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When do i have to use a gre over ipsec tunnel? i have heard that when i m using a routing protocol and vpn site to site i need a gre tunnel

i have configured a network with ospf and a vpn site to site without gre tunnel and it works very well. I want to know, when do i have to use gre tunnel over ipsec

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Cisco Employee

Jose, It sounds like you



It sounds like you currently have an IPsec Virtual Tunnel Interface (VTI) configured. By this, I mean that you have a Tunnel interface running in "tunnel mode ipsec ipv4" rather than having a crypto map applied to a physical interface. In the days before VTIs, it was necessary to configure GRE over IPsec in order to pass certain types of traffic across an encrypted channel. When using pure IPsec with crypto maps, you cannot pass multicast traffic without implementing GRE over IPsec. Today, IPsec VTIs and GRE over IPsec accomplish what is effectively the same thing with a few exceptions. For example, by using GRE over IPsec, you can configure multiple tunnels between two peers by means of tunnels keys, pass many more types of traffic rather than IP unicast and multicast (such as NHRP as utilized by DMVPN), and you can also configure multipoint GRE tunnels whereas VTIs are point to point.


Here's a document which discusses VTIs in more depth:




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