“I’ve configured the route. It shows in the table. So why is the traffic still not flowing?”
If you’ve asked yourself this question — probably while stress-refreshing a ping or blaming the firewall (again) — you’re not alone.
It’s one of the most frustrating networking moments: the route is there, but the packet doesn’t move.
Let’s walk through this together, one step at a time. By the end of this blog, you’ll understand six powerful, real-world reasons why routers don’t forward packets, even when the routing table looks just fine.
Let’s start with the most common trap. You’ve added a static route like:
ip route 10.1.1.0 255.255.255.0 192.168.1.2
You check the route table — it’s there! So what’s the problem?
Here’s the key: Does the router have a working path to 192.168.1.2?
Just because the next-hop IP is configured doesn’t mean it’s reachable.
Try:
ping 192.168.1.2
If that fails, the router has nowhere to send the packet — even if the route exists.
Real-World Fixes:
- Is the interface connected and in the right VLAN?
- Is the other side (next-hop) powered on and configured correctly?
- Is there a missing Layer 2 switchport config?
A route to a dead next-hop is like having directions to a house that’s been demolished — your packet just gets lost.
Here’s a classic case of tunnel vision. You’re so focused on the route and the ping that you forget to check if the exit interface is even up.
Run:
show ip interface brief
Look at the “Status” and “Protocol” columns. Both should say up/up.
If they say administratively down, someone shut the interface manually. If it says down/down, there could be:
- A disconnected cable
- A switchport that’s disabled
- A port in the wrong VLAN
The route might be alive, but if the exit door is locked, nothing gets out.
Tip:
Don’t just trust the routing table — always verify interface health during troubleshooting.
This one stings because it hides in plain sight.
You see the route, and you’re 100% sure your config is solid. But traffic still won’t go through.
What you didn’t notice? Someone applied an Access Control List (ACL) to the interface weeks ago — and it’s silently dropping your packets.
Check it:
show ip interface GigabitEthernet0/0
If it says:
Inbound access list is …
Outbound access list is …
Boom. Now you know where to look.
Also try:
show access-lists
Look for deny statements or a missing permit rule. Remember, implicit deny is always there at the end.
ACLs don’t tell you they’re killing your packets — they just do it quietly.
Pro Tip:
Always consider ACLs when pings fail or traffic is inconsistent, especially after recent firewall or network policy changes.
Here’s one that trips up even senior engineers.
You send traffic from Router A to Host B. But nothing comes back.
Why?
Because Host B (or its default gateway) doesn’t know how to get back to Router A.
You fixed the “going there” part — but forgot the “coming back” part.
This is common when:
- Using multiple ISPs
- Deploying static routes only
- Missing a default gateway on the destination side
Try pinging from the remote side back toward your source:
ping 192.168.1.1
If it fails, you have a return path problem — not a routing issue on your end.
You create a static route like this:
ip route 10.2.2.0 255.255.255.0 192.168.100.1
But 192.168.100.1 isn’t directly connected. Instead, it’s reachable through another route — maybe learned via OSPF.
Everything works — until that OSPF route disappears.
Now your static route still shows up, but the packet has nowhere to go. That’s a recursive route failure.
Run this:
show ip route 192.168.100.1
If no route exists for that next-hop, your static route breaks.
Solution:
- Ensure the path to the next-hop is always valid.
Consider using route tracking or floating static routes for redundancy.
Let’s say this happens:
- Ping works
- But your web app, file transfer, or VPN connection silently fails
That’s when you say, “Wait… it’s not the route?”
Nope — you may have an MTU problem.
What’s MTU?
MTU (Maximum Transmission Unit) is the max size of a packet that can be transmitted without fragmentation. For Ethernet, it’s usually 1500 bytes.
But if:
- A VPN adds encryption overhead, or
- A GRE/IPSec tunnel adds extra bytes, or
- A cloud route enforces smaller MTU…
…your regular packets might be too big to pass.
If the DF (Don’t Fragment) bit is set (which most OSs do by default), those oversized packets get dropped.
Your route exists. Your ping works. But your application fails. That’s classic MTU pain.
How to Detect MTU Issues:
Try a ping with size + DF bit set:
ping 10.1.1.1 size 1500 df-bit
If it fails, try:
ping 10.1.1.1 size 1400 df-bit
If that works — bingo! MTU mismatch confirmed.
How to Fix It:
- Use ip tcp adjust-mss 1360 on routers (especially on tunnels)
- Set proper MTU values on interfaces and endpoints
- Avoid setting DF bit when fragmentation is acceptable
- Use Path MTU Discovery tools like tracepath, mturoute, or MSS clamping
MTU issues are frustrating because everything “looks fine.” But they’re silent killers — and fixing them brings your apps magically back to life.
Here’s the truth:
Routing tables show you what should work.
But packet forwarding depends on what actually works.
Let’s recap the 6 silent killers of packet forwarding — even when your route exists:
Issue | Why It Breaks Routing |
1. Next-hop unreachable | Route points to a dead IP |
2. Interface down | Exit path is physically unavailable |
3. ACL applied | Packets dropped silently |
4. Return path missing | Packets go out but never return |
5. Recursive route failure | Static route depends on a missing route |
6. MTU mismatch | Packet too large → dropped silently |
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