hi all
i had a very sad encounter with an issue on Cisco ISR4331 with ios-xe 03.16.04b.S
this router is a dmvpn spoke and for some reasons the transport network ( ISP ) had an issue witch wouldn't allow you to send packets larger than 1350 bytes and i had a bad case of packet loss on my tunnel
so I changed the MTU in tunnel interface using ip mtu command but when I get show interface the mtu is still 1472 but when i try to change it again using mtu command i get this error message :
R-42_1(config-if)#mtu 1300
% Interface Tunnel1 does not support user settable mtu.
this is my running config on tunnel interface :
interface Tunnel1
ip address 172.18.1.42 255.255.0.0
no ip redirects
ip mtu 1300
ip hello-interval eigrp 500 20
ip hold-time eigrp 500 60
ip nhrp authentication KEY
ip nhrp map multicast dynamic
ip nhrp network-id 8532
ip nhrp holdtime 200
ip nhrp nhs 172.18.1.0 nbma 1.1.1.1 multicast
ip nhrp nhs 172.18.1.1 nbma 2.2.2.2 multicast
ip nhrp registration no-unique
ip nhrp shortcut
ip summary-address eigrp 500 10.42.0.0 255.255.0.0
ip tcp adjust-mss 1260
tunnel source Dialer1
tunnel mode gre multipoint
tunnel key 8532
tunnel protection ipsec profile DMVPN-SEC
end
this is the output of show interfaces tunnel 1:
R-42_1#show interfaces tunnel1
Tunnel1 is up, line protocol is up
Hardware is Tunnel
Internet address is 172.18.1.42/16
MTU 9972 bytes, BW 100 Kbit/sec, DLY 50000 usec,
reliability 255/255, txload 28/255, rxload 22/255
Encapsulation TUNNEL, loopback not set
Keepalive not set
Tunnel linestate evaluation up
Tunnel source 3.3.3.3 (Dialer1)
Tunnel Subblocks:
src-track:
Tunnel1 source tracking subblock associated with Dialer1
Set of tunnels with source Dialer1, 1 member (includes iterators), on interface <OK>
Tunnel protocol/transport multi-GRE/IP
Key 0x2154, sequencing disabled
Checksumming of packets disabled
Tunnel TTL 255, Fast tunneling enabled
Tunnel transport MTU 1472 bytes
Tunnel transmit bandwidth 8000 (kbps)
Tunnel receive bandwidth 8000 (kbps)
Tunnel protection via IPSec (profile "DMVPN-SEC")
Last input 00:00:17, output never, output hang never
Last clearing of "show interface" counters 3d21h
Input queue: 0/375/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/0 (size/max)
5 minute input rate 9000 bits/sec, 11 packets/sec
5 minute output rate 11000 bits/sec, 10 packets/sec
4439139 packets input, 450439115 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
5080822 packets output, 2360241834 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 unknown protocol drops
0 output buffer failures, 0 output buffers swapped out
