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Tunnel Transport MTU


i´m wondering how the router calculates the transport mtu of a tunnel-interface? what doesi t exactly mean?

Router#show interfaces tunnel 309

Tunnel301 is up, line protocol is up

  Hardware is Tunnel

  MTU 17867 bytes, BW 100 Kbit/sec, DLY 50000 usec,

     reliability 255/255, txload 1/255, rxload 1/255

  Encapsulation TUNNEL, loopback not set

  Keepalive not set

  Tunnel protocol/transport IPSEC/IP

  Tunnel TTL 255

  Tunnel transport MTU 1505 bytes

  Tunnel transmit bandwidth 8000 (kbps)

  Tunnel receive bandwidth 8000 (kbps)

Community Member

Tunnel Transport MTU


IP sec VPN uses PMTU discovery for calculating the transport mtu size....

Path MTU Discovery (PMTUD) is a standardized technique in computer networking for determining the maximum transmission unit (MTU) size on the network path between two Internet Protocol (IP) hosts, usually with the goal of avoiding IP fragmentation. PMTUD is performed by routers in Internet Protocol Version 4

Path MTU Discovery works by setting the Don't Fragment (DF) option bit in the IP headers of outgoing packets. Then, any device along the path whose MTU is smaller than the packet will drop it, and send back anInternet Control Message Protocol (ICMP) Fragmentation Needed (Type 3, Code 4) message containing its MTU, allowing the source host to reduce its Path MTU appropriately. The process repeats until the MTU is small enough to traverse the entire path without fragmentation.

If the Path MTU changes after the connection is set up and is lower than the previously determined Path MTU, the first large packet will cause an ICMP error and the new, lower Path MTU will be found.

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