Prevent the fragmentation of vpn
I've very fragmented with this configuration, and for this reason, the remote sites cannot receive a bandwidth good vpn.
2821 is the router of the seat and the location remote vpn 1721.
1721 features a module vpn.
What can I do?
Edgar,
This link is OK - lets do some math:
IP header - 20-byte
TCP header - 20 bytes
IPSEC header - 56 bytes
Standard LAN NIC MTU = 1500. When a syn tcp connection is launched - the TCP stack will do the following: -.
If the NIC MTU = 1500, 20 bytes of the TCP header takeaway, Takeaway 20 bytes of the TCP header - announce a MSS of 1460.
When you turn on PMTUD (enabled by default on all Microsoft OS) all packages have the DF bit set.
If you negotiate a TCP session, in 1460 with the DF bit is set, the packets arrive ready Firewall/VPN device for encryption...
but the device must add 56 bytes of encryption to the contents of the package... 1460 + 56 = 1516, but the interface's MTU 1500 right! Ooops!
If you start using a ping with the DF bit set - it is misleading because an ICMP packet is 20 bytes, with IP info - so the reported MTU willl
be 1480! not what you are looking for.
To be sure, I always do the following: -.
20 bytes of IP header
20 bytes of TCP header
28 bytes for the encapsulation of GRE (if I want to use dynamic routing over VPN protocols)
56 bytes for IPSEC
Up to present = 1356.
I always calculate a surcharge if I deal with VOIP: -.
12-byte RTP
All totaled = 1344
I also allow "fudge" so I use 1300 bytes as the MSS to heart... extermely workes fine for me.
HTH >
Tags: Cisco Security
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