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How the Tunnel Transport MTU being calculate? Already set ip mtu 1380 and tcp adjust-mss 1340

jackie_gx
Level 1
Level 1

Tunnel13 is up, line protocol is up
Hardware is Tunnel
Description: ### Tunnel 13 ###
Internet address is xx.xx.xx.xx/24
MTU 17850 bytes, BW 1000 Kbit/sec, DLY 1000 usec,
reliability 255/255, txload 2/255, rxload 1/255
Encapsulation TUNNEL, loopback not set
Keepalive not set
Tunnel linestate evaluation up
Tunnel source xx.xx.xx.xx (Loopback0), destination xx.xx.xx.xx
Tunnel Subblocks:
src-track:
Tunnel13 source tracking subblock associated with Loopback0
Set of tunnels with source Loopback0, 6 members (includes iterators), on interface <OK>
Tunnel protocol/transport GRE/IP
Key 0xD, sequencing disabled
Checksumming of packets disabled
Tunnel TTL 255, Fast tunneling enabled
Tunnel transport MTU 1410 bytes
Tunnel transmit bandwidth 8000 (kbps)
Tunnel receive bandwidth 8000 (kbps)
Tunnel protection via IPSec (profile "AFFBRSEC")
Last input 00:00:13, output 4d01h, output hang never
Last clearing of "show interface" counters 4d01h
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 1683
Queueing strategy: fifo (QOS pre-classification)
Output queue: 0/0 (size/max)
5 minute input rate 4000 bits/sec, 4 packets/sec
5 minute output rate 8000 bits/sec, 5 packets/sec
1417617 packets input, 567396259 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
1547150 packets output, 441681712 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 unknown protocol drops
0 output buffer failures, 0 output buffers swapped out

interface Tunnel13
description ### DMVPN Tunnel 13 ###
bandwidth 1000
ip address xx.xx.xx.xx 255.255.255.0
no ip redirects
ip mtu 1380
ip hello-interval eigrp 100 60
ip hold-time eigrp 100 180
ip nhrp authentication test
ip nhrp map xx.xx.xx.xx xx.xx.xx.xx
ip nhrp network-id 13
ip nhrp holdtime 600
ip nhrp nhs xx.xx.xx.xx
ip tcp adjust-mss 1340
delay 100
qos pre-classify
tunnel source Loopback0
tunnel destination xx.xx.xx.xx
tunnel key 13
tunnel protection ipsec profile XXX
end

1 Reply 1

Dmitri Popkov
Level 1
Level 1

Tunnel transport MTU - is actual calculated size for GRE packets with payload inside. At least, you should add 20 byte IP + 4 bytes GRE + 4 bytes GRE key. To better understand header size, its better to take packet capture. Can you provide  it?

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