L2TP - About General DSL Technology

Hi Fellas,

I am going to write something about xDSL Tech;








Consider those beautiful ATM Clouds as "IP" Please !

Cisco CCIE v5 exam makes us responsible to learn such DSL issues ; access group (a bba-group) that configuring broadband connection on ISP site. (I will share the lab in a couple of days)

Whatever...

In this scenerio, we use Cisco Router with (WIC card is using also it should be HWIC-1ADSL-M compatible ADSL, ADSL2 and ADSL2+ for Annex A) as home regulare CPE (modem) right?

The cloud represents there are lots of customers like us that is big, fluffy cloud.









Customer end is RJ 11, DSLAM connection is RJ 21. Thats how we connect multiple customers to single point on IP-DSLAM.


PPP Sessions Through Third-Party ISP







What is third party ISP  ?

ILEC - Independent Local Exchange Carrier - these are the companies that own the actual lines out on the pole or buried in the ground.

CLEC - Competitive Local Exchange Carrier - these are the companies that have the equipment to provide you 3rd party service over the ILEC lines.





The PPP session is initiated from the customer, it includes some authentication parameters, such as the CHAP username and password. Typically the username would contain a domain name such as stevegarbett@plusnet.net and some random password.

 Than we need a way of tunneling the layer 2 ppp session over towards the 3rd-party ISP’s route , after that  the ISP authenticates the customers PPP credentials, which then gives the customer access to the service provider network.

L2TP - Layer 2 Transport Protocol

Hi Fellas,

Today, we got chief's special recipe ;  L2TP protocol.

Mostly used on IP/MPLS Service Providers. Attention , I wrote IP/MPLS which provides IP and MPLS separately because L2TP uses just IP platform.

- Remote users can access a VPN over a public IP network without a gateway ( What ? Some kind of P2PTP VPN Tech without using VRF , RD,RT, COOL ! )

- We use L2TP as Point to Point Protocol

-  L2TP is a tunneling protocol, it's kinda like GRE or something but L2TP is able to tunnel L2 stuff.....Ethernet, frame-relay, ATM, etc. A tunnel like GRE can only encapsulate layer 3 (IP) traffic.

-We use it to tunnel L2 over a L3 network. For example, a service provider has a L3 network and a customer that wants to connect two sites. Let's say they want to bridge their LAN over the service provider network. With L2TP, you can do that...

- L2TP protocol uses UDP port 500 ( which is more easily blocked by NAT firewalls, and may therefore require port forwarding to get rid off such kind of problems

-  It doesn't provide any encryption stuff ( We can define IPSec, If we want to hide infos from customer network grid)


L2TP is generally deployed for such these cases;


---- Carrying PPP Session Through Service Provider Network



This shits is so old - legacy ATM WAN Network.









Originates L2TP Tunneling for PPP Session

LAC (L2TP Access Concentrator) : Originates L2TP Tunneling for PPP Session
LNS (L2TP Network Server) : Terminates L2TP Tunneling for PPP Session

The LAC and LNS are the two endpoints of the tunnel, normally the LAC is on the customer side and the LNS is the service provider side. It could be a router or another device.


To sum, L2TP is used to carry PPP sessions through IP/MPLS network.





Cust1 is able to see and connect Cust2 Local Routes as VPN services with using L2TPv3 .


-  L2TPv3 is an industry-standard Layer 2 tunneling protocol that ensures interoperability among vendors

- L2TPv3 does not require Multiprotocol Label Switching (MPLS) 

- L2TPv3 supports Layer 2 tunneling over IP for any payload 

- To enable Simple Network Management Protocol (SNMP) notifications of L2TP session up and down events, enter the snmp-server enable traps l2tun session command before configuring L2TPv3.