Chapter 17
| General IP Routing
Equal-cost Multipath Routing
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the traffic forwarded to the destination. ECMP uses either equal-cost multipaths
manually configured in the static routing table, or equal-cost multipaths
dynamically generated by the Open Shortest Path Algorithm (OSPF). In other
words, it uses either static or OSPF entries, not both. 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 681), or dynamically
generated by OSPFv2 (see “Configuring the Open Shortest Path First Protocol
(Version 2)” on page 716), then up to 8 paths with the same lowest cost can be used
to forward traffic to the destination.
Command Usage
◆ ECMP only selects paths of the same protocol type. It cannot be applied to both
static paths and dynamic paths at the same time for the same destination. If
both static and dynamic paths have the same lowest cost, the static paths have
precedence over dynamic paths.
◆ 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
and 8 for dynamic 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)
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