Lag And Ecmp Hashing; Per Flow Hashing - Alcatel-Lucent 7750 SR OS Interface Configuration Manual

LAG

LAG and ECMP Hashing

When a requirement exists to increase the available bandwidth for a logical link that exceeds the
physical bandwidth or add redundancy for a physical link, typically one of the methods is applied;
equal cost multi-path (ECMP) or Link Aggregation (LAG). A 7750 SR can deploy both at the
same time, meaning, using ECMP of two or more Link Aggregation Groups (LAG) and/or single
links. The supports up to 16 equal cost routes in ECMP and up to 16 ports per LAG.
Different types of hashing algorithms can be employed depending whether better loadspreading or
consistent per service forwarding is required. The Alcatel-Lucent implementation supports per
flow hashing used to achieve uniform loadspreading and per service hashing designed to provide
consistent per service forwarding. The following sub-sections describe these two hashing
algorithms.

Per Flow Hashing

Depending on the type of traffic that needs to be distributed into an ECMP and/or LAG, different
variables are used as input to the hashing algorithm that determines the next hop selection. There
are several traffic types to consider:
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VPLS known unicast traffic. This is hashed based on the IP source and destination
addresses, or the MAC source and destination addresses for non-IP traffic. Optionally
TCP and UDP traffic can include the source and destination port information in the hash
algorithm.
The hash used for LAG for VPLS services does not include the VPLS service ID. The
MAC SA/DA are hashed and then, if the Ethertype is IPv4 or IPv6, the hash is replaced
with one based on the IP source address/destination address. If Layer 4 hashing is enabled
on the ingress port, the Layer 4 source port and destination port are hashed. Packets for the
same SAP can be sprayed across different LAG members, if the result of this hash modulo
the number of LAG links is different.
Unicast IP traffic routed by a 7750 SR router uses the IP SA/DA or optionally TCP/UDP
port information.
By default, MPLS packet hashing at an LSR is based on the whole label stack, along with
the incoming port and system IP address. Note that the EXP/TTL information in each
label is not included in the hash algorithm. This method is referred to as "Label-Only
Hash" option and is enabled in CLI by entering the lbl-only keyword.
A couple of options to further hash on the header of an IP packet in the payload of the
MPLS packet are also provided.
The first method is referred to as the "Label-IP Hash" option and is enabled in CLI by
entering the lbl-ip keyword. In the first hash round for ECMP, the algorithm will parse
down the label stack and once it hits the bottom it checks the next nibble. If the nibble
7750 SR OS Interface Configuration Guide