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Firewpower 3140 - MTU on the Physical and Sub Interface

Hi Guys.

We are running Firepower 3140 Appliances. There are 2 * 40Gbps connection. The Physical (ETH2/1) 40Gbps interface has the MTU set as 9000 whilst the Sub Interface (ETH2/1.10) and (ETH2/1.20) has the MTU still at 1500

In this scenario,

1. Is there any need to change the MTU for the ETH2/1.10 & ETH2/1.20 to 9000, same as the Physical Interface.

2. Will the MTU for the Sub Interface be inherited from the Physical Interface or that we need to set it up manually.

3. If the MTU is still left at 1500 for the Sub Interface, would the Sub Interface still needs to Defragment if transmitting the Jumbo Frames.

4. Can the Sub Interface at 1500 MTU, still be able to receive Jumbo Frames.

5. Is there recommended MTU setting for the 40Gbps interfaces.

Looking forward to clear my doubts.

Regards

Adrian.

 

4 Replies 4

balaji.bandi
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If the Option to set MTU - then i would setup the MTU on sub-interfaces too.

if other side of the device and path can support jumbo frames.

 

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tvotna
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1. Yes.

2. No.

3. It will need to fragment large IPv4 packets if the packet size exceeds subinterface MTU, provided that DF bit is not set. If DF bit is set it will drop the packet and send ICMP unreachable, fragmentation needed to the sender.

4. Yes on this platform which supports jumbo frames by default. "Mtu" is a logical setting affecting transmitted packets.

5. No. Choose MTU supported by all devices in your network. Max supported FP3100 MTU is not documented. Command "show int" or something like that from "connect fxos" mode may help figure out.

 

1. Is there any need to change the MTU for the ETH2/1.10 & ETH2/1.20 to 9000, same as the Physical Interface. this better, otherwise the FW CPU will busy alot for frag and defrag packet

2. Will the MTU for the Sub Interface be inherited from the Physical Interface or that we need to set it up manually. NOT sure but I dont think so

3. If the MTU is still left at 1500 for the Sub Interface, would the Sub Interface still needs to Defragment if transmitting the Jumbo Frames. Yes and that so Bad
the Jambo in Network must use along the path 

4. Can the Sub Interface at 1500 MTU, still be able to receive Jumbo Frames. NO, it can send but not receive 

5. Is there recommended MTU setting for the 40Gbps interfaces. can you more elaborate

4. Can the Sub Interface at 1500 MTU, still be able to receive Jumbo Frames. NO, it can send but not receive

Why do you think so? Did you test?

 

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