Service Manual
During failure cases, when a VLT node goes down and comes back up all the ND entries learned via VLT interface must be
synchronized to the peer VLT node
Synchronization of IPv6 ND Entries in a Non-VLT Domain
L3 VLT provides a higher resiliency at the Layer 3 forwarding level. Routed VLT enables you to replace VRRP with routed VLT to
route the trac from L2 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.
In the present design the NDPM entries learned on non-VLT interfaces are synchronized with the peer VLT nodes in case the ND
entries are learned on spanned VLANs so that each node can do L3 forwarding on behalf of each other. Whenever a VLAN is
congured on VLT node, this information is communicated to the peer VLT node regardless of whether the VLAN congured is a VLT
or a non-VLT interface. If the VLAN operational state (OSTATE) is up, dynamically learned ND entry in VLT node1 is synchronized to
VLT node2.
Tunneling of 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 top of the rack (ToR) switch. The tunneled NA will carry
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 TOR through a LAG. The picture below 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 types of scenarios:
• Sometimes NA messages are sent by a node when its link-layer address is changed. This NA message is sent as an unsolicited
NA to advertise its new address and the destination address eld is set to the link-local scope of all-nodes multicast address. This
unsolicited NA packet need not be tunneled.
• 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 eld set to the unicast MAC address of the initial NS sender. This solicited NA need to be tunneled when
they reach the wrong peer.
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 must be tunneled to Unit 2 along with some control information. The control information
present in the tunneled NA packet is processed in such a way 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)
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