Link Aggregation
Over View of the IEEE 802.3ad-2000
(clause 43)
Authors: Anthony Mwingira, Jon
Sterritt
Bridge Functions Consortium
Introduction
Link Aggregation (LA): The combination of
multiple physical links to function as a single
logical link.
Link Aggregation Group (LAG): A group of
physical links that make up a single
aggregation. Each physical port may be a
member of a single LAG.
Link Aggregation Control Protocol (LACP): The
protocol specified by IEEE 802.3.ad-2000 to
outline standardized Link Aggregation.
Possible Problems and Issues

By combining multiple physical links to form a
single logical link two notable issues could
occur:


Frame duplication, “flooding”
Frame disordering
Both of these issues are addressed by LACP.
Goals and Objectives of LA





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Increased bandwidth – multiple links combined into one
logical link
Linearly incremental bandwidth – increase in unit
multiples
Increased availability – failure of a single link within aggr.
Need not cause MAC client failure
Load Sharing – MAC Client traffic may be distributed
across links
Auto Configuration – in even of changes in physical
connection
Rapid configuration and reconfiguration – also in the
event of changes in physical connection

Deterministic behavior – independent of
algorithm chosen( implementer can choose
internal algorithm). The configuration can be
made to resolve deterministically i.e.
resulting aggr. Can be made independent of
the order in which events occur, and be
completely determined by the capabilities of
the individual links and their physical
connectivity.

Low risk of duplication or mis-ordering – during
link reconfiguration or steady-state operation there
is a high probability that frames are neither
duplicated or mis-ordered.

Backwards compatibility with aggregationunaware devices - Link that can not take part in
an aggregation operate as normal, individual links.

Multipoint Aggregations – does not support
aggregations among more than two systems;
multiple aggregations are allowed, but 3 systems
cannot share a single Link Aggregation Group.
Limitations:
MACs – supported only on links using
the IEEE 802.3 MAC
 Half-duplex operation – supported
only on point-to-point links with MACs
operating in full duplex mode.
 Multiple data rates – All Links in a
Link Aggregation Group operate at
the same data rate (e.g. 10 Mb/s, 100
Mb/s or 1000 Mb/s )

LAG Operation






Frame Distribution - Responsible for taking frames from MAC Client and
submitting them for transmission on appropriate ports.
Frame Collection – Responsible for passing frames received from the various
ports to the MAC Client -> also includes Marker Responder ( Marker protocol
– allows the distribution function of an Actor’s Link Aggregation sub-layer to
request the transmission of a Marker PDU on a given link)
Aggregator Parser/ Multiplexers. When transmitting these blocks pass frame
requests to the appropriate port. When receiving, distinguish frames and pass
them to appropriate entity.
Aggregator – This is the combination of frame Distribution and Collection, along
with the aggregator Parser/ Multiplexers.
Aggregation Control – Responsible for configuration and control of the Link
Aggregation.
Control Parser/Multiplexers – On transmit -> pass frame transmission requests
from the Aggregator and Control entities to the appropriate port. On receive,
these blocks distinguish Link Aggregation Control PDUs from other frames,
passing LACPDUs to the appropriate sub-layer entity, and all other frames to the
Aggregator.
Principles of Link Aggregation

Note: Link Aggregation allows a MAC Client to treat a set
of one or more ports as if it were a single port.

A Mac Client communicates with a set of ports through
an Aggregator. An Aggregator binds to one or more ports
within a System.

It is the responsibility of an Aggregator to distribute frame
transmissions from the MAC Client to the various ports,
and to collect received frames from the ports and pass
them to the MAC Client transparently.

A given port will bind to (at most) a single Aggregator at
any time. A MAC Client is served by a single Aggregator
at a time.

The binding of ports to Aggregators within a
System is managed by the Link Aggregation
Control function for that System.

a unique, globally administered MAC address.

Binding may be under manual control through direct manipulation
of the state variables of Link Aggregation (e.g. Keys) by net
manager. Automatic determination, configuration, binding, and
monitoring my occur through the use of Link Aggregation Control
Protocol (LACP)

Frame ordering must be maintained. The distributor ensures that
all frames of a given conversation are past to a single port. They
are to be passed in order they were received from that port.

Conversations may be moved among ports within aggregation,
both for load balancing and to maintain availability in the event of
a link failures.

Each port is assigned a unique , globally administered MAC
address. ( Prospective of a MAC Client)

Each Aggregator is assigned a unique, globally administered
MAC address.
Link Aggregation Control
Link Aggregation Control configures and
controls the Link Aggregation sub layer using
static information local to the function and
dynamic information exchanged by means of
the Link Aggregation Control Protocol.
For Each aggregatable port in the System:
 Maintains Configuration information to
control aggregation


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Exchange configuration information with other
with other Systems to allocate the link to a Link
Aggregation Group. NOTE a given link is allocated
to, at most, one Link Aggregation Group at a
time.
Attaches the port to the Aggregator used by the Link
Aggregator Group, and detaches the port from the
Aggregator when it is no longer used by the Group
Uses information from the Partner Systems’ Link
Aggregation Control entity to enable or disable the
Aggregator or Distributor


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Checking that candidate links are actually be
aggregated.
Controlling link addition to Aggr. Group and
the creation of a Group if necessary
Monitoring the status of aggregated links
Removing a Link from a Link Aggr. Group if
its membership is no longer valid, and
removing the group if it no longer has any
member links.
Test Suite – General How To

See Cisco Document, most devices work just
like Cisco with different variations, all the
following could port a port into LAG1 in active
mode depending on the device.




“channel-group 1 mode active” (Cisco)
“channel-group 1 mode auto” (some Cisco)
“port-channel” 1 mode auto
“LACP mode auto”
“LACP key 1” (Foundry uses keys, no LAG’s)
How To Configure LAG’S
Different Strategies by Vendor
#1 Cisco - go into the interface prompt and put that
interface into a LAG, i.e.,“channel-group”
#2 Dell - command line, almost identical to Cisco, Web
Interface the ports are configured from the LAG
menu
#3 HP, Nortel - LAG’s configured as “trunks” that run
LACP as a protocol.
#4 3Com – From menu driven command line ports
must be activated for LACP (port menu) and then
added to an activated LAG (Link Agg. Menu).
#5 Other “odd” implementations require setting keys,
most devices require ports assigned to a LAG, but
some require setting the Key for each port, the result
is equivalent.
More About Configuration

#1 Configure LAG’s, some tests require multiple LAG’s

Complete Cabling/Setup

Ensure that for the device you are working on there is
not a way to configure the LAG as an interface – i.e.,
sometimes you can configure the group as if it was a
port – allow speed, enable/disable, and other
configurations to be executed on the entire group of
physical links that make up the LAG

See the Dell web Interface:
Ports->LAG Configuration
See the Cisco Interface:
int port-channel <PORT_CHAN>
Trouble-Shooting/Testing Continued

Line Taps: Very useful when diagnosing
issues with LACP



Verify that LACP PDU’s are being transmitted
Verify that both devices are correctly transmitting
traffic
Usually if inter-operability problems as suspect it
will be determined that one device is not
transmitting or receiving frames on it’s aggregated
links
Trouble-Shooting/Testing
When a port is enabled for active LACP, PDU’s
must be transmitted from that port:
MAC 01:80:C2:00:00:02
Type/Length 8809
Also, check the status of the LAG’s on both devices; LAG status
should be “bundl” (Cisco) “oper”, “operational”, “active”, or
equivalent and NOT “indep” (Cisco), “down”, “dis”, etc..
With STP DISABLED traffic should be passed over a multi-port
LAG without flooding or duplication, with multiple LAG’s flooding
should occur unless there is a protocol in place to prevent loops.
Note: Some devices will disable ports in the event of flooding.
Most LA testing is executed with STP disabled for this reason – if
LACP fails we want to note flooding.
Specific Device Configuration
Cisco – See My Cisco Document (word doc)
 The following is basic Dell CLI config.
Note: Web Interface Config is very easy, simply use the web interface
LAG menu and specify a LAG and “L” for LACP in the table.

console# conf
console(config)# int eth g1
console(config-if)# channel-group 1 mode auto
console(config-if)# exit
console(config)# int eth g2
console(config-if)# channel-group 2 mode auto
console# show lacp port 1
Console#show lacp port 1
Port-Channel ch1
Port Type Gigabit Ethernet
Produces the following
Attached Lag id:
LAG configuration.
Actor
System Priority:1
MAC Address: 00:30:ab:cd:66:55
Admin Key:
25
Oper Key:
25
Partner
System Priority:0
MAC Address: 00:00:00:00:00:00
Oper Key:
0
console# show lacp port 2
Port-Channel ch2
Port Type Gigabit Ethernet
Attached Lag id:
Actor
System Priority:1
MAC Address: 00:30:ab:cd:66:55
Admin Key:
26
Oper Key:
26
Partner
System Priority:0
MAC Address: 00:00:00:00:00:00
Oper Key:
0
3Com 4400 (RSTP) Config
Login: admin
Password:
Menu options: --------------3Com SuperStack 3 Switch 4400--------------bridge
- Administer bridge-wide parameters
feature
- Administer system features
gettingStarted - Basic device configuration
logout
- Logout of the Command Line Interface
physicalInterface - Administer physical interfaces
protocol
- Administer protocols
security
- Administer security
system
- Administer system-level functions
trafficManagement - Administer traffic management
Type ? for help
----------------------------------------- (1)--------------------------Select menu option: bridge
Menu options: --------------3Com SuperStack 3 Switch 4400--------------addressDatabase - Administer bridge addresses
broadcastStormCont - Enable/disable broadcast storm control
linkAggregation - Administer aggregated links
multicastFilter - Administer multicast filtering
port
- Administer bridge ports
resilientLinks - Administer resilient links
spanningTree
- Administer spanning tree
summary
- Display summary information
vlan
- Administer VLANs
Type "quit" to return to the previous menu or ? for help
----------------------------------------- (1)--------------------------Select menu option (bridge): linkA
Menu options: --------------3Com SuperStack 3 Switch 4400--------------detail
- Display detailed information
modify
- Modify aggregated links
summary
- Display summary information
Type "quit" to return to the previous menu or ? for help
----------------------------------------- (1)---------------------------
Select menu option (bridge/linkAggregation): mod
Menu options:
addPort
linkState
partnerID
removePort
--------------3Com SuperStack 3 Switch 4400--------------- Add a port to an aggregated link
- Enable/disable an aggregated link
- Set the partner ID for an aggregated link
- Remove a port from an aggregated link
Type "quit" to return to the previous menu or ? for help
----------------------------------------- (1)--------------------------Select menu option (bridge/linkAggregation/modify): addPort
Select aggregated link index (1-4): 1
Select ports (unit:port...,?): 1:1
Select menu option (bridge/linkAggregation/modify): q
Menu options: --------------3Com SuperStack 3 Switch 4400--------------addressDatabase - Administer bridge addresses
broadcastStormCont - Enable/disable broadcast storm control
linkAggregation - Administer aggregated links
multicastFilter - Administer multicast filtering
port
- Administer bridge ports
resilientLinks - Administer resilient links
spanningTree
- Administer spanning tree
summary
- Display summary information
vlan
- Administer VLANs
Type "quit" to return to the previous menu or ? for help
----------------------------------------- (1)--------------------------Select menu option (bridge): port
Menu options: --------------3Com SuperStack 3 Switch 4400--------------defaultPriority - Sets the default 802.1D priority for untagged frames
detail
- Display detailed information
lacpState
- Enable/disable Link Aggregation Control Protocol
stpCost
- Set the Spanning Tree path cost parameter
stpFastStart
- Enable/disable Spanning Tree fast start
summary
- Display summary information
Type "quit" to return to the previous menu or ? for help
----------------------------------------- (1)--------------------------Select menu option (bridge/port): lacpS
Select ports (unit:port...,?): 1:1
Enter new value (enable,disable)[enable]: en
Select menu option (bridge/port):
HP Configuration
HP ProCurve Switch 4172GL
8-Jan-1990 0:35:50
==========================- CONSOLE - MANAGER MODE -==========================18
Main Menu
1. Status and Counters...
2. Switch Configuration...
3. Console Passwords...
4. Event Log
5. Command Line (CLI)
6. Reboot Switch
7. Download OS
8. Run Setup
9. Stacking...
0. Logout
Displays the event log.
To select menu item, press item number, or highlight item and press <Enter>.
Enter 2 to get to the following menu:
HP ProCurve Switch 4172GL
8-Jan-1990 0:38:
==========================- CONSOLE - MANAGER MODE -==========================
Switch Configuration Menu
1. System Information
2. Port/Trunk Settings
3. Network Monitoring Port
4. Spanning Tree Operation
5. IP Configuration
6. SNMP Community Names
7. IP Authorized Managers
8. VLAN Menu...
0. Return to Main Menu...
Configures system-level information including system identification.
To select menu item, press item number, or highlight item and press <Enter>.
Enter 2 to get to the following menu:
HP ProCurve Switch 4172GL
8-Jan-1990 0:42:13
==========================- CONSOLE - MANAGER MODE -==========================18
Switch Configuration - Port/Trunk Settings
Port Type
Enabled
Mode
Flow Ctrl Group Type
---- --------- + ------- ------------ --------- ----- ----A1 100FX | Yes
100FDx
Disable
A2
| Yes
Disable
A3
| Yes
Disable
B1 10/100TX | Yes
Auto
Disable
B2 10/100TX | Yes
Auto
Disable
B3 10/100TX | Yes
Auto
Disable Trk1 LACP
B4 10/100TX | Yes
Auto
Disable Trk1 LACP
B5 10/100TX | Yes
Auto
Disable Trk2 LACP
B6 10/100TX | Yes
Auto
Disable Trk2 LACP
B7 10/100TX | Yes
Auto
Disable
B8 10/100TX | Yes
Auto
Disable
B9 10/100TX | Yes
Auto
Disable
Actions-> Cancel
Edit
Save
Use the arrow keys
and space bar to
configure.
Notice Two LAG’s of
Two ports each.
Help
Cancel changes and return to previous screen.
Use arrow keys to change action selection and <Enter> to execute action.