Synchronization of IPv6 ND Entries in a Non-VLT DomainLayer 3 VLT provides a higher resiliency at the Layer 3 forwarding level. Routed VLT allows you to replace VRRP with routed VLT to routethe traffic from Layer 2 access nodes. With ND synchronization, both the VLT nodes perform Layer 3 forwarding on behalf of each other.Synchronization of NDPM entries learned on non-VLT interfaces between the non-VLT nodes.NDPM entries learned on non-VLT interfaces synchronize with the peer VLT nodes in case the ND entries are learned on spanned VLANsso that each node can complete Layer 3 forwarding on behalf of each other. Whenever you configure a VLAN on a VLT node, thisinformation is communicated to the peer VLT node regardless of whether the VLAN configured is a VLT or a non-VLT interface. If theVLAN operational state (OSTATE) is up, dynamically learned ND entry in VLT node1 synchronizes to VLT node2.Tunneling IPv6 ND in a VLT DomainTunneling an NA packet from one VLT node to its peer is required because an NA may reach the wrong VLT node instead of arriving at thedestined VLT node. This may occur because of LAG hashing at the ToR switch. The tunneled NA carries some control information alongwith it so that the appropriate VLT node can mimic the ingress port as the VLT interface rather than pointing to VLT node’s interconnectinglink (ICL link).The overall tunneling process involves the VLT nodes that are connected from the ToR through a LAG. The following illustration is a basicVLT setup, which describes the communication between VLT nodes to tunnel the NA from one VLT node to its peer.NA messages can be sent in two scenarios:• NA messages are almost always sent in response to an NS message from a node. In this case, the solicited NA has the destinationaddress field set to the unicast MAC address of the initial NS sender. This solicited NA must be tunneled when they reach the wrongpeer.• Sometimes NA messages are sent by a node when its link-layer address changes. This NA message is sent as an unsolicited NA toadvertise its new address and the destination address field is set to the link-local scope of all-nodes multicast address. This unsolicitedNA packet does not have to be tunneled.Consider a sample scenario in which two VLT nodes, Unit1 and Unit2, are connected in a VLT domain using an ICL or VLTi link. To the southof the VLT domain, Unit1 and Unit2 are connected to a ToR switch named Node B. Also, Unit1 is connected to another node, Node A, andUnit2 is linked to a node, Node C. When an NS traverses from Unit2 to Node B(ToR) and a corresponding NA reaches Unit1 because ofLAG hashing, this NA is tunneled to Unit 2 along with some control information. The control information present in the tunneled NA packetis processed in such a way so that the ingress port is marked as the link from Node B to Unit 2 rather than pointing to ICL link throughwhich tunneled NA arrived.Virtual Link Trunking (VLT) 979
