Reference Guide

vlt domain domain-id
2. Enable peer-routing.
VLT DOMAIN mode
peer-routing
3. Configure the peer-routing timeout.
VLT DOMAIN mode
peer-routing—timeout value
value: Specify a value (in seconds) from 1 to 65535.
VLT Multicast Routing
VLT multicast routing is supported on the S4820T platform.
VLT Multicast Routing provides resiliency to multicast routed traffic during the multicast routing protocol convergence
period after a VLT link or VLT peer fails using the least intrusive method (PIM) and does not alter current protocol
behavior.
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 cannot configure a VLT node as a rendezvous point (RP), but any PIM-SM compatible VLT node can serve
as a designated router (DR).
• 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
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