Symptom
On ASR920, over GRE tunnel complete traffic drop if packet size > 1472 bytes.
Conditions
IPV4 Unicast GRE tunnels are configured and the tunnels are up.
Packet size > 1472 bytes. end to end GRE tunnel configured and MPLS in the core.
Workaround
It's good to configure ip mtu in the core interfaces
Fix Limitations:
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The fix will not be applicable in a particular case.
i.e If the access interface MTU is less than or equal to core interface MTU.
Here, the pkts are not decided to be fragmented as they have same MTU (OR) the core interface MTU is greater than access MTU.
But it can fail sue to the reason that, GRE tunnel will be adding extra 24 bytes (default case, without tunnel key) when the pkt is sent out of core interface.
So, the packet size can exceed the configured MTU on the interface and it fails.
To address this situation, we would need to configure "ip mtu " on the core interfaces.., where value = (Ethernet MTU - 20).
