Cell Mode MPLS Over ATM – Manually Specify VPI/VCI Range

In my previous post, I have shown how to manually configure control VC at LC-ATM interface. In this post, we will see how we can manually configure the VPI/VCI in MPLS ATM networks. This is mainly used at that time when SP doesn’t want to add the individual entries at each time for customers. By using command “mpls atm vpi 10 vci-range 33-100”, we actually tell the router to use LVC for these VPI/VCI values. Encapsulation type will be used as MUX because these VPI/VCI values are used for MPLS datagram’s.

In the above out, still the control VC is using VPI 0 and VCI 32 because it is not changed. If you want to change the control VC values simply add the following command:-

“mpls atm control-vc 1 100”

Make sure both the commands are used in both ends.

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Cell Mode MPLS Over ATM

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.

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Difference Between VC Based Multiplexing And Logical Link Control Encapsulation in ATM?

I have been stumbling since last week to understand the concept behind AAL5Snap and AAL5Mux encapsulation. Finally I made my self clear by reading RFC 1483 which explicitly depicts about the difference and usage of both Atm Adaptation Layer 5 encapsulations. These encapsulations techniques are used when user willing to carry the multiple protocol traffic over PVC. It is defined in two ways:-
1. VC based Multiplexing (AAL5Mux)
2. Logical Link Control Encapsulation (AAL5Snap)

VC based Multiplexing (AAL5Mux) is used to carry one protocol per PVC. e.g. If we want to forward IP and IPX over ATM, in this case we need to define two dedicated PVC. One PVC will carry IP and another will carry IPX. As the number of protocols will increase, PVC will also increase.

Logical Link Control Encapsulation (AAL5Snap) is used to carry multiple protocols in single PVC. In this case user multiplex all the protocols in one PVC. To differentiate PDU, Logical Link Control header is added.
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Basics of ATM (Asynchronous Transmission Mode)

ATM packet is also known as CELL. It has a fixed size of 53 bytes, which consist 48 bytes of payload and 5 bytes of header. CELL header is further divided into types of format:-
1. UNI Cell Format
2. NNI Cell Format
UNI Cell Format aka User Network Interface, which is directly connected to any ATM switch and responsible to send and receive atm cells.

NNI Cell Format aka Network to Network Interface; the interface between any two ATM switches.

ATM is based on pure circuit and packet switching technique. ATM connection is having VPI and VCI concept. VPI stands for Virtual Path Identifier and VCI stands for Virtual Connection Identifier. The VPI/VCI field is 24 bits long in UNI 28 bits in NNI. VPI field is 8 bits long in UNI and 12 bits long in NNI. The VCI field is 16 bits long in UNI as well as in NNI. It means we can have only 2^8=256 VPI connections in UNI and 2^12=4096 connections in NNI. Each UNI and NNI VPI is having 65536 VCI connections.
VPI connections can be assigned from any number but some limitation is there for assigning VCI connections. VCI 0 – 15 are reserved by ITU-T, 16 – 31 are reserved by ATM forum and 32 to 65535 are free and could be used by users.
Virtual connection is the combination of both VPI and VCI. VPI and VCI values are always local to the ATM devices and have nothing to do with the remote. Intermediate ATM switches translate the incoming and outgoing VPI and VCI values along with their ingress and egress ports. This operation is referred as label swapping.


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ATM - Packet and Circuit Switching

Circuit switching was the first information transform mode which was used for telephone networks. It is more efficient for transport real time services which requires quality of service qurantees in terms of delay and loss. In fact,TDM doesnot offers no delay except for switching latency and no packet losses. But on the otherhand, circuit switching was found inefficient for the transport of bursty traffic.
For bursty traffic packet switching was developed. The packet switching offers transport of bursty traffic but no quality of services. It is essentially a best effort switching technique where network will make attempt to trasfer every packet.

ATM switching is the technology which was developed by merging the concept of both packet as well as circuit switching. It has the various advantages
1. It offers quality of services like circuit switching.
2. It offers the statistical multiplexing like pacet switching.

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