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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