Administrator Guide

allows you to replace VRRP with routed VLT to route the traffic from the Layer 2 access nodes. With neighbor discovery (ND)
synchronization, both the VLT nodes perform Layer 3 forwarding on behalf of each other.
The neighbor entries are typically learned by a node using neighbor solicitation (NS) and ND messages. These NS or neighbor
advertisement (NA) messages can be either destined to the VLT node or to any nodes on the same network as the VLT
interface. These learned neighbor entries are propagated to another VLT node so that the peer does not need to relearn the
entries.
IPv6 Peer Routing
When you enable peer routing on VLT nodes, the MAC address of the peer VLT node is stored in the ternary content
addressable memory (TCAM) space table of a station. If the data traffic destined to a VLT node, node1, reaches the other
VLT node, node2, owing to LAG-level hashing in the ToR switch, it is routed instead of forwarding the packet to node1. This
processing occurs because of the match or hit for the entry in the TCAM of the VLT node2.
Synchronization of IPv6 ND Entries in a VLT Domain
Because the VLT nodes appear as a single unit, the ND entries learned via the VLT interface are expected to be the same on
both VLT nodes. VLT V6 VLAN and neighbor discovery protocol monitor (NDPM) entries synchronization between VLT nodes is
performed.
The VLT V6 VLAN information must synchronize with peer VLT node. Therefore, both the VLT nodes are aware of the VLT
VLAN information associated with the peers. The CLI configuration and dynamic state changes of VLT V6 VLANs are notified to
peer VLT node. The ND entries are generally learned by a node from Neighbor advertisements (NA).
ND entries synchronization scenarios:
● When you enable and configure VLT on both VLT node1 and node2, any dynamically learned ND entry in VLT node1 be
synchronizes instantaneously to VLT node2 and vice-versa. The link-local address also synchronizes if learned on the VLT
VLAN interface.
● During failure cases, when a VLT node goes down and comes back up all the ND entries learned via VLT interface must
synchronize to the peer VLT node.
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.
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Virtual Link Trunking (VLT)