Arista L3 Leaf/Spine Step by Step (2c) - VxLAN Bridging, packet walkthru

Step 1: Ping hostX from host2. 

Before that we clear arp so force the host2 to send out broadcast arp req. Also run "bash tcpdump -nvvi vlan2000 arp" to see the arp packet behavior


Step2: host2 sends the arp to vtep2. 



Step 3: Vtep flood ARP req.

Besides flooding out the local vlan ports, the vtep will also do hardware replication to send 1 copy to each remote vtep, which is called HER (head-end-replication)

Please note that, only ONE mlag peer receive this arp request based on the hashing. Because 2 mlag peers share the same loopback, they appear as 1 destination with 2-way ecmp. 



Step 4: Remote Vtep learn and flood

When the remote VTEP receives this arp req, they will do:
1) flood all local ports to reach the destination; 
2) send it to mlag peer via peer link if applied; 
3) learn the MAC address of host2, like

wa462.vtepB1#sh mac address-table dynamic | grep 72b7
2000    444c.a897.72b7    DYNAMIC     Vx1        1       0:00:42 ago

wa463.vtepB2#sh mac address-table dynamic | grep 72b7
2000    444c.a897.72b7    DYNAMIC     Vx1        1       0:00:42 ago

Why flooding to MLAG peer? Say if there is one singly connected host but this arp request was hashed to the other MLAG peer. So we need this step to reach these orphan clients. 

Please note that both MLAG peers assign the srcHost's MAC to interface Vx1. Basically, MLAG peer never learns the MAC via peer link. Instead, they use MLAG message to sync MAC learning. 



Step 5. dstHost unicasts ARP reply back to srcHost

Since VtepX1 nad VtepX2 already learn srcMAC, it knows how to unicast it back to srcVtep

ckp342.vtepX2#sh mac address-table dynamic
Vlan    Mac Address       Type        Ports      Moves   Last Move
----    -----------       ----        -----      -----   ---------
2000    444c.a897.72b7    DYNAMIC     Vx1        1       0:00:03 ago