If your ethernet / ISP circuit is running at 1460, then try driving your VPN at (1460-40)=1420. If you were to take some packet captures outside of the tunnel, you'd see your ipsec packets are all being fragmented. This forces fragmentation to happen at the transport layer(s) underneath your tunnel. You can't really drive a larger MTU encapsulated ontop of a smaller MTU. Your understanding of how MTU works is incomplete or in error. So the VPN connection/leg is the only thing running at higher than normal MTU. Jumboframes is only on my IPsec tunneled VPN connection. We are running 1460 on the ISP and LAN connections (for the reasons you pointed out). Any suggestions you might have would be appreciated! I'm totally stumped as to what I can look at next from the Meraki side of things. We've never come close to hitting that 200 mbps in either direction. On the office side of things, we have a 200/200 direct ethernet connection to our ISP. I've done iPerf tests between by VPN clients and my servers and see pretty much the same speeds as listed above. I've done some MTU tuning per Meraki's KB article, and that didn't make a lick of difference. Beyond that they say it can't possibly have anything to do with the Meraki's. Meraki support has been less than helpful, only being able to suggest trying to white list the file servers and the client machines via device policy while connected. To quantify, I'm talking about transfer speeds via Finder/Windows Explorer of around 4 MB/s on the old setup, and now around 1 MB/s or less in many cases.
#MTU FOR VPN MAC UPDATE#
Let’s calculate our proper MTU size using the formula: MTU size - encapsulation overhead = interface MTUĭepending on the vendor used, we can update our MTU size to calculated value.Ever since implementing a Meraki stack (MX-67, MS-225 switches in a stack), the performance of our file shares (Windows Server 2019 hosted) for our end users (Both Mac and Windows) has been considerably worse than it was with our old Cisco ASA 5512 firewall and Catalyst 2960 stack. Now, let’s calculate the IPSec overhead based on encryption used: IPSec Transform SetĮsp-AES-(256 or 192 or 128) esp-SHA-hmac or md5 To fix the issue, we need to determine our MTU size in non-VPN enivorment: ~]# ping -M do -s 1472 8.8.8.8 1380 bytes (Cisco ASA).Īnother workaround (not fix! and it should be used as a last resort!) is to get edge routing device to clear DF-bit so fragmentation is allowed. Hence most of the firewall vendors clamp MSS connections to e.g. Most of the common causes that break PMTUD are blocked icmp, asymmetric routing or not enough bytes sent from the client side to trigger PMTDU.
#MTU FOR VPN MAC CODE#
If MTU size along the way to destination is too small, router/firewall will inform the host and drops the packet and sends an ICMP Fragmentation Needed Type 3 Code 4 packet back to the sending device with its MTU size. This works by setting DF-bit to 1 and forcing MTU size. Most networking devices use Path MTU to calculate proper MTU size on the entire path. IEEE 802.1Q tag adds 4 bytes (Q-in-Q would add 8 bytes).MPLS adds 4 bytes for each label in the stack.IPSec encryption performed by the DMVPN adds 73 bytes for ESP-AES-256 and ESP-SHA-HMAC overhead (overhead depends on transport or tunnel mode and the encryption/authentication algorithm and HMAC).Any encapsulation that takes place, adds overhead to the original packet size: MTU size for Ethernet is 1500 (1514 if we count 802.1 Ethernet header).When original IP packet gets encrypted by IPSec, there’s an overall increase in packet size.
#MTU FOR VPN MAC MANUAL#
Common example is when icmp ping works both way without any issues, or manual telnet to www port is open but the actual page won’t open or opens intermittently. This is caused by incorrect MTU size and encapsulation overhead. Very often when IPSec tunnel is used, throughput is affected or users are experiencing fragmentation issues. The workaround involves lowering the ICA/EDT MSS to a known value that will not cause fragmentation. This is a limitation of the VPN which is not handling IP fragmentation properly.
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