Administrator Guide

VLT Unicast Routing
VLT unicast routing is a type of VLT peer routing that locally routes unicast packets destined for the L3 endpoint of the VLT peer. This
method avoids sub-optimal routing. Peer-routing syncs the MAC addresses of both VLT peers and requires two local DA entries in TCAM.
If a VLT node is down, a timer that allows you to configure the amount of time needed for peer recovery provides resiliency. You can
enable VLT unicast across multiple configurations using VLT links. You can enable ECMP on VLT nodes using VLT unicast.
VLT unicast routing is supported on both IPv4 and IPv6. To enable VLT unicast routing, both VLT peers must be in L3 mode. Static route
and routing protocols such as RIP, OSPF, ISIS, and BGP are supported. However, point-to-point configuration is not supported. To enable
VLT unicast, the VLAN configuration must be symmetrical on both peers. You cannot configure the same VLAN as Layer 2 on one node
and as Layer 3 on the other node. Configuration mismatches are logged in the syslog and display in the show vlt mismatch command
output.
If you enable VLT unicast routing, the following actions occur:
• L3 routing is enabled on any new IP address / IPv6 address configured for a VLAN interface that is up.
• L3 routing is enabled on any VLAN with an admin state of up.
NOTE: If the CAM is full, do not enable peer-routing.
NOTE: The peer routing and peer-routing-timeout is applicable for both IPv6/ IPv4.
Configuring VLT Unicast
To enable and configure VLT unicast, 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 peer-routing timeout.
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.
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Virtual Link Trunking (VLT)