Ping is success with up to 1532 MTU but not with 1600MTU size.

[JAIRAM]

Hi Expert

Recently i install 10G-LR sfp and TCL link migrated to this SFP. earlier 1G SFP was installed and ping was successful with 1600MTU but now no luck.

MTU is configured 1600byte on router interface at both end.

TCL engineer has checked every possibility and 9600bye MTU is passing in backbone. There is no issue in TCL backbone.

As i checked on my router interface there is 20843 error inputs and 20843 giants detected. 

Please help with your observation and to get it resolve.

[DanielP211]

What configuration do you have on the router? which router is it? 

[JAIRAM]

On router:

MTU 1600

No negotiation auto

-What is the meaning of MTU 1600, is it including all required header with payload or what? When initiate ping what kind of header include with payload? 

-What is the meaning of giants and when is detect on interface?

[r.heitmann]

on IOS-XE devices - MTU-configuration might depend on device-type, interface-type, operation-system type, ...

... the interface-level command "mtu xxx" modifies the "ip mtu"-setting, which is the relevant parameter for IP-Packets.

In a Lab device "mtu 1600" changes the IP-MTU to "1600 byte", too.

You can verify, what happens at your device:

• show interface
• show ip interface 

What "show ip interface" says, rules usually.

When you "ping" directly from your IOS-XE device, a "ping ...target-ip... size 1500 df-bit" generates an IP-Packet containing an ICMP-Ping-Message which has an IP-Size of 1500 bytes.

When you start your "ping" from an Windows-Command-Shell, a "ping ...target-ip... -l 1472 -f" generates a similar packet.

• Windows-Ping adds 20 byte for IP-Header and 8 byte for the ICMP-Header, Cisco IOS-XE not.

Service-Providers, especially if they provide L2-Connectivity might specify the L2-MTU:

• an Ethernet/L2-Provider talking about an MTU of 1518 Byte will usually provide an IP-MTU of 1500 bytes if untagged ethernet frames without VLAN-ID have to get processed.

..."giants" are usually frames which have been dropped in inbound direction since they exceed the local Interface-MTU.

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[MHM Cisco World]

Really hard to find issue here'

Change physical have no any effect to router mtu size 

Change provider can lead to this' 

9600 passing' how he know that if you try and  1600 drop?

[r.heitmann]

if it's a routed backbone, it might be worth testing

• traceroute to discover the path
• ping with jumbo-MTU to each discovered hop

to find the L2-Segment between two hops which breaks the "large MTU"-Path