[nsp] GRE tunneling and MTU

From: Adam Rothschild (asr@latency.net)
Date: Thu Jan 27 2000 - 23:06:22 EST


Hi,

Most likely a lame question resulting from oversight on my end, but here
goes:

I have a GRE/IP tunnel between a 7206VXR [running 12.0(8)S] and a 2611
[running 12.0(8)]. On the 7206, 'sh in' output is:

  MTU 1514 bytes, BW 10000 Kbit, DLY 500000 usec, rely 255/255, load 1/255
  Encapsulation TUNNEL, loopback not set
  Keepalive set (10 sec)
  ...
  Tunnel protocol/transport GRE/IP, key disabled, sequencing disabled
  Checksumming of packets enabled, fast tunneling enabled

On the 2611:

  MTU 1472 bytes, BW 10000 Kbit, DLY 500000 usec, rely 255/255, load 1/255
  Encapsulation TUNNEL, loopback not set, keepalive set (10 sec)
  ...
  Tunnel protocol/transport GRE/IP, key disabled, sequencing disabled
  Checksumming of packets enabled, fast tunneling enabled

MTU cannot be set on these tunnel interfaces. This discrepancy is causing
bad transmission-related things to happen, as would be expected.

Any advice/insight would be greatly appreciated.

Thanks in advance,
-adam



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