L2TP-over-IPsec (may be off topic)

[Kok-Yong Tan <ktan () realityartisans com>]

 From what I've read here (especially figures 54 and 55):

<http://www.juniper.net/techpubs/software/erx/junose53/swconfig- 
routing-vol1/html/l2tp-over-ipsec-config4.html#1028288>

it appears that the L2TP payload is encapsulated within the IPsec  
structure.  As such, UDP port 1701 shouldn't need to be opened on any  
device in between the end points of an L2TP-over-IPsec VPN tunnel,  
only UDP ports 500 for IKE and 4500 for NAT-T. Also, Wireshark should  
only see IPsec packets if located anywhere except at the endpoints  
regardless of whether pure IPsec or L2TP-over-IPsec VPNs are operating.

However, I have a physically separate hardware firewall in between  
the endpoints (a L2TP-over-IPsec client and a L2TP-over-IPsec server)  
and I've discovered that the L2TP-over-IPsec VPN will only  
successfully connect if UDP port 1701 is open on the firewall.

Can anyone explain why UDP port 1701 needs to be opened on the  
hardware firewall if the L2TP payload is encapsulated within the  
IPsec packet and thus hidden?
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