Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.112.0P1 - Page 1005

Synchronization of IPv6 ND Entries in a Non-VLT Domain Layer 3 VLT provides a higher resiliency at the Layer 3 forwarding level. Routed VLT allows you to replace VRRP with routed VLT to route the 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 VLANs so that each node can complete Layer 3 forwarding on behalf of each other. Whenever you configure a VLAN on a VLT node, this information is communicated to the peer VLT node regardless of whether the VLAN configured is a VLT or a non-VLT interface. If the VLAN operational state (OSTATE) is up, dynamically learned ND entry in VLT node1 synchronizes to VLT node2. Tunneling IPv6 ND in a VLT Domain Tunneling 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 the destined VLT node. This may occur because of LAG hashing at the ToR switch. The tunneled NA carries some control information along with it so that the appropriate VLT node can mimic the ingress port as the VLT interface rather than pointing to VLT node's interconnecting link (ICL link). The overall tunneling process involves the VLT nodes that are connected from the ToR through a LAG. The following illustration is a basic VLT 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 destination address field set to the unicast MAC address of the initial NS sender. This solicited NA must be tunneled when they reach the wrong peer. • Sometimes NA messages are sent by a node when its link-layer address changes. This NA message is sent as an unsolicited NA to advertise its new address and the destination address field is set to the link-local scope of all-nodes multicast address. This unsolicited NA 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 south of the VLT domain, Unit1 and Unit2 are connected to a ToR switch named Node B. Also, Unit1 is connected to another node, Node A, and Unit2 is linked to a node, Node C. When an NS traverses from Unit2 to Node B(ToR) and a corresponding NA reaches Unit1 because of LAG hashing, this NA is tunneled to Unit 2 along with some control information. The control information present in the tunneled NA packet is 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 through which tunneled NA arrived. Virtual Link Trunking (VLT) 1005