Ping Over 1500 Bytes

[DM812]

We have a Cisco 3850 and when trying to ping with a byte size of over 1500, the ping fails (unable to ping across the WAN or within the same VLAN with anything over 1500 bytes).

This works on other 3850's with the same image so seems to be an issue with this specific switch.

I've looked through the config and can't see anything that would be preventing this from running (MTU is set the same as the other working switches).

Has anyone seen this issue before?

[balaji.bandi]

The default MTU for the interface set to 1500 only, you can view that show interface vlan X

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3850/software/release/37e/consolidated_guide/b_37e_consolidated_3850_cg/b_37e_consolidated_3850_cg_chapter_01000.pdf

MTU - need to be setup both the side to match, not one side.

[DM812]

I cannot ping between VLAN SVI's on the same problematic switch which means it isn't even going via a physical interface.

On the two switches that can ping eachother with over 1500 byes, the MTU is still 1500 on the physical interfaces that it passes through.

[balaji.bandi]

I cannot ping between VLAN SVI's on the same problematic switch which means it isn't even going via a physical interface.

show and tell us the issue.

[DM812]

As you can see below I am unable to ping between VLAN SVI's on the same device with a ping size of over 1500 bytes

#sh ip int brief
Interface IP-Address OK? Method Status Protocol
Vlan8 192.168.8.112 YES NVRAM up up
Vlan15 192.168.15.3 YES NVRAM up up

#ping 192.168.15.3 source 192.168.8.112 size 2000
Type escape sequence to abort.
Sending 5, 2000-byte ICMP Echos to 192.168.15.3, timeout is 2 seconds:
Packet sent with a source address of 192.168.8.112
.....
Success rate is 0 percent (0/5)

[balaji.bandi]

#ping 192.168.15.3 source 192.168.8.112 size 2000

as i believe this not going to work, since MTU if SVI is 1500 only

show interface vlan 15

show interface vlan 8

example :

#show interfaces vlan XXX
VlanXXX is up, line protocol is up , Autostate Enabled
Hardware is Ethernet SVI, address XXXXXXXXXXXXXXXXXX
MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,

[DM812]

I don't believe this is the issue as when I ping between SVI's on a different switch it's successful even with the VLAN interface MTU set at 1500, see below on the working switch...

#sh ip int brief

Vlan8 192.168.8.110 YES NVRAM up up

Vlan15 192.168.15.2 YES NVRAM up up

#sh int vlan 8
Vlan8 is up, line protocol is up
Hardware is EtherSVI, address is 0026.5206.3f42 (bia 0026.5206.3f42)
Internet address is 192.168.8.110/22
MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,

#sh int vlan 15
Vlan15 is up, line protocol is up
Hardware is EtherSVI, address is 0026.5206.3f43 (bia 0026.5206.3f43)
Internet address is 192.168.15.2/24
MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,

#ping 192.168.15.2 source 192.168.8.110 size 15000

Type escape sequence to abort.
Sending 5, 15000-byte ICMP Echos to 192.168.15.2, timeout is 2 seconds:
Packet sent with a source address of 192.168.8.110
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 16/18/26 ms

[balaji.bandi]

Applogies - i read the wrong in the first place.

coming back to the issue, you pining in same switch, so it should work as expected.

#ping 192.168.15.3 source 192.168.8.112 size 2000
Type escape sequence to abort.
Sending 5, 2000-byte ICMP Echos to 192.168.15.3, timeout is 2 seconds:
Packet sent with a source address of 192.168.8.112
.....
Success rate is 0 percent (0/5)

can you try below :

#ping 192.168.8.112  source 192.168.15.3  size 2000

#ping 192.168.8.112  source 192.168.15.3  size 1500

#ping 192.168.8.112  source 192.168.15.3  size 20000

[DM812]

As below...

FAILED #ping 192.168.8.112  source 192.168.15.3  size 2000

PASSED #ping 192.168.8.112  source 192.168.15.3  size 1500

FAILED #ping 192.168.8.112  source 192.168.15.3  size 20000

[vencislav.metev]

Hi,

If you ping this way packets are fragmented again.

Try to ping with df bit set

ping 192.168.15.2 source 192.168.8.110 size 15000 df

I guess it won`t work.

Regards,

Ventsi

[Joseph W. Doherty]

What's the MTU?  If 1500, and you going beyond that, it depends on whether DF is set on your packets, or not, (I don't recall the default) and also end-to-end MTU.