multi-path routing (ECMP) is a routing strategy where next-hop packet
forwarding to a single destination can occur over multiple "best paths" which
tie for top place in routing metric calculations. Multipath routing can be used
in conjunction with most routing protocols, since it is a per-hop decision that
is limited to a single router. It potentially offers substantial increases in
bandwidth by load-balancing traffic over multiple paths.
protocols, including Open Shortest Path First (OSPF), Intermediate System to
Intermediate System (ISIS), Enhanced Interior Gateway Routing Protocol (EIGRP),
and Border Gateway Protocol (BGP) allow ECMP routing.
between ECMP paths is performed on IOS-XE based CEF object called loadbalance.
ECMP Per-Flow Load
Load balancing is a
forwarding mechanism that distributes traffic over multiple links based on
certain parameters. ECMP Per-Flow Load Balancing distributes packets across
multiple links based on Layer 3 routing information. If the router discovers
multiple paths to a destination, the routing table is updated with multiple
entries for that destination. Per-flow load balancing allows the router to use
multiple paths to achieve load sharing across multiple source-destination host
pairs. Packets for a given source-destination host pair are guaranteed to take
the same path, even if multiple paths are available. Traffic streams destined
for different pairs tend to take different paths.
of Per-Flow Load Balancing
Incoming data traffic is evenly distributed over multiple equal-cost connections.
Incoming data traffic is evenly distributed over multiple equal-cost connections member links within a bundle interface.
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ECMP Load Balancing
Both 4 ECMP and
8 ECMP paths are supported.
is supported on global IPv4 and IPv6 traffic. For global IPv4 and IPv6 traffic,
the traffic distribution can be equal among the available 8 links.
Per packet load
balancing is not supported.
Label load balancing is supported.
BGP multi-path is not supported with ECMP.
BGP multi-path with PIC Edge is not supported
When BGP PIC is configured, the L3VPN prefixes scale reduces by 1/4th of the supported value (Supported scale value/4), for better convergence value at the PIC core. For example, for RSP1A the supported L3VPN scale is 2000, if 4 ECMP path with PIC is enabled, then the maximum supported scale value is reduced to 5000 (20000/4).
following steps to configure ECMP load balancing.
3.platform loadbalance max-paths8
Command or Action
Enables privileged EXEC mode.
Enter your password if prompted.
Device# configure terminal
platform loadbalance max-paths8
Device(config)# platform loadbalance max-paths 8
loadbalance maximum paths. Select
Yes to save the configuration and reload the router.
default supports only 4 paths. To increase ISIS max-paths, use the command
config-maximum-paths 8 under router ISIS. IGP by
default supports only 4 paths. To increase IGP max-paths, use the command
config-maximum-paths 8 under respective IGP (OSPF
and ISIS) process.
privileged EXEC mode.
Examples for ECMP Load Balancing
This section shows
sample configurations for ECMP load balancing.