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IEEE 802.3ad Link Aggregation (LAG) : What It Is, and What It Is Not

IEEE 802.3ad Link Aggregation (LAG) aggregates multiple network links into a single logical trunk to provide increased bandwidth and redundancy. It distributes traffic across the links on a per-packet basis while ensuring packets from the same conversation stay in order. LAG does not require changing packet formats, reordering packets, or adding significant latency. However, it does not necessarily increase bandwidth linearly and incurs higher costs proportional to the number of links. Improving LAG further would be difficult without changing packet formats or adding large buffers.

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Tariq Mahmood
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181 views13 pages

IEEE 802.3ad Link Aggregation (LAG) : What It Is, and What It Is Not

IEEE 802.3ad Link Aggregation (LAG) aggregates multiple network links into a single logical trunk to provide increased bandwidth and redundancy. It distributes traffic across the links on a per-packet basis while ensuring packets from the same conversation stay in order. LAG does not require changing packet formats, reordering packets, or adding significant latency. However, it does not necessarily increase bandwidth linearly and incurs higher costs proportional to the number of links. Improving LAG further would be difficult without changing packet formats or adding large buffers.

Uploaded by

Tariq Mahmood
Copyright
© Attribution Non-Commercial (BY-NC)
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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IEEE 802.

3ad Link Aggregation (LAG)


what it is, and what it is not
Howard Frazier Broadcom Schelto Van Doorn Intel Robert Hays - Intel Shimon Muller Sun Microsystems Bruce Tolley Solarflare Communications Paul Kolesar CommScope Geoff Thompson Nortel Brad Turner Juniper Networks 17-April-2007 Ottawa

IEEE 802.3 HSSG

Outline
Overview of 802.3ad Link Aggregation Current status and future plans Improving Link Aggregation Summary

IEEE 802.3 HSSG

802.3ad Link Aggregation


Specified in Clause 43 LAG is performed above the MAC LAG assumes all links are:
full duplex point to point same data rate

Provides graceful recovery from link failures Traffic is distributed packet by packet All packets associated with a given conversation are transmitted on the same link to prevent mis-ordering
IEEE 802.3 HSSG
3

802.3ad Link Aggregation

IEEE 802.3 HSSG

802.3ad Link Aggregation


MAC Client distributor MAC PHY MAC PHY collector MAC PHY MAC PHY MAC Client distributor MAC PHY MAC PHY collector MAC PHY MAC PHY

IEEE 802.3 HSSG

802.3ad Link Aggregation


43.2.4 Frame Distributor This standard does not mandate any particular distribution algorithm(s); however, any distribution algorithm shall ensure that, when frames are received by a Frame Collector as specified in 43.2.3, the algorithm shall not cause a) Mis-ordering of frames that are part of any given conversation, or b) Duplication of frames. The above requirement to maintain frame ordering is met by ensuring that all frames that compose a given conversation are transmitted on a single link in the order that they are generated by the MAC Client; hence, this requirement does not involve the addition (or modification) of any information to the MAC frame, nor any buffering or processing on the part of the corresponding Frame Collector in order to re-order frames.

IEEE 802.3 HSSG

802.3ad Link Aggregation


Does not change packet format
No added headers or sequence numbers Type/Length interpretation unchanged

Does not require added buffers


No fragmentation or reassembly

Does not re-order or mis-order packets Does not add significant latency Does not increase the bandwidth for a single conversation Achieves high utilization only when carrying multiple simultaneous conversations Is not transparent to some 802.1 sub-layers
IEEE 802.3 HSSG
7

802.3ad Link Aggregation


Is a very good thing
It does what it was intended to do It is relatively easy to implement and use

Does not always provide a linear multiple of the data rate of a single link
N aggregated links usually do not provide N times the bandwidth

Incurs a linear multiple of the cost of a single link


N aggregated links cost N times as much as a single link, because everything must be replicated

Appears to the user as N individual links, which must be individually managed


IEEE 802.3 HSSG
8

Current status
In the process of being spun out of 802.3 802.3ax project creating IEEE P802.1AX No technical changes After the transfer, 802.1 may undertake enhancements or revisions

IEEE 802.3 HSSG

IEEE 802.3 HSSG

10

Improving Link Aggregation


Can link aggregation be fixed?
Inspect headers deep into packet

or
Add sequence number to packet
Change the packet format

and
Add LARGE buffers to receiver
IEEE 802.3 HSSG
11

Improving Link Aggregation


Why not inspect headers deep into a packet?
Futile if packets are encrypted Layer 2 LAG would need to parse upper layer protocol header formats (layering violation)

Why not change the packet format?


Requires a new 802.3 MAC definition Requires a new 802.3 MAC client interface

Why not add LARGE buffers to the receiver?


Adds LONG delay

Fixing LAG is neither easy, or rewarding


IEEE 802.3 HSSG
12

Summary

LAG is good, but its not as good as a fatter pipe

IEEE 802.3 HSSG

13

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