In Cell Mode MPLS, ATMLSR is also responsible for participating in LDP/TDP with the adjacent neighbor routers. Edge ATM routers are connected with LC-ATM (Label Control) LSR routers over LC interface. Cell mode MPLS uses VPI/VCI field in the ATM header as label value. Cell Mode MPLS uses downstream on demand for label propagation.
PE1#show mpls ldp neighbor
Peer LDP Ident: 4.4.4.4:1; Local LDP Ident 1.1.1.1:1
TCP connection: 10.10.10.4.11037 – 10.10.10.1.646
State: Oper; Msgs sent/rcvd: 64/64; Downstream on demand
Up time: 00:48:08
LDP discovery sources:
ATM1/0.1, Src IP addr: 10.10.10.4
OSPF is used as IGP with single area. Basic configuration of LC-ATM interface is given below which is using the default values of VPI/VCI:-
Configuration of LC- ATM Edge
interface ATM1/0
no ip address
no atm enable-ilmi-trap
no clns route-cache
!
interface ATM1/0.1 mpls
ip address 10.10.10.1 255.255.255.0
no atm enable-ilmi-trap
mpls label protocol ldp
mpls ip
LC-ATM LSR
interface ATM1/0
no ip address
no atm enable-ilmi-trap
no clns route-cache
!
interface ATM1/0.1 mpls
ip address 10.10.10.4 255.255.255.0
no atm enable-ilmi-trap
mpls label protocol ldp
mpls ip
In the above configuration, default VPI/VCI values are used. To check the VPI/VCI values, run “show atm vc” command on LC-ATM Edge router.
The above output reveals lot of information about the VPI and VCI values along with the encapsulation used. First VPI/VCI is 0/32 which is default and used as control vc. It uses the LLC/SNAP encapsulation as defined in RFC 2684. It SHOULD be possible to configure an LC-ATM interface with additional VPI/VCIs that are used to carry control information or non-labeled packets. In that case, the VCI values MUST NOT be in the 0-32 range.(RFC 3035 Section 7)
The other LVC uses VPI/VCI values are 1/33, 1/34, 1/35,1/36 and 1/37 with encapsulation MUX. The LVC uses MUX encapsulation because they carry MPLS datagram.