CCDE TOPIC: L2TPv3 & ATOM

L2TPv3 is a tunneling protocol similar to GRE
ATOM is L2TPv3 over MPLS

Here are some highlights:

  • Does not support Layer 3 protocols – this is Layer 2 only. GRE supports Layer 3
  • Does not require MPLS
  • Can change encapsulations between CE routers. For example, PPP to Ethernet is supported (this is called interworking)
  • Has support for ‘cookies’ which help avoid spoofing
  • Data/Control plane separation
  • Supports local switching as well

Technical details:

  • IP protocol type 115
  • L2TPv3 adds several bytes of overhead (4)
  • Fragmentation is supported pre-tunnel
    • important to do this at the edge
    • 12.0(24)S introduces the pre-tunnel fragmentation
      • this avoids the remote PE reassembly
    • ip pmtu in the pseudowire-class
    • ip dfbit set in the pseudowire-class (forces a drop / ICMP if packet too big)
  • Tunnel selection supported
    • Unidirectional – similar to TE
    • destinations must be /32 loopbacks
    • preferred-path under pseudowire-class
    • must configure traffic engineering

Quality of Service

  • Supported under MQC
    • classification based on CoS or VLAN only – no support for DSCP
    • maps to EXP in MPLS or IP DSCP
    • marks on layer 2 fields – Ethernet 802.1p, FR = FECN/BECN (outbound to CE only)
  • Supports multiple color policers

Interworking

  • Ethernet
    • native service is Ethernet
    • CEs may be required to use bridging (if using FR/ATM/HDLC/PPP) – IRB or RBE
    • Supports IP and other protocols
  • IP
    • Supports only IP
    • Simpler CE configuration possibly

Debugs

  • show mpls l2transport vc
  • debug mpls l2transport signaling message

References

  • http://www.faqs.org/rfcs/rfc3931.html

Here’s a basic example of L2TPv3 tunneling:

PE1:
l2tp-class CU1
password 7 11081B06464058
!
pseudowire-class CU1-PW
encapsulation l2tpv3
sequencing both
interworking ip
protocol l2tpv3 CU1
ip local interface Loopback0
!
interface FastEthernet1/0
no ip address
duplex auto
speed auto
no cdp enable
no clns route-cache
xconnect 192.168.1.2 103 encapsulation l2tpv3 pw-class CU1-PW

PE2:
l2tp-class CU1
password 7 00051105550958
!
pseudowire-class CU1-PW
encapsulation l2tpv3
interworking ip
protocol l2tpv3 CU1
ip local interface Loopback0

interface FastEthernet1/0
description connnection to CU1B f0/0
no ip address
duplex auto
speed auto
no cdp enable
no clns route-cache
xconnect 192.168.1.1 103 encapsulation l2tpv3 pw-class CU1-PW

Here’s a basic example of AToM, bridging Frame-Relay to Ethernet

PE1:

frame-relay switching
!
l2tp-class CU1
password 7 00051105550958
!
pseudowire-class CU1-PW
encapsulation l2tpv3
interworking ip
protocol l2tpv3 CU1
ip local interface Loopback0
!
interface Serial2/0
no ip address
encapsulation frame-relay
serial restart-delay 0
clockrate 2016000
frame-relay intf-type dce
!
connect ethernet-fr Serial2/0 100 l2transport
xconnect 192.168.1.2 300 pw-class CU1-PW

CU1A:

interface Serial2/0
ip address 172.16.111.1 255.255.255.252
encapsulation frame-relay
ip ospf network broadcast
serial restart-delay 0
frame-relay map ip 172.16.111.2 100 broadcast

PE2:
l2tp-class CU1
password 7 1513090F557878
!
pseudowire-class CU1-PW
encapsulation l2tpv3
interworking ip
protocol l2tpv3 CU1
ip local interface Loopback0
!
interface FastEthernet1/0
description connnection to CU1B FastEthernet0/0
no ip address
no cdp enable
xconnect 192.168.1.1 300 encapsulation l2tpv3 pw-class CU1-PW

CU1B:
interface FastEthernet0/0
ip address 172.16.111.2 255.255.255.252

Changes for PPP -> Ethernet

PE1:
interface Serial2/0
no ip address
encapsulation ppp
serial restart-delay 0
clockrate 2016000
no cdp enable
xconnect 192.168.1.2 300 encapsulation l2tpv3 pw-class CU1-PW


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