MPLS AToM – Pseodowires
MPLS AToM is very different to L2TP in that it does not utlize an IP header in anyway to carry data accross an MPLS network.
AToM uses LDP extended discovery ie targeted hellos to establish an LDP session with a non-directly connected neighbor.
Lets examine the topology below.
To setup a AToM circuit using the configuration below.
R1
pseudowire-class ZARAR-AToM
encapsulation mpls
interface FastEthernet1/1.1
encapsulation dot1Q 10
xconnect 3.3.3.3 10 pw-class ZARAR-AToM
R3
pseudowire-class ZARAR-AToM
encapsulation mpls
interface FastEthernet1/1.1
encapsulation dot1Q 10
xconnect 1.1.1.1 10 pw-class ZARAR-AToM
When you configure the xconnect it triggers an LDP extended discovery. A unicast LDP targetted hello is sent from R1 to R3. When R3 responds an LDP TCP session is then setup between R1 and R3. you can see the details of the session using the command below.
R1#show mpls ldp neighbor
Peer LDP Ident: 2.2.2.2:0; Local LDP Ident 1.1.1.1:0
TCP connection: 2.2.2.2.21278 - 1.1.1.1.646
State: Oper; Msgs sent/rcvd: 37/38; Downstream
Up time: 00:26:31
LDP discovery sources:
FastEthernet1/0, Src IP addr: 10.0.0.2
Addresses bound to peer LDP Ident:
10.0.0.2 10.0.0.5 2.2.2.2
Peer LDP Ident: 3.3.3.3:0; Local LDP Ident 1.1.1.1:0
TCP connection: 3.3.3.3.39795 - 1.1.1.1.646
State: Oper; Msgs sent/rcvd: 32/32; Downstream
Up time: 00:20:18
LDP discovery sources:
Targeted Hello 1.1.1.1 -> 3.3.3.3, active, passive
Addresses bound to peer LDP Ident:
10.0.0.6 3.3.3.3
As you can see R1 has 2 neighbors, R2 and R3. Looking at the detail for neighbor 3.3.3.3 you can see it was established using an LDP targeted hello.
Traffic forwarding
When ethernet frames are received from CE1, they are double tagged as follows.
The inner tag or bottom tag is associated with the pseodowire itself. Looking at the mpls forwarding table this will be 19.
R1#show mpls forwarding-table
Local Outgoing Prefix Bytes Label Outgoing Next Hop
Label Label or VC or Tunnel Id Switched interface
16 Pop Label 2.2.2.2/32 0 Fa1/0 10.0.0.2
17 17 3.3.3.3/32 0 Fa1/0 10.0.0.2
18 Pop Label 10.0.0.4/30 0 Fa1/0 10.0.0.2
19 No Label l2ckt(10) 48090 none point2point
The outer tag or the tag used to switch the mpls frame to its final destination is the tag associated with the loopback addresses of the remote PE so in this case a label of 17 is stacked on top of the AToM label.
The traffic is then label switched using the outer label to the final destination ie R3.
Click here to see a packet capture for traffic forwarded using AToM
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