There are two ways of configuring LAG:

• Static configuration
• Using LACP protocol
  • Verifies port settings between devices (speed, duplex, switchport…)

LACP configuration:

interface Ethernet42
   channel-group 10 mode active      # can be "active" or "passive"

Verification:

show port-channel dense

How does switch know that all links are connected to the same device on other end? It uses system-id to identify the switch (looks like a MAC address).

Operation:

• STP - one peer will be elected as Primary and will be sending STP BPDUs
• Secondary will start sending STP BPDUs when Primary goes down. This happens when Secondary stops receiving keepalive messages from Primary over the peer link
• System MAC inside BPDUs remains the same if Primary goes down, essentially it's an MLAG System ID

Conditions:

• Same or compatible EOS version
• Exactly the same interfaces configuration

• Two switches know they are MLAG peers if the share the same domain-id.
• Two switches need to be connected via peer-link (either L2 port or L2 port channel, with SVI)
  • Control plane (e.g. MAC address table) is synchronised via peer link

no spanning-tree vlan 4094
!
vlan 4094                        # VLAN ID 4094 is recommended 
   name VLAN4094
   trunk group m1peer            # Prevents this VLAN to be added to normal trunk ports
!
interface Vlan4094
   ip address 10.0.0.1/30
!
interface Port-Channel100
   switchport trunk allowed vlan 233,4094
   switchport mode trunk
   switchport trunk group m1peer
!
mlag configuration
   domain-id mlagDomain
   heartbeat-interval 2500
   local-interface Vlan4094      # Peer-link
   peer-address 10.0.0.2
   peer-link Po100
   reload-delay 150
!
interface Port-Channel1
   switchport access vlan 10
   mlag 1                        # Port-channel number on the peer switch
!
interface Ethernet1
   switchport access vlan 10
   channel-group 1 mode active
!

#show mlag
MLAG Configuration:
domain-id                          :               MLAG1
local-interface                    :            Vlan4094
peer-address                       :          10.0.0.130
peer-link                          :      Port-Channel10
hb-peer-address                    :             0.0.0.0
peer-config                        :        inconsistent     # config is wrong!
 
MLAG Status:
state                              :              Active
negotiation status                 :           Connected
peer-link status                   :                  Up
local-int status                   :                  Up
system-id                          :   2a:99:3a:8e:e8:cd
dual-primary detection             :            Disabled
dual-primary interface errdisabled :               False
 
MLAG Ports:
Disabled                           :                   0
Configured                         :                   0
Inactive                           :                   0
Active-partial                     :                   0
Active-full                        :                   2

See what's wrong with the configuration:

#show mlag config-sanity
No global configuration inconsistencies found.
 
Interface configuration inconsistencies:
    Feature                       Attribute       Interfaces       Local value    Peer value
-------------- ------------------------------- ---------------- ----------------- ----------
   bridging       trunk-allowed vlan mlag23             Po23           168-172    82,168-172
   bridging       trunk-allowed vlan mlag24             Po24           168-172    82,168-172

#show mlag interfaces

VARP is an Arista's flavour of FHRP, where both switches in MLAG are active.

!
ip virtual-router mac-address aaaa.bbbb.cccc
!
interface Vlan100
   ip address 10.0.0.252/24
   ip virtual-router address 10.0.0.254
!

Same idea as VARP, but does not require a unique SVI IP on each switch.

!
ip virtual-router mac-address aaaa.bbbb.cccc
!
interface Vlan100
   ip address virtual 10.0.0.254/24
!