L2TP explanation

[carl_townshend]

Hi all

Can anyone tell me about L2TP, why people use it, and does it encapsulate the IP header or does it need ppp to do this ?

[Laurent Aubert]

Hi,

L2TPv2 allows you to transport PPP session over an IP backbone via an L2TP tunnel.

One common scenario is when a ISP wants to provide Internet access to users where it doesn't control the dial network (PSTN/DSL or ISDN). In this case it will negotiate with a local wholesale Dial SP so it will forward the PPP session of the user who bought the service from the ISP via L2TP. L2TP is mandatory as between the wholesale SP and the ISP, there is only an IP backbone.

It's also useful when you want to use DSL access to connect commercial customers site to their VPN.

L2TPv3 can encapsulate any Layer 2 protocols like Ethernet for example. Common application of L2TPV3 is to provide L2VPN services when you are not using MPLS in your core.

HTH

Laurent.

[Laurent Aubert]

I forgot here is a link for L2TPv2 configuration example: http://www.cisco.com/en/US/tech/tk827/tk369/technologies_white_paper09186a00800a43e9.shtml

Laurent.

[carl_townshend]

hi

i still need to see the benifits for this, does it hide every hop between 2 endpoints ?

and why would we need to encapsulate ethernet frames, would this so we can extend vlans over the tunnel, and bridge my lan etc?

also how do you say send a certain subnet over the tunnel? or would it need to be all traffic ?

cheers

Carl

[Nelson Serrao]

Hello Carl,

I found quite a few benefits of using L2TP v3 on our network one of which you already mentioned. The list goes like this:

1) Minimal configuration when comparing to GRE where you need several tunnel interfaces (one for each VLAN).

2) After configuration the 2 sites are bridged together. So DHCP broadcasts and CDP info is exchanged between the local and remote site as if there was no WAN in between.

Sending a subnet over the tunnel can be done using VLANs. Whatever you allow on the trunk is what will pass through the tunnel.

Hope this helps.

Nelson