VPC Traffic Flows

VPC Traffic Flows

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

• VPC traffic flow via ARP Request
• VPC traffic flow via ARP Reply
• Data Traffic between Source & Destination

ARP Request:

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

• Let’s suppose Host A With MAC A ( 10.10.10.2 ) want to talk to Host B with MAC B ( 10.10.10.3) 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: 10.10.10.2
  4. DIP: 10.10.10.3
• 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 S2 will not 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: 10.10.10.3
  4. DIP: 10.10.10.2