ETEP Configuration

Ignore DF Bit

When the ETEP is configured for use in Layer 3 IP encryption policies, its default behavior is to enable DF Bit handling on the local port. This tells the ETEP to ignore the “do not fragment” (DF) bit in the IP header, and fragment outbound packets that exceed the MTU of the system. This setting should be used under the following conditions:

●Reassembly mode is set to gateway

●ICMP is blocked at the firewall

●PMTU path discovery isn’t working

A symptom of a PMTU problem is when the network operates normally when traffic passes in the clear but loses packets when encryption is turned on.

You can override the default behavior by disabling the DF Bit handling on the local port. The ETEP will then discard packets in which the DF bit is set and the packet length, including the encryption header, exceed the PMTU.

Table 89

Ignore DF Bit settings

 

 

 

Setting

 

Description

Enabled

 

The ETEP ignores the DF bit in the IP header and fragments outbound

 

 

packets greater than the MTU of the system. This setting is automatically

 

 

enabled when the reassembly mode is set to gateway.

Disabled

 

The ETEP acts in accordance with the DF bit setting in the IP header.

 

 

 

Related topic:

●“Reassembly Mode” on page 310

Reassembly Mode

The reassembly mode setting applies to packets entering the ETEP’s local port that are subject to fragmentation. This setting specifies whether packets are fragmented before or after they are encrypted and who performs the reassembly of the fragmented packet: the destination host or gateway.

The reassembly mode option is available only when the ETEP’s Encryption Policy Setting is set to Layer 3:IP. When the Encryption Policy Setting is set to Layer 2:Ethernet, packets that are subject to fragmentation are encrypted prior to fragmentation. Layer 2 jumbo packets that exceed the PMTU are discarded. The Encryption Policy Setting is configured on the Features tab.

Table 90

Reassembly mode settings

 

 

 

Setting

 

Description

Gateway

 

This setting is recommended for ETEP-ETEP encryption. Packets are

 

 

encrypted first and then fragmented based on the new packet size, which

 

 

includes the encryption header. This behavior is consistent with RFC 2401.

 

 

The gateway (ETEP) performs the reassembly.

 

 

When the reassembly mode is set to gateway, the Ignore DFBit setting is

 

 

automatically enabled.

Host

 

This setting is required for the ETEPs to interoperate successfully with some

 

 

security gateways. Packets are fragmented before they are encrypted, and

 

 

the encryption header is added to the packet fragments. The destination

 

 

host performs the reassembly.

 

 

 

310

EncrypTight User Guide

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Black Box ET0010A, ET1000A, EncrypTight, ET0100A manual Reassembly Mode, Ignore DF Bit settings, Reassembly mode settings
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EncrypTight, ET0100A, ET0010A, ET1000A specifications

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