed from the multipath
group.
Parameters
These parameters are displayed:
◆ ECMP Number – Sets the maximum number of equal-cost paths to the same
destination that can be installed in the routing table. (Range: 1-8; Default: 8)