Technical White Paper for IP Leased Line

Transcription

1 Technical White Paper for IP Leased Line

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3 Contents Technical White Paper for IP Leased Line Preface Background of the IP Leased Line Technology Associated Technical Standards Technical Introduction Layer 2 Circuit Emulation Technical Scheme Pseudo Wire Technical Scheme MPLS L3VPN Technical Scheme Key Technologies Leased Line Bearing Technology End to End Quality Assurance for IP Leased Line Service Visualized Deployment of IP Leased Line IP SLA Visualized Report on the Quality of the Leased Line Visualized Detection of Leased Line Faults Protection Technology of the Leased Line Services Traditional Leased Line Migration Technology Typical Application Interconnection of Branches within an Enterprise Carrier Wholesale Service Migration of the Traditional Leased Line Conclusion References Acronyms and Abbreviations...43

4 Figures Figure 1-1 Revenue and development trend of the leased line of the enterprise... 2 Figure 2-1 Basic structure of the MPLS L2VPN...5 Figure 2-2 PWE3 basic transmission components...6 Figure 2-3 BGP/MPLS VPN model...8 Figure 3-1 VLL networking in the Martini mode...12 Figure 3-2 VLL networking in the Kempella mode...13 Figure 3-3 Topology model of PW protection...15 Figure 3-4 MS-PW reference model...16 Figure 3-5 Inter-AS BGP MPLS IP VPN Option A...18 Figure 3-6 Inter-AS BGP MPLS IP VPN Option B...19 Figure 3-7 Inter-AS BGP MPLS IP VPN Option C...20 Figure 3-8 Figure 12 Multicast VPN networking model...22 Figure 3-9 VLAN HQoS service and bandwidth model...23 Figure 3-10 MPLS HQoS service and bandwidth model...24 Figure 3-11 Deployment of the PWE3 service through the U Figure 3-13 Using a template to deploy services...26 Figure 3-12 Formulating a template...26 Figure 3-14 Replicating services...27 Figure 3-16 Creating an SLA monitoring instance...28 Figure 3-15 U2520 monitoring center...28 Figure 3-17 Checking the SLA detection result...29 Figure 3-18 Fault and alarm management...29 Figure 3-19 Presentation of the service topology...30 Figure 3-20 Test diagnosis...30 Figure 3-21 Scenario that the E-Trunk implements the redundant access at the access side...31 Figure 3-22 PW Redundancy in the dual-homing scenario...32 Figure 3-23 PW redundancy in the scenario where the E-Trunk accesses the VPLS network...33 Figure 4-1 Interconnection of branches in an enterprise in point-to-point mode Figure 4-2 Interconnection of branches in an enterprise in point-to-multipoint mode...37 Figure 4-3 Interconnection of branches in an enterprise in point-to-multipoint mode...38 Figure 4-4 Wholesale service...38 Figure 4-5 ATM leased line migration...39

5 Technical White Paper for IP Leased Line Keywords VPWS, VPLS, MPLS L3VPN, IP leased line OAM, ATM/TDM leased line migration, HQoS Abstract The ALL-IP service bearer technology has been widely recognized in the industry. The leased line service, as a high value-added service, attracts attention from carriers worldwide. More and more leading carriers worldwide adopt the IP technology to carry the leased line service. On the broadband MAN, more and more new leased line services are using the IP technology. At the same time, services carried over the traditional ATM/TDM leased line are migrating to the IP-based leased line. This document describes the background, development trend, implementation, key technology, and typical service scenarios of the IP leased line to explain the understandings and technical viewpoints of Huawei on the IP leased line service. 1

6 1 Preface The leased line service has long been the most important value-added service for the carrier. With the development of the IP technology and changes of the requirement of the leased line service, carriers need to carry all services uniformly over the IP network. The enterprise leased line service also needs to be carried in an IP network. Through the IP bearer network, enterprise sites interconnection, service wholesale, and data migration from the traditional ATM/TDM network to the IP network can be implemented to provide the high-quality and high value-added leased line services. 1.1 Background of the IP Leased Line Technology The Multi-protocol label switching (MPLS) technology implements high-speed data forwarding through the label over the IP network. Its value lies in that the feature of the connection mode can be used on the connectionless network and that multiple value-added mechanisms (such as the differentiated service, traffic engineering (TE), fault recovery, and path display) can be provided. This helps extend new applications flexibly and meets the new requirements. With these characteristics, the MPLS makes the high-quality and high-valued leased line services feasible. The statistics indicate that the IP-based leased line services are ever increasing, and meanwhile the ATM/TDM-based traditional leased line services are decreasing. For details, see the following figure. Enterprise Data Services Revenue, Worldwide Millions of U.S.Dollars Source: Gartner ( June 2008 ) 120, ,000 80,000 60,000 40,000 Ethernet WAN IP-VPN Legacy Packet Leased Lines Equipment Market Space 20, Figure 1-1 Revenue and development trend of the leased line of the enterprise 2

7 As shown in Figure 1-1, the IP leased line service in the enterprise data market is increasing rapidly, and the traditional ATM/TDM leased line service is decreasing very fast. In this case, carriers are all optimizing the existing network, that is, slowing down or stopping the traditional leased line service and promoting the MPLS-based IP leased line service. In the development of the MPSL-based IP leased line service, carriers, however, face a lot of troubles. During the service migration from the traditional leased line to the IP leased line, carriers need to protect the original investment, reuse the original ATM/TDM network transmission resources, and implement data migration on the network without affecting the user service. This document describes these issues in detail and also describes the smooth migration and the migration solution. 1.2 Associated Technical Standards The MPLS-based IP leased line involves the MPLS basic protocols, VPWS, VPLS, MPLS L3VPN, ATM over PSN, and TDM over PSN. The associated standards are as follows: RFC 2283-Multiprotocol Extensions for BGP-4 RFC 3107-Carrying Label Information in BGP-4 RFC 2547-BGP/MPLS VPNs RFC Pseudo Wire Emulation Edge-to-Edge RFC 4553-Structure-Agnostic Time Division Multiplexing (TDM) over Packet (SAToP) RFC 5086-Structure-Aware Time Division Multiplexed (TDM) Circuit Emulation Service over Packet Switched Network (SoPSN) 3

8 2 Technical Introduction 2.1 Layer 2 Circuit Emulation Technical Scheme Principle With the VPN service based on the MPLS network, carriers can provide the Layer 2 interconnection services based on different media on the same MPLS network, such as ATM, FR, VLAN, Ethernet, and PPP. The Layer 2 interconnection service is used to transmit the user Layer 2 data transparently on the MPLS network. From the perspective of the user, the MPLS network is a L3 switching network, and can set up Layer 2 connections between different nodes. The VLL mode and VPLS mode are mainly involved. VLL Virtual leased line (VLL) is the emulation of the traditional leased line. The IP network is used to emulate the leased line, providing the asymmetric point-to-point (PTP) interconnection service at a low cost. The VLL is a PTP virtual leased line technology, and can transmit all the user data transparently. From the perspective of users on the two ends of the virtual leased line, this virtual leased line is similar to the traditional leased line. VPLS Virtual private LAN service (VPLS) technology connects multiple Ethernet LAN network segments through the packet switched network (PSN) to make the network segments work as a LAN. The VPLS is also called transparent LAN service (TLS) or virtual private switched network service (VPSNS). Different from the ordinary L2VPN PTP service, the VPLS can help carriers provide the Ethernet-based point-to-multipoint (P2MP) service for the users through the MPLS network. Service Element The MPLS L2VPN contains three sections, AC, VC, and tunnel, as shown in the following figure. 4

9 AC VC AC Tunnel MPLS Network Figure 2-1 Basic structure of the MPLS L2VPN Attachment circuit (AC) is the access circuit. The AC is an independent link or circuit that connects the to the. The AC interface can be a physical interface or a logical interface. Attributes of the AC include interface parameters, such as the encapsulation type, maximum transmission unit (MTU), and specific link type. Virtual circuit (VC) is a logical connection between two nodes. Network tunnel is used to transmit the user data transparently. 2.2 Pseudo Wire Technical Scheme Principle PWE3 is a bearer technology that implements the end-to-end Layer 2 service bearing. It is a PTP L2VPN. Between two s on the PSN, the PWE3 takes the LDP/RSVP-TE as a signaling, and emulates various Layer 2 services of the end through the tunnel (MPLS tunnel, GRE, L2TPv3, or other types of tunnels) to implement the transparent transmission of the Layer 2 data on the end on the PSN network. The Layer 2 services include Layer 2 data packet and bit stream. Service Element The basic transmission components and functions of the PWE3 network are as follows: Attachment circuit (AC) is a link or virtual circuit that connects the and. All user packets on the AC need to be directly forwarded to the peer site, including Layer 2 and Layer 3 protocol packets. Pseudo wire (PW) is a VC plus a tunnel. The tunnel can be LSP, 5

10 L2TPv3, GRE, or TE. The virtual connection has direction. The virtual connection of the PWE3 is set up by the VC message transfer through the signaling (LDP or RSVP). The VC message and tunnel is managed to form a PW. For the PWE3 system, the PW is direct connection channel from the local AC to the remote AC to implement transparent transmission of the Layer 2 data. Forwarders select the PW for forwarding the packet after the receives the data frame sent from the AC. The forwarder is actually the forwarding table of the PWE3. Tunnels are used to bear the PW. One tunnel can bear multiple PWs. The tunnel is generally the MPLS tunnel. The tunnel is actually a direct connection channel from the local to the remote to implement transparent transmission of the data between s. Encapsulation: The packet transmitted on the PW is encapsulated through the standard PW encapsulation format and technology. There are multiple encapsulation modes for the PWE3 packet on the PW, which are defined in the draft-itef-pwe3-iana-allocation-x. Pseudo wire signaling is the basis of the PWE3. It is used to create and maintain the PW. Currently, the PW signaling protocols are mainly LDP and RSVP. Quality of service (QoS): Based on the priority carried in the Layer 2 packet header, the packet is forwarded on the network according to the QoS priority. In this case, the MPLS QoS needs to be supported. The following figure shows the location of the PWE3 basic transmission components on the network. VPN1 Site 1 C PW C VPN1 Site 2 U -AGG N VPN2 Site 1 C C VPN2 Site 2 Tunnel AC PW Figure 2-2 PWE3 basic transmission components 6

11 The PWE3 provides a Layer 2 technology for the PTP transmission. With the PWE3, the user can access the IP network smoothly without changing the original access mode, and interconnection between different access modes can be also implemented, thus implementing the service interconnection between the traditional and IP leased line services. 2.3 MPLS L3VPN Technical Scheme Principle The BGP/MPLS IP VPN is a layer 3 virtual private network (L3VPN) technology. It uses the boarder gateway protocol (BGP) to advertise the VPN route information on the IP network of the carrier and forward the VPN packet on the IP network of the carrier. Service Element The basic model of the BGP/MPLS IP VPN contains,, and P. Customer edge () provides an interface for directly connecting to the network of the service provider (SP). The can be a router, a switch, or a host. Generally, the can not sense the existence of the VPN and need not support the MPLS. Provide edge () is an edge device of the SP and connected to the directly. On the MPLS network, all the processing of the VPN is performed on the, and therefore there is a high requirement on the performance. Provide (P) is a backbone router on the SP network, and is not directly connected to the. The P device only needs to support MPLS forwarding, but does not maintain the VPN information. The following figure shows the networking of a BGP/MPLS VPN. 7

12 VPN2 Site 2 VPN1 Site 1 P P VPN2 Site 3 VPN2 Site 1 P P VPN1 Site 2 Figure 2-3 BGP/MPLS VPN model Site 1 and site 5 belong to VPN1. Site 2, site 3, and site 4 belong to VPN2. The and are divided according to the management scope of the carrier and user. The is the boarder between the management scopes of the carrier and the user. The is generally a router. After the adjacency relationship is set up between the and the that is directly connected to the, the advertises the VPN route of this node to the and learns the route to the remote VPN from the. The and exchange the route information by using the BGP/IGP and the static route can be also used. 8

13 3 Key Technologies 3.1 Leased Line Bearing Technology VPLS In the P2MP leased line service, direct interconnection between sites is required. In addition, the correlation between the carriers needs to be minimized. Therefore, the VPLS is recommended to bear the service. In the case of leased line service carried by the VPLS, to provide P2MP Layer 2 communication service, the following issues need to be considered. Prevention of the VPLS loop On the Ethernet, to prevent the loop, the Layer 2 protocols such as STP, RRPP, and REP need to be enabled. Considering the minimum correlation between the user and carrier network, the IP protocol of the user network must not be used on the carrier network, but runs only between the devices on the user network. This prevents the loop on the user network. In the VPLS, "full connection" and "spilt horizon" are used to prevent the loop. Each must create a tree to all the routers under this instance for a VPLS forwarding instance. Each router must support spilt horizon to prevent the loop. That is, the cannot forward packet between the PWs that have the same VPLS instance. Generally, in the same VPLS instance, the s are connected through the PW. Accordingly, spilt horizon means that the data packet received from the PW are not forwarded to other PWs but only forwarded to the AC. The full connection and spilt horizon between s ensure that the VPLS forwarding is reachable and free of loops. When multiple s are connected to the, or different s connected to the same VPLS VPN are connected between each other, the VPLS cannot ensure that the loop does not occur. Therefore, various mechanisms are needed to prevent the loop. User access mode User access modes can be classified into the VLAN access and 9

14 Ethernet access modes. The meanings are as follows: VLAN access: If the VLAN access mode is used, the frame header of the packet that is transmitted from the to the or from the to the carries a VLAN tag. This VLAN tag is a service boarder identifier marked for differentiating the users. It is also called Provider Tag (P-Tag). Ethernet access: If the Ethernet access mode is used, the frame header of the packet that is transmitted from the to the or from the to the does not carry a P-Tag. If the VLAN tag is carried in the frame header, it is only the inner VLAN tag in the user packet, and is called U-Tag (user-tag). The U-Tag is carried in the packet before the packet is transmitted but is not added by the during packet transmission. It is used for the to differentiate the VLAN of the packet and is of no significance for the. The user access mode in the actual scenario can be specified through the configured mode. Hierarchical VPLS (H-VPLS) The VPLS requires the full connection between the s. Therefore, the relation between the number of PWs of one VPLS instance and the number of s is the number of PWs = the number of s x (the number of s - 1)/2. In a large scale VPLS network, the number of PWs and the overhead of the PW signaling are both great. Therefore, network management and extension become complicated. To simplify the network management and improve the extensibility of the network, the H-VPLS networking is introduced. The H-VPLS divides the into U and N. The U mainly functions as the MTU when the user accesses the VPN and is used to connect the and the SP network. The N is on the edge of the core of the VPLS network and provides the transparent transmission of the user packet on the core network. The U need not set up connections with all Ns, but need to set up connections between Ns. Through hierarchy, the H-VPLS reduces the number of PWs and the load of the PW signaling. The advantages of the VPLS are as follows: The VPLS uses the Ethernet port at the user network side, which 10

15 simplifies the LAN/WAN boarder and supports the rapid and flexible service deployment. The VPLS hands over the control and management rights of route policies on the user network to the user, which simplifies the management of the carrier network. All the user router s in the VPLS service are part of the same subnet, which simplifies the IP addressing planning. The VPLS service neither need to sense nor need to participate in the IP addressing and routing. VLL/PWE3 VLL In the P2MP leased line service, direct interconnection between multiple sites is required. In addition, the correlation between the carriers needs to be minimized. Therefore, the VLL is recommended to bear the service. According to the protocols of setting up the VLL, two modes of VLLs are defined in the relevant standards: Martini mode This mode complies with RFC4762 specified by the IETF. In this mode, the LDP is used to transmit the signaling of the VC messages. In the Martini mode, the LDP remote session is set up between the s. The allocates a VC label to each link of the, and transmits the VC label to the remote through the LSP set up using the LDP. In this case, a virtual link is set up on the LSP. In this mode, an LSP between a pair of s on the carrier network can be shared to multiple virtual links. As shown in the following figure, site 1 and site 2 on VPN1 use the LSP1 and LSP2, site 1 and site 2 on VPN2 can also use LSP1 and LSP2. 11

16 VPN 1 Site 1 VLAN 10 VCI601 VPN 2 Site 2 VCI P P VLAN P P VPN 1 Site 2 VCI605 VPN Site VCI705 Figure 3-1 VLL networking in the Martini mode In the Martini mode, the on the carrier network needs to maintain a few VC labels and mapping information of the LSP. The P device needs not process any L2VPN information, and therefore the extensibility is good. In addition, when a new virtual link needs be set up, unidirectional virtual links only need to be configured on the s on the two ends. Therefore, network running is not affected. The Martini mode is applicable to the Layer 2 connection, such as star connection. Kompella mode The Kompella mode complies with FRC4761. Through the setup of the IBGP session between the s, the sites of the L2VPN are detected automatically, and the VPN information is transmitted. The L2VPN information is transmitted between the s through the extended BGP. In label allocation, the MPLS L2VPN uses the label block manner in the Kompella mode. The size of the label block is equal to the range (specified by the user) for the convenience of allocating labels to multiple connections. This mode allows the user to allocate some extra label to the VPN, which reduces the configuration workload during the VPN deployment and capacity expansion. In the Kompella mode, the vpn-target is used to differentiate the VPNs. This brings great flexibility to the VPN networking and is applicable to various VPN network topologies. Different from the MPLS L2VPN in the Martini mode, in the Kompella mode, no operation is performed for the link between the s, but the SP network is divided into different VPNs. The is numbered within the VPN. When the connection needs to be set up between 12

17 two s, the IDs of the local and the remote need only to be configured on the, and the virtual link ID allocated to the link of the local needs to be specified. Site 4 VLAN 10 VOI Site 1 VLAN 11 P P Site 3 VOI Site 2 VLAN 12 P P Site 0 Figure 3-2 VLL networking in the Kempella mode In Kempella mode, the VLL user can be reserved for configuration. When the L2VPN site needs to be added in the future, only the of the new site needs to be configured, and other s need not be configured. As shown in Figure 3-2, in the initial phase, the number of labels allocated to the s on the s is greater or equal to six, and the link to 6 is bound in advance. In this case, when site 4 is added to 6, only 6 and other s bound to 6 need to be added. The advantages of the leased line service through the VLL are as follows: High extensibility Overcoming the complexity of the extension on the traditional ATM or FR network, the MPLS VPN uses the label technology to reuse multiple virtual circuits on the same LSP. Therefore, the only needs to maintain information of one LSP, which greatly improves the extensibility of the system. Clear administrative responsibility The MPLS L2VPN carrier provides the Layer 2 connectivity, and the customer provides the L3 connectivity. Route oscillation caused by the incorrect configuration of the user does not affect the stability of the carrier network. Better security and confidentiality 13

18 The security and confidentiality equivalent to the ATM or RF VPN network are provided. The user maintains its own route information. Therefore, the carrier need not consider IP address overlay of each user, and also need not concerns about route information leakage to the private networks of other uses. This reduces the management workload of the carrier and also improves the security of the user information. Supports from multiple network protocols The carrier provides only the Layer 2 connection. The customer can use any L3 protocol, such as IP, IPv6, IPX, and SNA. Smooth evolution of the L2VPN The MPLS L2VPN is transparent to users. When the carrier upgrades the traditional L2VPN such as ATM and FR to the MPLS L2VPN, the users need not perform any reconfiguration. Except temporary data loss during the switchover, the user services are not affected. PWE3 It is recommended that you deploy the PWE3 to carry services in the traditional leased line transition scenario. Sites with various media can be interconnected through PWE3. In deploying PWE3, you need to consider different networking modes: Networking of Static-and-Dynamic Multi-Hop The static-and-dynamic multi-hop PW indicates that a static PW is set up on one end and a dynamic PW is set up on the other end. The static PW and dynamic PW can be multi-hop PWs. They, however, cannot alternate between each other. The processing of the PW on the U- and the S- is the same as the processing of the static PW or dynamic PW, but is different from the processing of the PW on the S- where the static PW and the dynamic PW meet. On the S- where the static PW and the dynamic PW meet, for the dynamic PW, the static PW can be regarded as the AC of the dynamic PW; the status change of the static PW can be regarded as the status change of the AC of the dynamic PW. The parameters such as the PW 14

19 type and interface MTU need to be specified for signaling negotiation and these parameters must be the same as those configured for the interface of the static PW. For the static PW, it is Up if a tunnel exists; for a dynamic PW, the remote PW is Up if a tunnel exists. If the type and the MTU of the remote PW have the same setting as the local ones, the dynamic PW is Up. PW Protection The PW protection indicates that when a PW fails, such as a PW tunnel is deleted, the traffic can be rapidly switched to another PW. The fast switching on the data plane thus can be implemented. As shown in the following figure: Primary PW U-1 PW1 PW5 PW4 S- PW2 PW3 U-2 PW6 PW7 PW8 S- Secondary PW Figure 3-3 Topology model of PW protection To protect the PW, you need to do the following (in the case of the multi-hop): Configure two PWs on two U-s respectively. The two PWs map each other. Configure one PW (PW5) on one U- (U-1) as the secondary PW. Configure PWs on the S-s that are passed. The PWs configured on the S-s and on the U-s thus form multihop PWs, as shown in figure Figure 3-3. Both the primary PW and the secondary PW need to negotiate and process the signaling. This is the same as the processing of ordinary dynamic multi-hop PWs. If the primary PW fails (for example, the LDP session is Down or the tunnel is deleted), the secondary PW must be immediately 15

20 informed of the fault. If the secondary PW is Up, it upgrades to a primary PW. MS-PW Generally, a PW is a Single-Segment PW, referred to as an SS-PW. The SS-PW is applied to most bearer scenarios. But some scenarios have limitations and problems. The service pressure on the core and the convergence node service is great. When the services traverse the PSN network with heterogeneous media, the tunnel cannot be established in end to end mode. When the inter-area services are required, the tunnel cannot be established in end to end mode. The capabilities of fast protection on many times of faults are low. The MS-PW can solve the above mentioned problems. Native Service (AC) < Multi-Segment Pseudowire > PSN PSN < -Tunnel- > < -Tunnel- > V V 1 V V 2 V V Native Service (AC) 1 T1 S1 T2 PW.Seg t1 PW.Seg t3 PW.Seg t2 PW.Seg t4 2 < Provider Edge 1 Provider Edge 2 < < PW switching point Emulated Service > Figure 3-4 MS-PW reference model 16

21 The preceding figure shows the MS-PW reference model. T-1 and T-2 are respectively connected to 1 and 2 and provide the end-to-the-end Layer 2 service simulation for the s. The MS-PW, traversing two or multiple PSN networks, is comprised of two or multiple PWs. When each PW is established on the tunnel of the PSN network, the switching of these PWs occurs on one or multiple S- nodes. According to the way in which the PSN tunnel is established and the switching relationship is configured on the S- node, MS-PW can be set up in the following ways: Statically configure the PW label and PW switching relationship on each T-/S-. Establish each PW dynamically and configure the PW switching relationship statically on the S- node. Establish the end-to-end MS-PW dynamically and the PW switching on the S- node is automatically completed. The configurations must support the mode 1 and mode 2 and can totally meet requirements for the MS-PW application. But the mode 3 only simplifies users' configurations and requires more protocol extensions. At present, the mode 3 has not become mature yet in the aspect of the standard. It remains at the stage of the private draft and the commercial products supporting the mode 3 have not been unveiled in the industry, in particular, in the extension for the PW reachable information advertisement. BGP/MPLS IP VPN Multiple sites are required to be directly interconnected for the scenario of the point-to-multipoint leased line services. If large scale interconnected sites faces the extension problem or the leasedline clients are not capable of maintaining the information about the Layer 3 network and the IP router, it is expected that carriers can maintain the information in place of the leased line clients and can also charge them for the value-added services. At this time it is recommended that you deploy the BGP MPLS IP VPN and provide the Layer 3 interconnection service between multiple sites. In this case, you need to consider the following: 17

22 Inter-AS VPN In the actual networking, the sites of some leased line client at multiple branches may be connected to many carriers' network with different AS numbers or to multiple ASs of one carrier (for example, multiple sites of the leased line clients are located in different MANs and different WANs distribute different AS numbers). This application mode that the VPN spans multiple autonomous systems is called inter- AS VPN (Multi-AS BGP/MPLS VPN). At present, there are three technological solutions to the above mentioned inter-as problems: VRF-TO-VRF (back-to-back mode) -ASBR1 VPN-1 VPN-1 -ASBR2 AS-X VPN-2 VPN-2 AS-Y VPN-1 Site 1 VPN-1 Site 2 VPN-2 Site 1 VPN-2 Site 2 Figure 3-5 Inter-AS BGP MPLS IP VPN Option A For an inter-as VPN client, the VPN-A needs to configure a same VPN in the -ASBR of the AS. As shown in the preceding figure, configure VPN-AS on the -ASBR-1 and the -ASBR-2, create a logic link (or a physical link) between the two ASBRs and then associate VPN-As with this link. In this networking, the ASBR, as a device, creates a same VPN on the and regards the peer ASBR as its own device. In this case, two ASs play the role as operating the MPLS VPN services in one AS. Specifically, advertise the VPN information in the AS to the ASBR ( device), and advertise the VPN information to the remote ASBR (considered as the ASBR's device). After the VPN on the ASBR receives the VPN information (considered as another device of the AS), it advertises the information to the device of the VPN AS. Finally, the VPN is advertised to the device and then the VPN 18

23 routing information is exchanged between the two ASs. MP-EBGP (single-hop MP-EBGP mode) -ASBR1 MP-eBGP -ASBR2 AS-X AS-Y VPN-1 Site 1 VPN-1 Site 2 VPN-2 Site 1 VPN-2 Site 2 Figure 3-6 Inter-AS BGP MPLS IP VPN Option B The MP-EBGP runs between two ASBRs and transmits all the VPN information of an AS to another AS. The transmitted information is about VPN routes and labels. When the MP-EBGP transmits the route, it changes the next hop of routes. According to the principle of the label distribution, when the next hop of an FEC is changed, the label must be replaced in the local ASBR. So when the ASBR receives the information about the VPN router of the AS and advertises the information, the ASBR must redistribute labels for the information about the VPN router. After the information is advertised with new labels, new and old labels form a switching operation of labels in the local ASBR. The remote ASBR receives the information about the VPN route advertised through MP-EBGP, the information is saved in the local end and continues to be advertised to the device of the ASBR AS. When this ASBR advertises the route to MP-IBGP neighbors of the AS, it can choose not to change the next hop of the route or change the next hop of the route to itself. If the next hop of the route is changed, the ASBR also needs to redistribute the labels for the VPN routes and the switching operation of labels is formed in the local ASBR, as shown in the preceding principle of the label distribution. 19