Dell S4048–ON Configuration Manual page 947

Consider a situation in which NA for VLT node1 reaches VLT node1 on non-VLT interface and NA for VLT node1 reaches VLT node2
on non-VLT interface. When VLT node1 receives NA on VLT interface, it learns the Host MAC address on received interface. This
learned neighbor entry is synchronized to VLT node2 as it is learned on ICL. If VLT node2 receives a NA packet on non-VLT interface
which is destined to VLT node1, node 2 will lift the NA packet to CPU using an ACL entry then it adds a tunnel header to the
received NA and forwards the packet to VLT node1 over ICL. When VLT node1 received NA over ICL with tunnel header it learns the
Host MAC address on ICL. Host entries learned on ICL will not be synchronized to the VLT peer.
If NA is intended for VLT peer and DIP is LLA of peer, it is lifted to CPU and tunneled to the peer. VLT nodes will drop NA packet, If
NA is received over ICL without tunneling header.
Traffic Destined to VLT Nodes
Hosts can send traffic to one of the VLT nodes using global IP or Link-Local address. When host communicates with VLT node using
LLA and traffic reaches wrong peer due to LAG level hashing in TOR, the wrong peer should route the packet to correct VLT node
though the destination IP is LLA. Consider a case in which traffic destined for VLT node1 reaches VLT node1 on VLT interface and
traffic destined for VLT node1 reaches VLT node2 due to LAG level hashing in TOR.
When VLT node1 receives traffic on VLT interface, it consumes the packets and process it based on the packet type. If VLT node2
receives packet on VLT interface which is destined to VLT node1, it routes the packet to VLT node1 instead of switching the packet
owing to the match that occurs for the neighbor entry in the TCAM table.
If the destination IP address is peers' link-local advertisement (LLA), wrong VLT peer will switch the traffic over ICL. This is achieved
using switching egress object for peers LLA.
VLT host to North Bound traffic flow
One of the VLT peer is configured as default gateway router on VLT hosts. If VLT node receives L3 traffic intended for the other VLT
peer it routes the traffic to next hop instead of forwarding the traffic to the VLT peer. If neighbor entry is not present VLT node will
resolve the next hop. There may be traffic loss during neighbor resolution period.
North-Bound to VLT host traffic flow
When a VLT node receives traffic from north bound intended to the VLT host, it does neighbor entry lookup and routes traffic to VLT
interface. If VLT interface is not operationally up the VLT node routes the traffic over ICL. If neighbor entry is not present the VLT
node will resolve the destination. There may be traffic loss during neighbor resolution period.
VLT host to Non-VLT host traffic flow
When VLT node receives traffic intended to non-VLT host, it routes the traffic over non-VLT interface. If the traffic intended to non-
VLT host reaches wrong VLT peer due to LAG hashing in TOR, the wrong VLT node will resolve the destination over ICL and routes
the traffic over ICL. When Correct VLT node receives this routed traffic over ICL it will switch traffic to non-VLT interface.
Non-VLT host to VLT host traffic flow
When VLT node receives traffic from non-VLT host intended to VLT host, it routes the traffic to VLT interface. If VLT interface is not
operationally up VLT node will route the traffic over ICL.
Non-VLT host to North Bound traffic flow
When VLT node receives traffic from non-VLT host intended to north bound with DMAC as self MAC it routes traffic to next hop.
When VLT node receives traffic from non-VLT host intended to north bound with DMAC as peer MAC it will not forward the packet
to VLT peer instead it will route the traffic to north bound next hop.
North Bound to Non-VLT host traffic flow
When VLT node receives traffic from north bound intended to the non VLT host, it does neighbor entry lookup and routes traffic to
VLT interface. If traffic reaches wrong VLT peer, it routes the traffic over ICL.
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Virtual Link Trunking (VLT)