Alcatel-Lucent 7750 Configuration Manual page 122

LAG and ECMP Hashing
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In addition to the above outlined per-flow hashing inputs SROS supports multiple option to
modify default hash inputs.
For all cases that involve per-packet hashing, the NPA produces a 20-bit result based on hashing
the relevant packet data. This result is input to a modulo like calculation (divide by the number of
routes in the ECMP and use the remainder) to determine the ECMP index.
If the ECMP index results in the selection of a LAG as the next hop, then the hash result is hashed
again and the result of the second hash is input to the modulo like operation (divide by the number
of ports in the LAG and use the remainder) to determine the LAG port selection.
Note however that when the ECMP set includes an IP interface configured on a spoke-SDP (IES/
VPRN spoke interface), or a Routed VPLS interface, the unicast IP packets—which will be
sprayed over this interface—will not be further sprayed over multiple RSVP LSPs (part of the
same SDP), or multiple LDP FEC next-hops when available. In this case, a single RSVP LSP or
LDP FEC next-hop will be selected based on a modulo operation of the service ID. The second
round of the hash is exclusively used for LAG link selection. IP unicast packets from different
IES/VPRN services or Routed VPLS services will be distributed across RSVP LSPs or LDP FEC
next-hops based on the modulo operation of their respective service ID.
Changing Default Per Flow Hashing Inputs
For some traffic patterns or specific deployments, per-flow hashing is desired but the hashing
result using default hash inputs as outlined above may not be produce a desired distribution. To
alleviate this issue, SROS allows operators to modify default hash inputs as outlined in the
following subsections.
LSR Hashing
The LSR hash routine operates on the label stack only. However, there is also the ability to hash on
the IP header if a packet is IP. An LSR will consider a packet to be IP if the first nibble following
the bottom of the label stack is either 4 (IPv4) or 6 (IPv6). This allows the user to include an IP
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VLL traffic from a service access point is not sprayed on a per-packet basis, but as for
VPLS flooded traffic, the service ID is used to pick one of the ECMP/LAG paths. The
exception to this is when shared-queuing is configured on an e-pipe SAP, i-pipe SAP, or
f-pipe SAP, or when H-POL is configured on an e-pipe SAP. In those cases, traffic
spraying is the same as for VPLS known unicast traffic. Packets of the above VLL
services received on a spoke-SDP are sprayed the same as for VPLS known unicast traffic.
Note that a-pipe and c-pipe VLL packets are always sprayed based on the service-id in
both directions.
Multicast IP traffic is hashed based on an internal multicast ID, which is unique for every
record similar to VPLS multicast traffic with IGMP snooping enabled.
7750 SR Interface Configuration Guide