TTL = 0 When routers are configured incorrectly..

 

❌ “When TTL is 0 then routing loops”

This is not the cause of routing loops. It is the result of a routing loop in Layer 3.

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✅ Correct Concept

Routing loops cause packets to circulate between routers repeatedly, and as a result the TTL (Time To Live) decreases until it reaches 0. When TTL becomes 0, the packet is dropped.

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🧠 How TTL Actually Works

• Every IP packet starts with a TTL value (e.g., 64, 128, 255)
• Each router that forwards the packet:
  • Decreases TTL by 1
• When TTL = 0:
  • ❌ Packet is dropped
  • Router sends ICMP Time Exceeded

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🔁 Routing Loop Example

      R1
    ↙    ↘
  R2  ←→  R3

If routing is misconfigured:

1. Packet goes R1 → R2
2. R2 sends it to R3
3. R3 sends it back to R2
4. Loop continues…

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📉 TTL Decreases in Loop

Hop	Router	TTL
1	R1	64
2	R2	63
3	R3	62
4	R2	61
...	...	...
final	any router	0 ❌ Packet dropped

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🚨 Key Point

✔ Routing loop = problem

✔ TTL = protection mechanism

TTL is NOT the loop. TTL is the lifesaver that stops infinite looping.

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🧠 Why TTL exists

Without TTL:

• Packet would loop forever
• Network would get flooded
• CPU usage on routers would spike
• Bandwidth would be wasted

So TTL prevents:

❌ Infinite routing loops

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🔧 What actually causes routing loops?

• Incorrect static routes
• Misconfigured dynamic routing (OSPF, EIGRP, RIP)
• Missing route updates
• Redistribution errors
• Layer 3 topology issues