Setup Guide

redirected to the designated router (DR) if they are incorrectly hashed. In addition to being first-hop or last -hop routers, the
peer node can also act as an intermediate router.
On a VLT-enabled PIM router, if any PIM neighbor is reachable through a Spanned Layer 3 (L3) VLAN interface, this must be
the only PIM-enabled interface to reach that neighbor. A Spanned L3 VLAN is any L3 VLAN configured on both peers in a VLT
domain. This does not apply to server-side L2 VLT ports because they do not connect to any PIM routers. These VLT ports can
be members of multiple PIM-enabled L3 VLANs for compatibility with IGMP.
To route traffic to and from the multicast source and receiver, enable PIM on the L3 side connected to the PIM router using the
ip pim sparse-mode command.
Each VLT peer runs its own PIM protocol independently of other VLT peers. To ensure the PIM protocol states or multicast
routing information base (MRIB) on the VLT peers are synced, if the incoming interface (IIF) and outgoing interface (OIF) are
Spanned, the multicast route table is synced between the VLT peers.
To verify the PIM neighbors on the VLT VLAN and on the multicast port, use the show ip pim neighbor, show ip igmp
snooping mrouter, and show running config commands.
You can configure virtual link trunking (VLT) peer nodes as rendezvous points (RPs) in a Protocol Independent Multicast (PIM)
domain.
If the VLT node elected as the designated router fails and you enable VLT Multicast Routing, multicast routes are synced to the
other peer for traffic forwarding to ensure minimal traffic loss. If you did not enable VLT Multicast Routing, traffic loss occurs
until the other VLT peer is selected as the DR.
VLT Routing
VLT Routing refers to the ability to run a dynamic routing protocol within a single VLT domain or between VLT domains (mVLT).
In a single VLT domain, VLT routing allows routing adjacencies to be formed across the VLTi link. In eVLT, routing adjacencies
are formed across the port-channel that connects the two VLT domains.
Because VLT ports are Layer 2 ports and not IP interfaces, VLT Unicast and VLT Multicast routing protocols do not operate
directly on VLT ports. You must add the VLT ports as a member of one or more VLANs and assign IP addresses to these VLANs.
VLT Unicast and VLT Multicast routing protocols require VLAN IP interfaces for operation. Protocols such as BGP, ISIS, OSPF,
and PIM are compatible with VLT Unicast Routing and VLT Multicast Routing.
Layer 2 protocols from the ToR devices to the server are intra-rack and inter-rack. Although no spanning tree is required,
interoperability with spanning trees at the aggregation layer is supported to prevent switching loops from forming due to any
incorrect configuration. . Communication between devices is active-active, with no blocked links. MAC tables are synchronized
between VLT nodes for bridging and you can enable IGMP snooping.
Spanned VLANs
Any VLAN configured on both VLT peer nodes is referred to as a Spanned VLAN. The VLT Interconnect (VLTi) port is
automatically added as a member of the Spanned VLAN. As a result, any adjacent router connected to at least one VLT node on
a Spanned VLAN subnet is directly reachable from both VLT peer nodes at the routing level.
Peer Routing
Peer routing enables one VLT node to act as a proxy gateway for the other peer in a VLT domain. When you enable routing on
VLT peers, you can also enable the peer routing feature.
The following figure shows how a packet is routed when peer routing is not enabled. Due to the hashing algorithm in the port
channel, a packet from a host is sent to either of the VLT port-channel members. If a packet is sent to Peer-1 which is not the
destined gateway for the hosts under the ToR Switch, the packet is switched to the destined VLT peer (Peer-2) using the VLTi
link. Peer-2 then routes the packet to its destination.
Virtual Link Trunking (VLT)
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