Alcatel-Lucent 9500 MXC User Manual page 467

Layer 2 Link Aggregation
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For L2 aggregation a session is established between source and destination MAC addresses. Once
established all traffic will use the established link, which will be over one link, or the other - it cannot be
split over both.
3DB 23063 ADAA - Rev 004 July 2007
• Layer 2 (L2) link aggregation uses source and/or destination MAC address data
in the Ethernet frame MAC/LLC header. It provides excellent load balancing
between the physical links under normal LAN traffic densities i.e. where there
are multiple concurrent sessions between different MAC source and destination
addresses.
• Layer 1 (L1, physical layer) aggregation acts on the byte-level data stream; data
is split between the physical links on a byte-by-byte basis. It should be used
where there is just one source and destination MAC address session in play,
such as between two routers, or where there are just a few sessions in play,
particularly so if one session dominates the available bandwidth.
The same rapid-transport-channel failure detection mechanism (RFD) used with
RWPR operation is also used to sense a physical link failure, to result in traffic
re-assignment times of typically less than 50 ms.
Note that two aggregated links (physical links) may be operated on the same
frequency channel (CCDP) using RAC 40s.
For applications where just two physical links are to be aggregated L1
aggregation is recommended as it supports equitable loading (load balancing)
regardless of the number of data sessions in play. It also supports higher burst
speeds compared to L2 aggregation.
Traffic streams transiting the virtual link are split between the physical links
based on their source and destination MAC addresses and the 'aggregation key'
allocated to each of the physical links, the aggregation group
prevents the occurrence of an IP loop, even though all traffic is sent and received
on a common LAN interface at each end of the virtual link.
• The aggregation key process sets the weighting (load balancing) of traffic
between the links in the aggregated group. For the DAC GE there are 16 keys,
and traffic (source / destination MAC address based) is allocated between the
keys on a random basis.
• Keys are only assigned to the channels and ports used to connect to the links in
the aggregated link group.
• The number of keys assigned to the channels / ports is based on the split of
capacity between the links. For example, if the aggregated group comprises two
links of equal capacity, the keys are assigned as 8/8. But for a 300 Mbps link
aggregated with a 150 Mbps link, the former should be allocated 11 keys, and
the latter 5 keys. The number of keys applied must always total 16.
9500 MXC User Manual
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Vol. IV-7-77