LAG & MCLAG

LAG is a method of bundling ethernet interfaces to logically form one higher bandwidth interface between two network devices. In Cisco and Arista this feature is known as etherchannel.

Below description is from the official Alcatel-Lucent documentation for SR 7750.

LAG

Link Aggregation Groups (LAGs) can be configured to increase the bandwidth available between two network devices, depending on the number of links installed. LAG also provides redundancy in the event that one or more links participating in the LAG fail. All physical links in a given LAG links combine to form one logical interface.

Hardware capabilities:

• The LAG load sharing is executed in hardware, which provides line rate forwarding for all port types.

+++Software capabilities:
• The Alcatel-Lucent solution conforms to the IEEE LAG implementation including
dynamic costing and LAG port threshold features. The dynamic cost and LAG port
threshold features can be enabled even if the second node is not an Alcatel-Lucent router.

Dynamic cost

Dynamic cost can be enabled with the config>lag dynamic-cost command or by the
action specified in the config>lag>port-threshold command.
If dynamic cost is enabled and the number of active links is greater than the port
threshold value (0-7 or 0-15)

LAG port threshold

The LAG port threshold feature allows configuration of the behavior, once the
number of available links in a LAG falls below or is equal to the specified threshold.
Two options are available:

LAG configurations should include at least two ports. Other considerations include:

• A maximum of eight ports can be included in a LAG. All ports in the LAG must share the same characteristics (speed, duplex, hold-timer, etc.). The port characteristics are inherited from the primary port.

• Autonegotiation must be disabled or set limited mode for ports that are part of a LAG to guarantee a specific port speed.

• Ports in a LAG must be configured as full duplex.

Configuration Example;

7750-LAG.jpg
#7750-R1

 port 1/1/1
        description "to-7750-R2 p 1/1/1"
        ethernet
            no autonegotiate
        exit
        no shutdown
    exit
    port 1/1/2
        description "to-7750-R2 p 1/1/2"
        ethernet
            no autonegotiate
        exit
        no shutdown

!---Make sure the ports are not assigned to any router interface and are in shutdown state. Then configure the LAG in the configure mode. 

lag 1
        port 1/1/1 
        port 1/1/2 
        dynamic-cost
        port-threshold 1 action down
        no shutdown
    exit

!--- Make sure the port-threshold value is equal to or less then the number of ports joining the link aggregation group. I have configured 1 so if 1 out of the two ports goes down the LAG interface will go down. If a value of more then two is configured this LAG interface will never become operational.

Now do the similar configuration on 7750-R2

#7750-R2
port 1/1/1
        description "to-7750-R1 p 1/1/1"
        ethernet
            no autonegotiate
        exit
        no shutdown
    exit
    port 1/1/2
        description "to-7750-R1 p 1/1/2"
        ethernet
            no autonegotiate
        exit
        no shutdown
#
lag 1
        port 1/1/1 
        port 1/1/2 
        dynamic-cost
        port-threshold 1 action down
        no shutdown
    exit

Once the ports are configured do not forget to configure the L3 interface (router interface) if the port was assigned to it before, but now instead of adding the port number we will add the lag-id.

    router 
        interface "R1-to-R2"
            address 10.100.100.1/30
            port lag-1         <------------------lag-id inserted here
            no shutdown
        exit

Now we can verify the LAG-1 interface is up and operational.

A:7750-R1# show lag 1 detail 

===============================================================================
LAG Details
===============================================================================
Description        : N/A
-------------------------------------------------------------------------------
Details
-------------------------------------------------------------------------------
Lag-id              : 1                     Mode                 : network
Adm                 : up                    Opr                  : up
Thres. Exceeded Cnt : 1                     Port Threshold       : 1
Thres. Last Cleared : 03/10/2021 00:02:55   Threshold Action     : down
Dynamic Cost        : true                  Encap Type           : null
Configured Address  : da:00:ff:00:01:42     Lag-IfIndex          : 1342177281
Hardware Address    : da:00:ff:00:01:42
Hold-time Down      : 0.0 sec               Port Type            : standard
Per-Link-Hash       : disabled              
Include-Egr-Hash-Cfg: disabled              
Per FP Ing Queuing  : disabled              Per FP Egr Queuing   : disabled
Per FP SAP Instance : disabled              
LACP                : disabled              
Standby Signaling   : lacp                  
Port weight speed   : 0 gbps                Number/Weight Up     : 2
Weight Threshold    : 0                     Threshold Action     : down

-------------------------------------------------------------------------------
Port-id        Adm     Act/Stdby Opr     Primary   Sub-group     Forced  Prio 
-------------------------------------------------------------------------------
1/1/1          up      active    up      yes       1             -       32768
1/1/2          up      active    up                1             -       32768
===============================================================================

From the above show lag 1 detail we can see that the port is administratively UP and Operational state is also UP. Both interfaces participating to form this lag interface are shown at the bottom of the output.

MC-LAG

Multichassis Link Aggregation Group is a form of interface bundling where a network element can connect to two separate physical devices to virtually act as one.

More on this topic coming soon ……

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