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
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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