Dell OS10 VLT Peer Routing: Active-Active Layer 3 Gateway

Dell SmartFabric OS10 does not support stacking; instead, two identical physical switches are paired with Virtual Link Trunking (VLT) to act as one logical switch. By default that virtual switch is still a Layer 2 construct, which means routed traffic often has to cross the VLT interconnect (VLTi) to reach the peer that owns the gateway MAC. VLT peer routing removes that hairpin by letting each peer locally route traffic destined to the other peer's Layer 3 endpoint. This guide walks through the full VLT foundation and then enables peer routing for an active-active default gateway.

VLT Foundation: Domain, VLTi and Port Channel

Before peer routing can work, the VLT domain must be healthy. Start by removing switchport mode from the VLTi ports and creating the domain with the same ID on both peers:

OS10# configure terminal
OS10(config)# interface range ethernet 1/1/1-1/1/2
OS10(conf-range-eth1/1/1-1/1/2)# no switchport
OS10(config)# vlt-domain 1
OS10(conf-vlt-1)# discovery-interface ethernet 1/1/1
OS10(conf-vlt-1)# discovery-interface ethernet 1/1/2

Then set the primary/secondary roles with primary-priority (for example 4096 on the primary peer and 8192 on the secondary), configure a backup link over the out-of-band management network, and build the LAG toward the downstream device. Keep spanning-tree enabled: RPVST+ (default) or RSTP is supported on VLT ports and prevents loops in the VLT domain.

Enabling VLT Peer Routing

Peer routing is enabled inside the VLT domain configuration on both nodes and is applied domain-wide rather than per VLAN or per routing protocol:

OS10# configure terminal
OS10(config)# vlt-domain 1
OS10(conf-vlt-1)# peer-routing enable
OS10(conf-vlt-1)# exit
OS10(config)# exit
OS10# show vlt 1 detail

Peer routing synchronizes the router MAC address across the VLT domain, so a server can send a frame to either peer's gateway MAC and the packet is forwarded locally without crossing the VLTi. The result is an active-active Layer 3 gateway with no dependence on VRRP for normal forwarding.

Design Rules and Restrictions

• Both VLT peers must operate in Layer 3 mode for VLT unicast routing to be enabled.
• VLAN configuration must be symmetrical on both peers: you cannot configure the same VLAN as Layer 2 on one node and Layer 3 on the other.
• Peer routing supports IPv4 and IPv6 unicast; it is not restricted to VLANs running a dynamic routing protocol.
• If the peer link is lost, traffic behavior depends on the VLT failure mode (primary/secondary election), which is why the backup link and consistent priority settings matter.

When both peers route their own traffic and each keeps a default route to the upstream, static or dynamic routing (OSPF/BGP) must be configured on both nodes so the upstream sees consistent next hops. See how the concept compares to multi-chassis designs in our Arista EOS MLAG peer-link and domain setup article.

Verification and Monitoring

OS10# show vlt 1
OS10# show vlt 1 detail
OS10# show vlt mac-inactive
OS10# show vlt role
OS10# show lacp port-channel 1

Confirm the VLT role shows primary on one node and secondary on the other, verify the VLTi is in the up state, and check that the downstream LAG bundles on both peers. If peer-routing traffic is not forwarding, confirm the VLT peer-routing status is enabled on both switches and that the VLANs exist as Layer 3 on both peers. For more background on the underlying LAG mechanics on OS10, read our guide to Dell OS10 port-channel and LACP configuration.

原文链接：https://www.dell.com/support/kbdoc/en-us/000102901/dell-emc-networking-os10-how-to-set-up-virtual-link-trunking-vlt