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An IPSec VPN between ASA and ASR with a GRE tunnel inside that (GRE tunnel source and destination as the interesting traffic) and OPSF running over the GRE tunnel. I have used this later option for connections between various sites for the last few years with no real issues. Alternatively, if you did want to complicate it a little you can run BGP through the tunnel or GRE and run a routing protocol over the GRE tunnel. If you need simple tunnel between sites for a few networks they are perfect as you can get away from any unnecessary complexity such as running routing protocols. They can be problematic between different vendors using a mix of policy-based (ACL interesting traffic) and route-based (tunnel interface) VPNs, but if you stick to the basics they are pretty solid. Site-to-site VPNs using crypto maps with an interesting traffic ACL seem to have got a lot of bad press over the years. For more complex environments or cloud connectivity you are probably going to need to use VTIs, this post goes through the process of building VTI VPNs between an ASR and ASA. For a simple solution to join small sites with no need for routing these work great and keep the complexity down to a minimum. When you want to read more on Path MTU Discovery and MSS clamping look at this article.Over the years I have built numerous IPsec VPNs on ASAs using crypto maps and an ACL for the interesting traffic. The best solution for these type of issues is to use MSS clamping, which means that the gateway will, on the fly, adjust the value that the SYN and SYN-Ack packet contain from 1460 (default) to the value you set in the gateway config. This is part of the Path MTU Discovery mechanism, the ICMP packet should cause the sending machine to lower the MSS value and resend the packet with the lower number of bytes.
#Mtu for vpn tunnel code#
This will cause the device to issue a ICMP packet back to the sender with a code 3 (destination unreachable) type 4 (fragmentation needed but DF bit set) message with the header of the original packet in it. Now when you set the DF bit, you tell all devices along the way that the packet cannot be split into multiple packets, which will cause a device wich will have to add a header (like the IPSEC header) into the packet and the packet comes with 1460 bytes of data, that new header will not fit in the same packet as that would make the packet 1562 (in your case) bytes in size. The point here is that the 1500 value minus 20 bytes IP header and 20 bytes TCP header, comes down to 1460 bytes MSS, whicxh is what you will see in most SYN and SYN-Ack packets for a new TCP connection. The MSS is the Maximum Segment Size, which in fact is the actual data that can pass through a connection.
#Mtu for vpn tunnel mac#
The MTU is the Maximum Transmision Unit, this includes all headers except the 18 bytes for the outer header (where the MAC addresses are). You ask questions that do not reflect the reality. Please consider the model offered by other vendor as an example.
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if those 62 bytes represent the IPSEC header, please help me figure out also what is the breakdown structure of IPSEC overhead used by Checkpoint, considering R80.10 version that is used.
![mtu for vpn tunnel mtu for vpn tunnel](https://community.cisco.com/kxiwq67737/attachments/kxiwq67737/5991-discussions-wan-routing-switching/194370/1/129775-HeadEnd.png)
why central Checkpoint gw computes a new MTU of 1438 when considering sending packets to the remote gw 11.0.0.1? (There is a dynamic decrease of 62 bytes from the static value, defined on the interface eth0).Ģ. Please also find the following information retrieved from the Central ip r get 11.0.0.1ġ1.0.0.1 via 10.0.0.2 dev bond2 src 10.0.0.1Ĭache ipid 0x10 advmss 1460 hoplimit ping 11.0.0.1 -s 1410 -c 1 IPSEC VPN is configured between 2 gateways, tunnel mode, AES-128 and SHA 256 Please find attached the general network diagram consisting of:Ģx Checkpoint firewalls with 2 external interfaces, eth0 on the Hub, eth1 on the Remote Please assist me figuring out the following behaviour related with the MTU setup, used by Checkpoint.