ECMP Load Sharing for IP

ECMP Hashed Mode Overview

Hashed mode uses hashing of source and destination addresses to determine which of the available paths in the ECMP set to use. Hashed mode is the default ECMP mode of operation.

Defining the Maximum Parallel Routes Supported by the Routing Protocol

You can add routing table entries manually (as static routes), or they are formed as routers discover their neighbors and exchange routing tables (via OSPF, BGP, and other routing protocols).

To control the maximum number of parallel routes that the routing protocol (BGP, IS-IS, OSPF, or RIP) can support:

• Issue the maximum paths command in Router Configuration mode.
  host1(config-router)#maximum-paths 2

  The maximum number of routes can be in the range 1–16 for BGP, IS-IS, OSPF, or RIP. Use the no version to restore the default value, 1 for BGP or 4 for IS-IS, OSPF, or RIP.

ECMP Round-Robin Mode Overview

Round-robin mode distributes packets equally among the available paths in the ECMP set.

If all the ECMP links are configured for the ip multipath round-robin command and their next hops are direct next hops, the round-robin mode uses the random algorithm for traffic distribution.

In round-robin mode, if a packet uses a path, the next packet can choose the same path or the previous path, or the next path based on the random value generated. The random algorithm does not guarantee equal distribution of the packets among the ECMP links.

Configuring ECMP Round-Robin Load Sharing

ECMP uses the round-robin mode when you have configured all interfaces in the set to round-robin. Otherwise, ECMP defaults to hashed mode because round-robin mode can cause reordering of packets. You must explicitly ensure that the possible reordering is acceptable on all the member interfaces by setting them to round-robin mode.

If one of the ECMP next hops is an indirect next hop, ECMP uses hashed mode load balancing.

To specify round-robin as the mode for ECMP load sharing:

• Issue the ip multipath round-robin command in Subinterface Configuration mode.
  host1:router_0(config-subif)#ip multipath round-robin

  Use the no version to set the ECMP mode to the default, hashed.

ECMP Fast Reroute Protection Overview

If a link goes down, ECMP uses fast reroute protection to shift packet forwarding to use operational links, thereby decreasing packet loss. Fast reroute protection updates ECMP sets for the interface without having to wait for the route table update process. When the next route table update occurs, a new ECMP set can be added with fewer links or the route might point to a single next hop.

Caution: To provide ECMP fast reroute functionality in the event of an interface failure, the members of an equal cost multipath must be resolved to corresponding interfaces. If the member is an indirect next hop, the interface is obtained by using the forwarding equivalence class (FEC) to which the member points. This method of resolving members occurs only if the FEC, pointed to by the indirect next hop, is either an interface or a direct next hop. An indirect next hop member is not resolved to an interface if it points to another indirect next hop or to an equal cost multipath. ECMP fast reroute functionality is not available if any interfaces that correspond to unresolved indirect next hop members go down. If you modify an indirect next hop member to point to a different FEC (that is, a different interface, direct next hop, indirect next hop, or ECMP), the indirect next hop member is not resolved for the new changes.