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--- networking:mtu [2025/09/11 01:12] – aperez
+++ networking:mtu [2025/10/04 15:48] (current) – aperez
@@ Line 41: / Line 41: @@
 </code>
+----
+<note tip>
+**MTU & PMTU Validation (side note)**
+Quick reference on how to confirm the effective MTU and Path MTU (PMTU) end-to-end. Use these tests when enabling jumbo frames or troubleshooting connectivity.
+----
+**1) ICMP Check with ping**
+Command:
+<code bash>
+ping -M do -s 1472 8.8.8.8
+</code>
+Expected output (OK, MTU 1500):
+<code>
+PING 8.8.8.8 (8.8.8.8) 1472(1500) bytes of data.
+bytes from 8.8.8.8: icmp_seq=1 ttl=114 time=8.0 ms
+bytes from 8.8.8.8: icmp_seq=2 ttl=114 time=7.9 ms
+--- 8.8.8.8 ping statistics ---
+packets transmitted, 2 received, 0% packet loss
+</code>
+👉 If **1472 passes**, effective MTU ≈ **1500**.
+👉 If it fails (e.g., *Frag needed*), the link is forcing MTU <1500.
+----
+**2) Path MTU Discovery with tracepath**
+Command:
+<code bash>
+tracepath -n 8.8.8.8
+</code>
+**Case A — Standard network (PMTU 1500):**
+<code>
+: 192.168.1.1          0.45ms
+: 192.168.1.1          0.39ms pmtu 1500
+: 10.10.10.1           3.21ms
+: 8.8.8.8              7.84ms reached
+     Resume: pmtu 1500 hops 3 back 3
+</code>
+**Case B — Jumbo end-to-end (PMTU ~9000):**
+<code>
+: 10.0.0.1             0.22ms
+: 10.0.0.1             0.21ms pmtu 9000
+: 172.20.0.1           0.95ms
+: 8.8.8.8              6.74ms reached
+     Resume: pmtu 9000 hops 3 back 3
+</code>
+👉 If you see **`pmtu 1500`**, the path is limited to standard frames.
+👉 If you see **`pmtu 9000`** (or similar), jumbo frames are preserved across the path.
+👉 If it drops (e.g., 9000 → 1500 mid-path), a hop does not support jumbo.
+----
+**Validation summary**
+^ Test                  ^ Expected outcome                           ^
+| `ping -M do -s 1472`  | Successful reply ⇒ effective MTU ≈ 1500    |
+| `tracepath -n`        | Reports PMTU 1500 (standard) or 9000 (jumbo)|
+</note>
 ----
@@ Line 88: / Line 156: @@
+----
+> **Note: PMTU and PMTUD (Path MTU / Path MTU Discovery)**
+The *Path Maximum Transmission Unit (PMTU)* is the **largest IP packet size that can travel end-to-end without fragmentation**.
+It is determined by the **smallest MTU along the entire path**.
+Example:
+  * Link 1: MTU = 9000
+  * Link 2: MTU = 1500
+  * Link 3: MTU = 1400
+  → **PMTU = 1400 bytes**
+---
+**Path MTU Discovery (PMTUD):**
+A dynamic mechanism to discover the PMTU using the **DF (Don’t Fragment)** bit.
+Steps:
+  - Source sends a large packet with DF=1.
+  - If a router cannot forward due to its MTU, it discards the packet and replies with ICMP *Fragmentation Needed* (Type 3, Code 4).
+  - The source reduces the size until it learns the **real PMTU**.
+---
+**Verification on Linux**::
+  ping -M do -s 1472 8.8.8.8
+    → If it replies: path supports 1500 (1472+28 headers).
+    → If it fails: PMTU is smaller.
+  tracepath -n 8.8.8.8
+    → Displays estimated PMTU along the route.
+---
+**Common pitfalls**
+  * Blocking ICMP → breaks PMTUD (TCP sessions may hang).
+  * Tunnel/VPN overhead → reduces MTU (e.g., IPSec 1500 → ~1400).
+  * Misconfigured Jumbo Frames → one 1500 hop breaks 9000 end-to-end.
+---
+**In summary:**
+  * **PMTU** = max packet size without fragmentation.
+  * **PMTUD** = process to dynamically discover it using ICMP + DF.
@@ Line 198: / Line 312: @@
+{{ :aruba_networks:switch:6400:mtu_pmtu.pdf |}}
+{{pdfjs 46em >:aruba_networks:switch:6400:mtu_pmtu.pdf}}
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