Alcatel-Lucent 7750 SR OS, Interface Configuration Manual

Page: 72 / 444

Port Features
Page 72 7750 SR OS Interface Configuration Guide
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 two 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 7750 SR supports up to 16 equal cost routes in ECMP and up to eight ports per LAG.
To avoid out-of-sequence packets the algorithm for selecting the next hop in an ECMP or LAG
must be deterministic. The algorithm performs at line rate and is executed in the 7750 SR Network
Processor Array (NPA) when the packet ingresses the IOM, after determining that the next hop is
an ECMP and/or LAG.
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:
• 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 an SR series router uses the IP SA/DA or optionally TCP/UDP
port information.
• MPLS switched traffic is based on the whole label stack (up to 5 labels), 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.
• VPLS multicast, broadcast and unknown unicast traffic transmitted on SAPs is not
sprayed on a per-frame basis, but instead the SAP ID is used to pick ECMP and LAG
paths statically.
→
VPLS multicast, broadcast and unknown unicast traffic transmitted on SDPs is 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.