09-15-2003 12:27 PM - edited 03-02-2019 10:20 AM
Hi,
I have built a VPN tunnel between two routers. The tunnels work fine and get pings replies from either sides of the tunnels.
The problem is that the pings are fine until you send upto 1468bytes packets over 1468 and ping replies fail.
Any ideas why this could be failing.
Thanks in advance.
Jay
09-15-2003 05:19 PM
you have an MTU issue , when you do a sh cry ipsec sa , what does it show for path and media mtu ? also what does it show when you do a sh ip interface for ip mtu ?
09-15-2003 09:55 PM
when I do a sh cry ipsec sa
local crypto endpt.: X.X.X.X, remote crypto endpt.: Y.Y.Y.Y
path mtu 1500, media mtu 1500
#sh ip int fa0/1
FastEthernet0/1 is up, line protocol is up
Internet address is 10.188.64.21/28
Broadcast address is 255.255.255.255
Address determined by setup command
MTU is 1500 bytes
09-16-2003 01:25 AM
You must be pinging a specific device. Try pinging another device (different kind) and you will start seeing larger pings work.
Some devices do not allow ping more than a given threshold.
HTH
09-16-2003 01:42 AM
You should get it clear that the echo packet or the echo reply packet fails? Then find out the error point and error link hop by hop.
09-16-2003 01:43 AM
We have tried various devices from different lans and getting the same problems.
09-16-2003 05:03 AM
take a look at this
router#sh int tu 34
Tunnel34 is administratively down, line protocol is down
Hardware is Tunnel
Internet address is
MTU 1514 bytes, BW 128 Kbit, DLY 500000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation TUNNEL, loopback not set
Keepalive not set
Tunnel source (Loopback0), destination
Tunnel protocol/transport GRE/IP, key disabled, sequencing disabled
Checksumming of packets disabled, fast tunneling enabled
Last input never, output 33w5d, output hang never
Last clearing of "show interface" counters 16w0d
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
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
0 packets input, 0 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
0 packets output, 0 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 output buffer failures, 0 output buffers swapped out
router #sh ip int tu 34
Tunnel34 is administratively down, line protocol is down
Internet address is
Broadcast address is 255.255.255.255
Address determined by non-volatile memory
MTU is 1476 bytes
Helper address is not set
Directed broadcast forwarding is disabled
Multicast reserved groups joined: 224.0.0.10
Outgoing access list is not set
Inbound access list is not set
Proxy ARP is enabled
Security level is default
Split horizon is enabled
ICMP redirects are always sent
ICMP unreachables are always sent
ICMP mask replies are never sent
IP fast switching is enabled
IP fast switching on the same interface is disabled
IP Flow switching is disabled
IP CEF switching is enabled
IP Feature Fast switching turbo vector
IP Feature CEF switching turbo vector
IP multicast fast switching is enabled
IP multicast distributed fast switching is disabled
IP route-cache flags are Fast, CEF
Router Discovery is disabled
IP output packet accounting is disabled
IP access violation accounting is disabled
TCP/IP header compression is disabled
RTP/IP header compression is disabled
Probe proxy name replies are disabled
Policy routing is disabled
Network address translation is disabled
WCCP Redirect outbound is disabled
WCCP Redirect inbound is disabled
WCCP Redirect exclude is disabled
BGP Policy Mapping is disabled
IP multicast multilayer switching is disabled
note that the MTU on the interface is showing 1500 , but the ip mtu is only 1476, this is not correct
what you need to do is shut down or preferably delete the tunnels then rebuild them and add the command 'ip mtu 1500'
see below
interface Tunnel16
ip address
ip mtu 1500
tunnel source Loopback0
tunnel destination
crypto map x
09-16-2003 05:38 AM
The tunnel interface is already configured for 1500
I#sh ip int tu 100
Tunnel100 is up, line protocol is up
Interface is unnumbered. Using address of Loopback10 (X.X.x.X)
Broadcast address is 255.255.255.255
MTU is 1500 bytes
Helper address is not set
Directed broadcast forwarding is disabled
Multicast reserved groups joined: 224.0.0.5 224.0.0.6
Outgoing access list is not set
Inbound access list is not set
Proxy ARP is enabled
Local Proxy ARP is disabled
Security level is default
Split horizon is enabled
ICMP redirects are always sent
ICMP unreachables are always sent
ICMP mask replies are never sent
IP fast switching is enabled
IP fast switching on the same interface is disabled
IP Flow switching is disabled
IP CEF switching is disabled
IP Feature Fast switching turbo vector
IP multicast fast switching is enabled
IP multicast distributed fast switching is disabled
IP route-cache flags are Fast
Router Discovery is disabled
IP output packet accounting is disabled
IP access violation accounting is disabled
TCP/IP header compression is disabled
RTP/IP header compression is disabled
Policy routing is disabled
Network address translation is disabled
WCCP Redirect outbound is disabled
WCCP Redirect inbound is disabled
WCCP Redirect exclude is disabled
BGP Policy Mapping is disabled
#sh int loopback 10
Loopback10 is up, line protocol is up
Hardware is Loopback
Internet address is X.X.X.X/32
MTU 1514 bytes, BW 8000000 Kbit, DLY 5000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation LOOPBACK, loopback not set
Last input never, output never, output hang never
Last clearing of "show interface" counters 5d02h
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
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
0 packets input, 0 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
0 packets output, 0 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 output buffer failures, 0 output buffers swapped out
09-16-2003 06:15 AM
what about the other side ? also what about sh int tu 100 , what does the mtu show for the tunnel 100, you may also want to adjust the mtu to have the tunnel and loopback match
09-16-2003 06:56 AM
at the other end of the tunnel also the tunnel mtu is 1514 and the loopback is also 1514
09-16-2003 10:25 AM
Isn't there overhead with GRE, 24 bytes? And I'm probably wrong, but I think I read ICMP has 8 bytes overhead, that leaves 1468 bytes...
1500-24-8=1468
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