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GRE - fast tunneling enabled - what does this imply?

Hi - does anyone know the significance of the 'fast tunneling enabled' comment in the show interface tunnel'x' output below:

Tunnel120 is up, line protocol is up

Hardware is Tunnel


Interface is unnumbered. Using address of Loopback100 (x.x.90.101)

MTU 1514 bytes, BW 256 Kbit, DLY 500000 usec,

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

Encapsulation TUNNEL, loopback not set

Keepalive set (5 sec), retries 3

Tunnel source x.x.91.133 (Loopback50), destination x.x.136.19

Tunnel protocol/transport GRE/IP, key disabled, sequencing disabled

Tunnel TTL 255

Checksumming of packets disabled, fast tunneling enabled

Last input never, output 00:00:03, output hang never

Last clearing of "show interface" counters never

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 2

Queueing strategy: fifo

Output queue: 0/0 (size/max)

5 minute input rate 0 bits/sec, 0 packets/sec

5 minute output rate 0 bits/sec, 0 packets/sec

3630570 packets input, 174267377 bytes, 0 no buffer

Received 0 broadcasts, 0 runts, 0 giants, 0 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort

3641995 packets output, 175165679 bytes, 0 underruns

0 output errors, 0 collisions, 0 interface resets

0 output buffer failures, 0 output buffers swapped ou

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Re: GRE - fast tunneling enabled - what does this imply?


I found a paragraph which seems to explain this in terms of support for fast switching and CEF for GRE encapsulated traffic:

Encapsulation and the removal of encapsulation at the tunnel endpoints are slow operations; in general, only processor switching is supported. However, fast switching of GRE tunnels was introduced in Cisco IOS Release 11.1 for the Cisco 2500 series and the Cisco 4000 series of routers, and CEF switching of GRE tunnels was introduced in Cisco IOS Release 12.1.

If you want to see the original source here is the link:



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