Mapping IP Multicast Addresses to Ethernet and FDDI MAC Addresses

To support IP multicasting, the Internet authorities have reserved the multicast address range of 01-00-5E-00-00-00 to 01-00-5E-7F-FF-FF for Ethernet and Fiber Distributed Data Interface (FDDI) media access control (MAC) addresses. As shown in Figure 4.1, the high order 25 bits of the 48-bit MAC address are fixed and the low order 23 bits are variable.

To map an IP multicast address to a MAC-layer multicast address, the low order 23 bits of the IP multicast address are mapped directly to the low order 23 bits in the MAC-layer multicast address. Because the first 4 bits of an IP multicast address are fixed according to the class D convention, there are 5 bits in the IP multicast address that do not map to the MAC-layer multicast address. Therefore, it is possible for a host to receive MAC-layer multicast packets for groups to which it does not belong. However, these packets are dropped by IP once the destination IP address is determined.

For example, the multicast address 224.192.16.1 becomes 01-00-5E-40-10-01. To use the 23 low order bits, the first octet is not used, and only the last 7 bits of the second octet is used. The third and fourth octets are converted directly to hexadecimal numbers. The second octet, 192 in binary is 11000000. If you drop the high order bit, it becomes 1000000 or 64 (in decimal), or 0x40 (in hexadecimal). For the next octet, 16 in hexadecimal is 0x10. For the last octet, 1 in hexadecimal is 0x01. Therefore, the MAC address corresponding to 224.192.16.1 becomes 01-00-5E-40-10-01.

The 25th bit is always zero.

Understanding LDP

Label Distribution Protocol(LDP) is a signalling protocol that is used in an MPLS network to distinguish label information to LERs and LSRs. In this way, it is very similar to an IP routing protocol except that instead of distributing IP network address information, it distributed label information. Before LDP can be enabled on a router, the network must be running a routing protocol. The routing protocol allows LDP to know which destinations it needs to generate labels for. Once a peering session is established, the routers check their routing tables and send out a label associated with networks that they see.

Understanding MPLS

MPLS is a packet-forwarding technology which uses labels to make data forwarding decisions. With MPLS, the Layer 3 header analysis is done just once (when the packet enters the MPLS domain). Label inspection drives subsequent packet forwarding. MPLS provides these beneficial applications:

Virtual Private Networking (VPN)
Traffic Engineering (TE)
Quality of Service (QoS)
Any Transport over MPLS (AToM)

Additionally, it decreases the forwarding overhead on the core routers. MPLS technologies are applicable to any network layer protocol.

In MPLS network, routers are catagorized as Label Edge Routers(LERs) and Label Switched Routers(LSRs). The LERs are the endpoints of the MPLS tunnels, known as Label Switched Paths(LSPs)

and are normally at the edge of the network. The LSRs are at the core of the network and provide the connectivity between the LERs.

The purpose of MPLS is to provide a tunneling service to forward customer packets across the provider network based on information other than simply IP address destination. In order to provide the service, MPLS will mark packets inbound from the customer devices(CE) with a special header called label. The label is simply an additional header that is packets inbound to the provider network.

A label is a short, four-byte, fixed-length, locally-significant identifier which is used to identify a Forwarding Equivalence Class (FEC). The label which is put on a particular packet represents the FEC to which that packet is assigned.

                         

Label—Label Value (Unstructured), 20 bits
Exp—Experimental Use, 3 bits; currently used as a Class of Service (CoS) field.
S—Bottom of Stack, 1 bit
TTL—Time to Live, 8 bits

These labels are used to make forwarding decisions rather than the IP address. This basically turns the Layer 3 routed network into a switched network. This provides several advantages over traditional IP routing.

LERs mark the packets with label information based on provider configured policies. By marking the packets, the provider network can differentiate customer traffic based on the labels, for example, by forwarding differently labeled packets over a preferred path.

The MPLS enabled routers(LERs and LSPs) use a signalling protocol to distribute labeled packets across the network. The method for distributing labels through the network depends on the signalling protocol being used, either Label Distribution Protocol(LDP) or Resource Reservation Protocol(RSVP).

MPLS and VPN services - Basic Terms

Customer Edge Routers:

• Reside on customer permises
• Provides access to service provider network over a link or more Provider Edge Routers.
• End user typically owns and operates these devices.
• CE devices are unaware of tunneling protocols over VPN services provided by the service provider.
• Eg: Home networking router or wireless access point.

Provider Edge Router:

• Has at lease one interface that is directly connected to the CE device.
• PE device usually has at lease one interface that connects to the Service Provider Core devices.
• Must be able to connect to different CE devices over different access media, usually able to support many different interface types.
• Customer's gateway to the VPN services offered by the service provider.

Provider Router:

• Routers located in the Provider Core Network
• P router supports the service provider's bandwidth and switching requirements over a geographically dispersed area
• Does not connect directly to the customer equipement
• P routers does not care about the different types of services that are provided to individual customers and instead simply make forwarding decisions based on information attached to the IP packets by the PE devices.

Thus we can say that much of the intelligence in a service-based network is provided by the PE devices.It provides almost all of the critical setup and control that are needed to mark traffic and have it forwarded correctly throughout the rest of a service network.

Service Access Point(SAP): 

• Provides the logical entity that serves as the customer access to whatever services are provided by the PE.

Service Distribution Point(SDP):

This is the method that a service uses to connect to another router's services.

Important features of SDPs include:

• To provide the transport tunnel encapsulation that the service will be using such as MPLS/RSVP-TE/MPLS-LDP or IP/GRE
• SDP IDs are locally unique, the same SDP ID can be used on another router
• An SDP is not specific to one service. Many services can use the same SDP.