Inter-VLAN Routing Verification and Troubleshooting

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Inter-VLAN Routing Verification and Troubleshooting

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Lesson format19 sections2 code blocks1 practice itemUpdated Oct 2, 2026

After a weekend change window, users report intermittent ability to reach the file server in a different VLAN. Some work, some don't. You log into the Layer 3 switch and everything looks normal in `show run`. But one specific VLAN's users can't reach anything outside their VLAN. What's happening?

Intermittent inter-VLAN failures almost always trace back to one of a small set of causes. The verification workflow below isolates them quickly.

#The Verification Checklist#link

Is `ip routing` enabled? — check with `show ip route` for connected routes
Is the SVI up? — `show ip interface brief | include Vlan`
Is the SVI's VLAN active? — `show vlan brief`
Is at least one port in the VLAN up? — check access ports on the switch
Does the host have the right gateway? — check host config
Are there ACLs blocking? — `show ip access-lists`
Is the return path working? — route back to the source VLAN exists

#The Scenario Diagnosis#link

Check the SVIs first

root@vulnarex:~#show ip interface brief | include Vlan

Vlan30 is administratively down. Someone issued a `shutdown` on the SVI during the change window and forgot to bring it back. Users in VLAN 30 can talk to each other (Layer 2 still works) but can't route out — no gateway is up.

#The Fix#link

bash

SW1(config)# interface vlan 30
SW1(config-if)# no shutdown
SW1(config-if)# end

SW1# show ip interface brief | include Vlan30

One command. All VLAN 30 users regain inter-VLAN routing immediately. This is one of the most common causes of inter-VLAN failure after a change window — someone shut an SVI to isolate it during testing and never brought it back.

#Other Common Failures#link

Symptom	Likely cause	Fix
VLAN users can't reach other VLANs at all	`ip routing` disabled	Enable with `ip routing`
SVI shows down/down	No active port in that VLAN	Verify VLAN membership, bring up a port
Some hosts work, some don't	Wrong gateway on some hosts (DHCP scope issue, static config error)	Check DHCP scope or static config
Ping by IP works, by name fails	DNS issue, not routing	Check DNS resolver config
Traffic one-way only	Asymmetric routing or ACL blocking return	Check ACLs on both directions
Intermittent, correlated with time of day	STP topology change, or ARP aging issue	Check STP logs and MAC table

#Debugging a Host Perspective#link

★ 20 XPhard

#Structured Workflow#link

★ 20 XPhard

Decision Scenario · 20 XP

One-directional inter-VLAN failure

Hosts in VLAN 10 can reach hosts in VLAN 20, but hosts in VLAN 20 cannot reach VLAN 10. Pings from 10 -> 20 succeed, pings from 20 -> 10 fail. What do you check first?

Select the safest next action.

#Check Your Understanding#link

★ 15 XPmedium

quiz BLOCK (★ 15 XP)

Users in VLAN 40 can ping their gateway but cannot reach any other VLAN. Other VLANs work. What is the most likely cause?

Select your proof vectors above

tip

Inter-VLAN routing problems almost always come down to four things: `ip routing`, SVI state, SVI mask, and ACLs. Keep the checklist in your head. You'll solve 95% of these tickets in under five minutes.

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The Verification Checklist The Scenario Diagnosis The Fix Other Common Failures Debugging a Host Perspective Structured Workflow Check Your Understanding

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October 2, 2026

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MCP Server Config (mcp.json)

{
  "mcpServers": {
    "vulnarex": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-fetch"],
      "env": { "MCP_FETCH_URL": "https://vulnarex.com" }
    }
  }
}

MCP Server Card/.well-known/mcp.json A2A Agent Card/.well-known/agent-card.json API Catalog/.well-known/api-catalog robots.txt/robots.txt

Laboratory sanity code

Isolate active probes on matched virtual networks and keep execution streams sandboxed.

Symptom

Likely cause

Fix

VLAN users can't reach other VLANs at all

`ip routing` disabled

Enable with `ip routing`

SVI shows down/down

No active port in that VLAN

Verify VLAN membership, bring up a port

Some hosts work, some don't

Wrong gateway on some hosts (DHCP scope issue, static config error)

Check DHCP scope or static config

Ping by IP works, by name fails

DNS issue, not routing

Check DNS resolver config

Traffic one-way only

Asymmetric routing or ACL blocking return

Check ACLs on both directions

Intermittent, correlated with time of day

STP topology change, or ARP aging issue

Check STP logs and MAC table

Inter-VLAN Routing Verification and Troubleshooting

#The Verification Checklist#link

#The Scenario Diagnosis#link

#The Fix#link

#Other Common Failures#link

#Debugging a Host Perspective#link

#Structured Workflow#link

One-directional inter-VLAN failure

#Check Your Understanding#link

Users in VLAN 40 can ping their gateway but cannot reach any other VLAN. Other VLANs work. What is the most likely cause?

Ready to resolve this lesson?

Inter-VLAN Routing Verification and Troubleshooting

#The Verification Checklist#link

#The Scenario Diagnosis#link

#The Fix#link

#Other Common Failures#link

#Debugging a Host Perspective#link

Spot the inter-VLAN routing problem

#Structured Workflow#link

One-directional inter-VLAN failure

#Check Your Understanding#link

Users in VLAN 40 can ping their gateway but cannot reach any other VLAN. Other VLANs work. What is the most likely cause?

Ready to resolve this lesson?

Spot the inter-VLAN routing problem