Lag Hashing - Alcatel-Lucent 7210 SAS E OS Configuration Manual

LAG

LAG 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 Link Aggregation (LAG) one
of the methods is applied. The supports up to 16 equal cost routes in ECMP and up to two 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.
Depending on the type of traffic that needs to be distributed into LAG, different variables are used
as input to the hashing algorithm that determines the port to be used. There are several traffic types
to consider:
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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
value is 4 then it will assume it is an IPv4 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 known unicast traffic. This is hashed based on the IP source, destination addresses
and if Layer 4 traffic, hash will include (TCP or UDP) source and destination port
information, or for non-IP traffic, MAC source, destination addresses, VLAN, Ethertype
and port of entry are considered for hash.
7210 SAS E Interface Configuration Guide