Alcatel-Lucent 7750 SR OS Interface Configuration Manual page 105

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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.
7750 SR OS Interface Configuration Guide
value is 4 then it will assume it is an IPv4 packet. If the nibble value is 6 it will assume it
is an IPv6 packet. The result of the hash of the label stack, along with the incoming port
and system IP address, is fed into another hash along with source and destination address
fields in the IP packet header. Otherwise, it will just use the label stack hash calculated in
the first round like in the default "Label-Only Hash" option. If there are more than 6 labels
in the stack the algorithm will also use the result of the label stack hash only. The net
result will be used to select which LDP FEC next-hop to send the packet to using a
modulo operation of the net result with the number of next-hops. This same net result will
feed to a second round of hashing to select a LAG link on the egress port where the LSP
has its NHLFE programmed when applicable.
The second method is referred to as "IP-only Hash" and is enabled in CLI by entering the
ip-only keyword. It operates the same way as the "Label-IP Hash" method except that the
hash is performed exclusively on the source and destination address fields in the IP packet
header.
VPLS multicast, broadcast and unknown unicast traffic transmitted on SAPs is not
sprayed on a per-frame basis, but instead the service ID is used to pick ECMP and LAG
paths statically.
→ VPLS multicast, broadcast and unknown unicast traffic transmitted on SDPs is and
hashed on a per packet basis in the same way as VPLS unicast traffic. However, per
packet hashing is applicable only to the distribution of traffic over LAG ports, as the
ECMP path is still chosen statically based on the service ID.
Data is hashed twice to get the ECMP path. If LAG and ECMP are performed on the
same frame, the data will be hashed again to get the LAG port (three hashes for LAG).
However, if only LAG is performed, then hashing will only be performed twice to get
the LAG port.
→ VPLS multicast traffic transmitted on SAPs with IGMP snooping enabled is load-
balanced based on the internal multicast ID which is unique for every (s,g) record.
This way, multicast traffic pertaining to different streams is distributed across
different LAG member ports.
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 Etherpipe SAP or Frame
Relay pipe SAP. In this case, traffic spraying is the same for VPLS known unicast traffic.
IP multicast is sprayed over LAG based on the unique multicast ID.
IP multicast Layer 4 UDP traffic will not be hashed.
Interface Configuration
Page 105