Application Note 264

The continuing expansion and popularity of the Internet is forcing routers in the core network to support the interconnection of more and more networks. Based on layer 3 (IP) parameters, these essential devices route information from one logical network to another according to the destination IP address.

However, as the number of interconnected networks grows, so does the strain on the processing power of these devices. Advances in hardware logic have enabled routers to keep up with the increased IP address ranges; however the routing decisions could still have an impact on the traffi c fl ow of the interconnected network.

Multiprotocol label switching (MPLS) is a traffi c-directing technology that promises a more efficient routing scheme based on the assignments of labels to routed packets. This allows for a more effi cient routing process as well as the ability to control the fl ow of traffi c within the network—a process commonly known as traffi c engineering.

The purpose of this application note is to discuss the basic technical aspects of MPLS and the testing needs associated with deploying and maintaining MPLS networks.

LABEL SWITCHING VS. TRADITIONAL ROUTINGRouting is defi ned as the transfer of information across interconnected networks—between an origin and a destination network—through at least one network component called a router. Routing will occur mainly based on the destination IP address found in layer 3 of the OSI model, or the network layer.

Traditional routers exchange information and build routing tables, determining the lowest-cost next hop that a frame should take in order to reach the destination IP address. This is accomplished using routing algorithms such as BGP (Border Gateway Protocol) and OSPF (Open Shortest Path First).

The traditional routing process is a straightforward but strenuous process. Once a packet is received by a router, it is inspected in order to obtain the destination IP address. This address is then compared to an internal database of IP address ranges, and the next best hop to attain this destination is calculated. This process can be further complicated by the possibility of having multiple next-best-hop destinations. In such a case, a router must perform additional analysis in order to identify a more specifi c route.

As stated above, routing algorithms are only concerned with the lowest-cost route and do not take into consideration quality-affecting parameters such as latency or links with lower utilization.

MPLS, on the other hand, is a frame-forwarding mechanism based on the application, treatment and exchange of labels that provide effi cient forwarding of traffi c within an MPLS-enabled network. These labels are inserted as the packets enter the MPLS network, and removed as they exit the network through label edge routers.

MPLS is not designed to replace IP or IP routing protocols, but instead works in conjunction with IP routing protocols to provide a simple and less process-intensive approach to determining the next best hop. External routing protocols such as BGP are still used to determine connectivity to the edge routers, while label switching avoids complex routing tables through the use of simple and fi xed length labels. These labels are easy to search in lookup tables, and they are easier to treat and manipulate than complex IP addresses and their associated subnet masks.

MPLS BASICS

Network ArchitectureThe MPLS network is typically composed of two main devices, the label edge router (LER) and the label switch router (LSR).

The label edge router is, as the name implies, located at the edge of the MPLS network and is responsible for the insertion of labels before transmission in the MPLS network. The label switch router is a core device that performs label operation and packet forwarding through the MPLS-enabled network.

Packets travel across the MPLS-enabled network via a specifi c route referred to as the label switched path (LSP). This path is unidirectional and is defi ned between ingress edge routers to an egress edge router. In bidirectional communication, return traffi c does not necessarily take the same path as the original traffi c, therefore independent LSP assignment is necessary for each direction.

MPLS Basics and Testing with the NetBlazer SeriesHammadoun Dicko, Datacom Product Specialist

© 2012 EXFO Inc. All rights reserved.

Application Note 264

The LabelThe MPLS label is inserted between the layers 2 and 3, and is 32 bits long.

The MPLS label contains the following parts:

› Label: The label itself is 20 bits long, which allows 2^20 -1 combinations (about 1 million different labels).

› Class of service (CoS): These three bits allow to classify the traffi c according to seven levels of priority. These have the same function as the IP ToS class of service bits.

› Stack bit: This bit is used to indicate if the MPLS label is the last label, as labels can be stacked on top of other labels.

› Time to live (TTL): This value determines how many MPLS routers can a packet traverse before being discarded.

An Ethernet frame can contain more than one label, as MPLS allows label stacking. In label-stacking operations, a label is pushed onto an existing label, thus creating an inner and an outer label. As the stacked label is forwarded within the MPLS cloud, LSRs are only aware of the outermost label.

In turn, this creates a form of security as the inner label is only treated when it becomes the last label. This method is typically used in VPN applications.

LDP, LIB and FECLabel distribution protocol (LDP) is an MPLS protocol designed to distribute labels between label edge and label switch routers. LSRs use LDP in order to build routing and forwarding databases called the label information base (LIB). LERs use LDP in order to establish forward equivalence class (FEC) tables, which label incoming packets as they enter the MPLS cloud via the LERs.

Once LIB and FEC tables are built, MPLS routing and forwarding is a straightforward process:

1. At the LER, the incoming packet is inspected and labeled using the information found in the FEC tables. It is then forwarded to the next hop.

2. When the next hop receives the packet, it inspects the label and compares it to its internal LIB. Then, it performs the labeling operation and forwards the packet to the next hop according to the LIB entry.

3. The process repeats until the packet reaches the far-end LER. The labels are then removed and the packet is forwarded to its fi nal destination.

The Advantages of MPLSThe forwarding process clearly shows one of the major strengths of MPLS—the forwarding mechanism. In MPLS, the routing decision is performed at the edge as packets enter the core, while effi cient packet switching occurs in the core. The routing decision is only performed once. Once it is inserted, the packet is simply forwarded according to the label, and its fi xed length ensures that it is quickly analyzed and processed.

Another major strength of MPLS is the traffi c engineering capabilities of label insertion. Since frames are forwarded via labels, carriers can easily control the route that packets take and even design QoS mechanisms using MPLS labels. This type of fl exibility is not available in traditional routing protocols and provides management and control functions to carriers on MPLS-enabled networks.

Example of Fibre Channel SAN

Layer 5-7 Higher Layer Applications

Layer 4 TCP-UDP

Layer 3 IPv4-IPv6-Rw Data

Layer 2.5 MPLS Label

Layer 2 PPP-Ethernet-HDLC-ATM-Frame delay

Layer 1 Optical-Electrical

DestinationsMAC

Source MAC

Ethertype0x88470x8848

MPLSLabel

IPPacket

EthernetFCS

MPLS Label COS StackBit TTL

© 2012 EXFO Inc. All rights reserved.

Application Note 264

TESTING WITH THE NETBLAZER SERIESTesting MPLS networks usually involves ensuring connectivity and resiliency, as well as measuring performance. The following scenarios represent typical MPLS edge to MPLS edge, MPLS core to customer edge, and VPN/stacking tests.

Customer Edge to Customer EdgeThis basic test scenario involves sending untagged packets from the customer edge to ensure they are properly tagged and serviced through the MPLS network. This test can be used to measure end-to-end performance or to ensure that the network is properly confi gured via a network loading test.

The best-suited test application is the EtherSAM dual test set, in which the NetBlazer series simulates a simultaneous bidirectional link between the two testers. Untagged traffi c from multiple streams is generated and sent in both directions. The traffi c generation, BER and RFC 2544 tests can also be used.

MPLS Edge to MPLS EdgeIn this test scenario, traffi c is sent from the originating MPLS edge router to the destination MPLS edge router to measure performance and ensure that traffi c can fl ow within the MPLS network. It ensures that the label information base (LIB) is properly provisioned and a label switch path (LSP) can be established.

One specifi c case of MPLS edge to MPLS edge testing is VPN emulation. Traffi c that is already tagged is sent through an MPLS edge or core to verify that edge and switch routers properly service these tagged frames by stacking a supplementary label and properly forwarding them.

Just as in the previous scenario, the NetBlazer series can be used to perform this test. In this case though, MPLS tagged traffi c is generated in both directions. At the MPLS layer, streams can be generated with up to two MPLS layers with all fi elds of the MPLS label available for confi guration. Bidirectional EtherSAM is once again the best-suited test, even though traffic generation and monitoring can be used as well.

Customer Edge to MPLS CoreIn this test scenario, traffi c is sent from the customer standpoint to the MPLS core to test the forward equivalence class (FEC) found at the entrance edge router, and verify that all packets are properly labeled and forwarded to the MPLS core.

This test scenario can also be performed from the MPLS core to the customer edge, ensuring that the destination MPLS edge router properly strips labels and forwards packets to the proper customer edge.

In this case, the NetBlazer series is used the same way as in the previous scenario, except that the tagged traffi c is only sent in one direction, at the core end. The customer edge tester will transmit untagged traffi c.

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Application Note 264

APNOTE264.1AN © 2012 EXFO Inc. All rights reserved. 2008

Printed in Canada 12/04

MPLS Traffic Analysis with the NetBlazer SeriesThe NetBlazer series provides comprehensive test suites for analyzing and qualifying MPLS networks. Analysis is performed on incoming traffi c with specifi c statistics on MPLS tagged traffi c.

CONCLUSIONMultiprotocol label switching efficiently enhances the traffic-forwarding process while still implementing essential routing processes across the core network. However, the deployment of MPLS requires unique testing scenarios to assess the performance and reliability of the network, as well as to guarantee service levels. This becomes even more important as networks become increasingly complex with many types of traffi c. EXFO’s NetBlazer series offers a comprehensive MPLS test solution to effi ciently qualify Ethernet services from end to end, validating metro and core tunneling technologies.