VPC Traffic Flows

VPC Traffic Flows

Posted on Jan 24, 2020 (0)

VPC Traffic Flows

In order to understand the traffic flow, for data to flow, following steps are Nexus follows:

  • ARP Request
  • ARP Reply
  • Data Traffic between Source & Destination

ARP Request:

We will use the following topology to send the traffic from Server A to Sever B

  • Let’s suppose Server With MAC A ( ) want to talk to Server with MAC B ( and Host A will search for Host B MAC address ins it ARP table and will be a MISS.
  • Now Host A will Send ARP Request with following header details:
    1. DMAC : FFFFF
    2. SMAC : MAC A
    3. SIP:
    4. DIP:
  • Now when this ARP request will hit to S3 on Eth1/5, Source MAC A will be learned on this interface on its MAC table.
  • Now as this is ARP, HASH algo will be used between DMAC FFFF and SMAC MAC A, and will find one link from Port-channel vPC 10 and let suppose it will select the port E1/1 and ARP will traverse on it.
  • ARP request will hit to Eth1/1 of S1 and puts its entry on MAC table and because of CFS running the Same SMAC MAC A will also be learned on Eth1/2 or port-channel interface on S2.
  • The ARP request will be forwarded to Eth1/2 and Po100 peer-link on S1.
  • Switch S2 and S4 will receive this broadcast and update its MAC table with its respective receiving port-channel interface also S4 will forward the ARP request to any of vPC member port due to Loop avoidance. Likewise S4 will also not forward this ARP request in vPC 20 but will forward this packet to Eth1/4 where destination Host B resides.

ARP Reply:

  • Now when ARP request is received by Host B , Host B will reply it With ARP Reply and provide its MAC address for which the ARP request has come with following header details:
    1. SMAC : MAC B
    2. DMAC: MAC A
    3. SIP:
    4. DIP:

  • Now when ARP reply is received on S4 Eth1/4 interface, S4 updates its MAC address table and then search for DMAC MAC A in MAC table where it will found the MAC A is reachable on PO20 and then it does the HASH also to find the port member in Port-channel Po20, let’s suppose HASH results to Eth1/1 of vPC 20.
  • S4 Will forwards the ARP Reply on ETH1/1 of S4 and then the reply reached to S2 and on interface E1/1 and the S2 will update the MAC B on receiving interface and on same time the MAC B will also be learned on S1 Eth1/2 vPC member interface and is updated in MAC address Table.
  • On S2, S2 will look up its MAC address table for DMAC A and find the hits on port Eth1/2 and will forwards the packet on same interface.
  • Finally S1 will receive this ARP reply on Eth1/2 of vPC port-channel Interface 10 and will also update its MAC table and will forwards the ARP Reply to HOST A.

Data Traffic between Source & Destination

  • Now once MAC Table is correctly populated then HOST A will send the data packet to HOST B,
  • S1 will receive this data packet and will look for DMAC MAC B on its MAC Table, where it will be hit on Port-channel 10.
  • S1 will do HASH also to find the one port of the Port-channel member ports and let’s say that HASH results to ETH1/2 of S1
  • S1 will forward that data packet to Eth1/2 and Data packet reaches to S2 and S2 will again look the MAC address table and find that MAC B is reachable via Eth1/1 of Port-channel 10 and will forwards that packet to S4
  • S4 will receive that packet and finally send to Host B.

Same ways the Return will flow from Host B to Host A.


  • Super Duper Like


Please login here to comment.