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VLT DOMAIN mode
peer-routing—timeout value
value: Specify a value (in seconds) from 1 to 65535. The default value is infinity (without configuring the timeout).
VLT Multicast Routing
VLT multicast routing is a type of VLT peer routing that 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 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.
NOTE:
Reduce the multicast peer-routing-timeout value to 10 seconds to clear the (S,G) entry in mroute in primary VLT
peer. Also, the MLD leave packet must be sent after the unicast route convergence.
4. Configure a PIM-SM compatible VLT node as a designated router (DR). For more information, refer to Configuring a
Designated Router.
5. Configure a PIM-enabled external neighboring router as a rendezvous point (RP). For more information, refer to Configuring
a Static Rendezvous Point.
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Virtual Link Trunking (VLT)