Unlike VLT unicast routing, a normal multicast routing protocol does not exchange multicast routes between
VLT peers. When you enable VLT multicast routing, the multicast routing table is synced between the VLT
peers. Only multicast routes configured with a Spanned VLAN IP as their IIF are synced between VLT peers.
For multicast routes with a Spanned VLAN IIF, only OIFs configured with a Spanned VLAN IP interface are
synced between VLT peers.
The advantages of syncing the multicast routes between VLT peers are:
• VLT resiliency — After a VLT link or peer failure, if the traffic hashes to the VLT peer, the traffic continues
to be routed using multicast until the PIM protocol detects the failure and adjusts the multicast
distribution tree.
• Optimal routing — The VLT peer that receives the incoming traffic can directly route traffic to all
downstream routers connected on VLT ports.
• Optimal VLTi forwarding — Only one copy of the incoming multicast traffic is sent on the VLTi for
routing or forwarding to any orphan ports, rather than forwarding all the routed copies.
Important Points to Remember
• You can only use one spanned VLAN from a PIM-enabled VLT node to an external neighboring PIM
router.
• If you connect multiple spanned VLANs to a PIM neighbor, or if both spanned and non-spanned VLANs
can access the PIM neighbor, ECMP can cause the PIM protocol running on each VLT peer node to
choose a different VLAN or IP route to reach the PIM neighbor. This can result in issues with multicast
route syncing between peers.
• Both VLT peers require symmetric Layer 2 and Layer 3 configurations on both VLT peers for any
spanned VLAN.
• For optimal performance, configure the VLT VLAN routing metrics to prefer VLT VLAN interfaces over
non-VLT VLAN interfaces.
• When using factory default settings on a new switch deployed as a VLT node, packet loss may occur
due to the requirement that all ports must be open.
• ECMP is not compatible on VLT nodes using VLT multicast. You must use a single VLAN.
Configuring VLT Multicast
To enable and configure VLT multicast, follow these steps.
1 Enable VLT on a switch, then configure a VLT domain and enter VLT-domain configuration mode.
CONFIGURATION mode
vlt domain domain-id
2 Enable peer-routing.
VLT DOMAIN mode
peer-routing
3 Configure the multicast peer-routing timeout.
VLT DOMAIN mode
multicast peer-routing—timeout value
value: Specify a value (in seconds) from 1 to 1200.
4 Configure a PIM-SM compatible VLT node as a designated router (DR). For more information, refer to
Configuring a Designated Router.
Virtual Link Trunking (VLT) 1128
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