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 traffic 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 configured on 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 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 field 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 field 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.
1052
Virtual Link Trunking (VLT)