How does a router communicate with a switch to enable inter-VLAN routing?

[ProfRon]

I remember setting up inter-VLAN routing for the first time in my old job, and it clicked for me how routers and switches team up. You see, when you want devices in different VLANs to talk to each other, the switch handles the local traffic within each VLAN, but it needs the router to bridge those gaps. I always think of the switch as the traffic cop inside a building, directing cars (packets) on the same floor, but the router acts like the elevator operator connecting floors.

Let me walk you through it like I would if we were troubleshooting together over coffee. First off, you configure your switch ports to trunk mode so it can carry multiple VLANs over a single link to the router. I use 802.1Q tagging for that-it's the standard way. The switch slaps a VLAN ID tag on each Ethernet frame before sending it out the trunk port. That tag tells the router exactly which VLAN the frame belongs to. Without it, the router would be clueless about separating the traffic.

On the router side, I set up subinterfaces for each VLAN. For example, if you have VLAN 10 for sales and VLAN 20 for engineering, I create something like GigabitEthernet0/0.10 for VLAN 10 and .20 for VLAN 20. Each subinterface gets its own IP address in the respective subnet, say 192.168.10.1 for VLAN 10. The router's main interface, like Gi0/0, I configure as a trunk too, matching the switch's settings. That way, when the switch sends a tagged frame from VLAN 10, the router strips the tag on the subinterface, processes the packet at Layer 3, and routes it if needed.

You might wonder how the return traffic works. I make sure the default gateway on devices in each VLAN points to the router's subinterface IP. So, a PC in VLAN 10 sends a packet destined for VLAN 20 to its gateway, which is the router. The router receives it untagged on the subinterface (after decapsulating), looks up the route, and sends it back out the trunk to the switch, re-tagging it for VLAN 20. The switch then forwards it untagged to the end device in that VLAN. It's all about that encapsulation and decapsulation dance between them.

I ran into a snag once where the trunk wasn't negotiating properly, and traffic just looped or dropped. Turns out, I forgot to allow the VLANs on the trunk port with a switchport trunk allowed vlan command. You have to specify which VLANs can traverse that link, or else the switch ignores them. Also, I always enable trunking on both ends-on the switch with switchport mode trunk, and on the router with encapsulation dot1q. If you're using a Cisco router, that command is key. Mismatched MTU or speed/duplex can kill it too, so I double-check those.

Now, if you go with a multilayer switch that does Layer 3 routing itself, you might not need a separate router, but since your question is about router-switch combo, I'll stick to that. In a router-on-a-stick setup, everything funnels through that single trunk link, which can bottleneck if you have heavy traffic. I mitigated that in one network by upgrading to a faster port, like 10G, but you have to watch for that single point of failure. Redundancy with HSRP or VRRP on the router helps if you want failover.

You know, I love how this setup scales for small offices. I deployed it at a friend's startup where they had guest Wi-Fi on VLAN 30 isolated from the internal net. The router enforced ACLs on the subinterfaces to block unauthorized access. You can even add NAT if the VLAN needs internet without full routing. Just configure ip nat inside on the subinterface and tie it to your WAN.

One tip I give everyone: Test with pings from a host in one VLAN to another. I use extended pings with source interface specified to verify. If it fails, I jump on the switch with show interfaces trunk to see what's tagged and allowed. On the router, show ip interface brief confirms the subinterfaces are up. Debugging with debug ip packet helps, but I turn it off quick because it floods the logs.

I also think about security here. You don't want VLAN hopping, so I disable DTP on unused ports with switchport nonegotiate. And CDP or LLDP can leak info, so I prune those. In inter-VLAN, the router becomes the chokepoint for policy, so I apply access lists inbound on subinterfaces to filter what crosses.

Expanding on that, suppose you have VoIP phones in VLAN 40. I prioritize that traffic with QoS on the trunk-marking with CoS or DSCP so the router and switch honor it. The switch pushes the tagged frames, and the router respects the markings when routing. I set it up once for a call center, and it made a huge difference in call quality.

If you're simulating this in a lab, I recommend Packet Tracer or GNS3. You build the topology quick: connect switch to router with a straight cable on trunk ports, assign VLANs to switch ports, set subinterfaces, and boom, pings work across VLANs. I spent weekends tinkering like that early in my career, and it built my confidence.

Troubleshooting VLAN mismatches is common too. If a device can't reach the gateway, I check if its port is in the right VLAN with show vlan brief. Sometimes admins forget to trunk the right VLANs, so traffic arrives but gets dropped. I always document the VLAN database-export it or something simple.

In bigger setups, you might use EtherChannel for the trunk to bundle links, but that's overkill for basics. I keep it simple: trunk, tags, subinterfaces, routes. The router and switch chat via those tagged frames, and that's the magic.

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