MPLS10S04-Advanced MPLS Technology.ppt

1、Module 4,Advanced MPLS Technology,Objectives,Upon completion of this chapter, you will be able to perform the following tasks: Describe the concept of Label Switch Paths and the impact of route summarization on LSP Understand the basics of MPLS Traffic Engineering Understand the data-plane loop dete

2、ction in MPLS and how it relates to IP TTL Explain the benefits and drawbacks of IP TTL propagation Understand the data-plane loop detection in the ATM environment and how it affects troubleshooting tools such as traceroute Explain the impacts of configuring MPLS in networks running BGP Design simpl

3、ified BGP networks based on MPLS technology,Label Switch Paths in Unicast IP Routing, 2001, Cisco Systems, Inc.,MPLS v1.04-3,Objectives,Upon completion of this section, you will be able to perform the following tasks: Explain the concept of Label Switch Path Describe how the LSP is built in unicast

4、IP routing Describe the impact of IP aggregation on Label Switch Paths,LSP Tunnels,An LSP tunnel is a sequence of LSRs that forward labeled packets of a certain forwarding equivalence class. MPLS unicast IP forwarding builds LSP tunnels based on the output of IP routing protocols. LDP or TDP adverti

5、ses labels only for individual segments in the LSP tunnel. LSP tunnels are unidirectional. Return traffic uses a different LSP tunnel (usually the reverse path, as most routing protocols provide symmetrical routing). An LSP tunnel can take a different path from the one chosen by an IP routing protoc

6、ol (MPLS traffic engineering).,LSP Tunnel Building Example,IP routing protocol determines the path. LDP or TDP propagates labels to convert the path to an LSP tunnel.,LSP: AB D G I,A,B,C,D,E,F,G,H,I,IP routing protocol updates,A,B,C,D,E,F,G,H,I,LSP Tunnel Building Example (cont.),LDP or TDP propagat

7、es labels to convert the path to an LSP tunnel.,A,B,C,D,E,F,G,H,I,E,D,C,B,A,Effects of IP Aggregation on LSP Tunnels,IP aggregation breaks an LSP tunnel into two segments. Router C is forwarding packets based on Layer 3 information.,Aggregation point,10.1.1.1,10.1.1.1,IGP,LDP/TDP,ATM LSRs must not a

8、ggregate because they cannot forward IP packets. Aggregation should not be used where endtoend LSP tunnel are required (MPLS Virtual Private Networks VPNs).,Effects of IP Aggregation on LSP Tunnels (cont.),Summary,After completing this section, you should be able to perform the following tasks: Expl

9、ain the concept of Label Switch Path Describe how the LSP is built in unicast IP routing Describe the impact of IP aggregation on Label Switch Paths,Review Questions,What is an LSP? Which mechanism determines the path? What happens when IP aggregation (summarization) is used?,Explicit Label Switch P

10、aths (Traffic Engineering), 2001, Cisco Systems, Inc.,MPLS v1.04-12,Objectives,Upon completion of this section, you will be able to perform the following tasks: Explain the concept of explicit Label Switch Path Describe how an explicit LSP can be used for traffic engineering Describe the needs for r

11、unning LDP/TDP across explicit LSP,Explicit LSP Tunnels,LSP tunnels are usually determined by IP routing protocols. MPLS traffic engineering can be used to diverge from the IGP-determined path. Constraint-based Routing using Label Distribution Protocol (CR-LDP) or RSVP with extensions for traffic en

12、gineering is used to establish LSP tunnels. LSP tunnels can also be configured manually.,E,D,C,B,A,F,MPLS Traffic Engineering Example,IGP and LDP or TDP create an LSP tunnel based on the shortest path determined by IGP.,E,D,C,B,A,F,MPLS Traffic Engineering Example (cont.),RSVP creates a TE tunnel be

13、tween routers A and E. The new link can be included in the IGP shortest path calculation. RSVP uses downstream-on-demand label distribution. The tunnel creation is initiated from router A.,1.2.3.4/32 L=pop,1.2.3.4/32 L=19,1.2.3.4/32 L=54,1.2.3.4,E,D,C,B,A,F,MPLS Traffic Engineering Example (cont.),I

14、GP and LDP or TDP create a new LSP tunnel based on the shortest path determined by IGP. This LSP tunnel is going across the MPLS TE LSP tunnel.,10.0.0.0/16 L=44,E,D,C,B,A,F,MPLS Traffic Engineering Example (cont.),FIB: 10/8 44, 54 1.2.3.4 54,FIB: 10/8 23 1.2.3.4 19,LFIB: 54 19 16 23,FIB: 10/8 44 1.2

15、.3.4 pop,LFIB: 19 pop,FIB: 10/8 23 1.2.3.4 ,LFIB: 44 23,FIB: 10/8 pop,LFIB: 23 pop,10.1.1.1,10.1.1.1,Explicit LSP Tunnels,As seen in the previous example, MPLS TE can be used to implement load balancing across unequal paths. Explicit paths are almost transparent to LDP and TDP. LDP or TDP uses direc

16、ted hello packets to find nonadjacent neighbors.,Summary,After completing this section, you should be able to perform the following tasks: Explain the concept of explicit Label Switch Path Describe how an explicit LSP can be used for traffic engineering Describe the needs for running LDP/TDP across

17、explicit LSP,Review Questions,What is the purpose of using explicit LSPs? Which technology makes use of explicit LSPs? How does LDP/TDP find neighbors across an MPLS/TE tunnel? Which protocols can be used to establish MPLS/TE tunnels? What type of label propagation do these protocols use?,Loop Detec

18、tion in Packet Mode MPLS, 2001, Cisco Systems, Inc.,MPLS v1.04-22,Objectives,Upon completion of this section, you will be able to perform the following tasks: Describe loop detection in packet-mode MPLS Explain the implications of IP TTL propagation into the TTL field of the label header Explain the

19、 interactions between IP TTL propagation and traceroute diagnostic tools,Loop Detection,LDP or TDP relies on loop detection mechanisms built into IGPs that are used to determine the path. If, however, a loop is generated (that is, misconfiguration with static routes), the TTL field in the label head

20、er is used to prevent indefinite looping of packets. TTL functionality in the label header is equivalent to TTL in the IP headers. TTL is usually copied from the IP headers to the label headers (TTL propagation).,MPLS Domain,Normal TTL Operation,Cisco routers have TTL propagation enabled by default.

21、 On ingress: TTL is copied from IP header to label header. On egress: TTL is copied from label header to IP header.,IP TTL,Label TTL,5,1,C,B,A,D,MPLS Domain,C,B,A,D,Loop Detection,Labeled packets are dropped when the TTL is decremented to zero.,IP TTL,Label TTL,Routing loop,Disabling TTL Propagation

22、,TTL propagation can be disabled. IP TTL value is not copied into the labels and label TTL is not copied back into IP TTL. Instead, the value 255 is assigned to the label header TTL field on the ingress LSR. Disabling TTL propagation hides core routers in the MPLS domain. Traceroute across an MPLS d

23、omain does not show any core routers.,C,B,A,D,Traceroute with Disabled TTL Propagation (1),The first traceroute packet (ICMP or UDP) that reaches the network is dropped on router A. An ICMP TTL exceeded message is sent to the source from router A.,IP TTL,Label TTL,1,traceroute 10.1.1.1,C,B,A,D,Trace

24、route with Disabled TTL Propagation (2),The second traceroute packet that reaches the network is dropped on router D. An ICMP TTL exceeded message is sent to the source from router D.,IP TTL,Label TTL,2,1,traceroute 10.1.1.1 1 10 ms A,Effect of Disabling TTL Propagation,Traceroute across an MPLS dom

25、ain does not show core routers. TTL propagation has to be disabled on all LSRs. Mixed configurations (some LSRs with TTL propagation enabled and some with TTL propagation disabled) could result in faulty traceroute output. TTL propagation can be enabled for forwarded traffic onlytraceroute from LSRs

26、 does not use the initial TTL value of 255.,Summary,After completing this section, you should be able to perform the following tasks: Describe loop detection in packet-mode MPLS Explain the implications of IP TTL propagation into the TTL field of the label header Explain the interactions between IP

27、TTL propagation and traceroute diagnostic tools,Review Questions,How are routing loops prevented in MPLS networks? What is the purpose of the TTL field? What is TTL propagation? What is the result of disabling TTL propagation? What can happen when some LSRs have TTL propagation disabled and some do

28、not?,Loop Detection in Cell-Mode MPLS, 2001, Cisco Systems, Inc.,MPLS v1.04-33,Objectives,Upon completion of this section, you will be able to perform the following tasks: Explain the challenges of loop detection in cell-mode MPLS Describe how the label-distribution procedures enable loop detection

29、in cell-mode MPLS List loop detection mechanisms available during TDP/LDP label distribution,Loop Detection in Cell-Mode MPLS,VPI/VCI field in the ATM header is used for label switching. ATM header does not contain a TTL field. LDP or TDP still primarily relies on IGPs to prevent routing loops. Ther

30、e is an additional mechanism built into LDP or TDP to prevent loops.,LDP Hop Count TLV,LDP uses an additional type, length, value (TLV) attribute to count the number of hops in an LSP tunnel. The TTL field in the IP header or label header is decreased by the number of hops by the ingress ATM edge LS

31、R before being forwarded through an label VC . If the TTL field is zero, or less the packet is discarded. Maximum number of hops can also be specified for LDP.,A,D,LDP Hop CountExample,LSR A discovers the length of the LSP tunnel across the ATM domain to LSR D through LDP.,10.0.0.0/16 L=1/35 Hops=1,

32、10.0.0.0/16 L=1/34 Hops=2,10.0.0.0/16 L=1/43 Hops=3,B,C,A,D,B,C,Traceroute Through ATM LSRs Example (1),The first traceroute packet that reaches the network is dropped on router A. An ICMP TTL exceeded message is sent to the source from router A.,IP TTL,Label TTL,1,traceroute 10.1.1.1,A,D,B,C,Tracer

33、oute Through ATM LSRs Example (2),The second traceroute packet that reaches the network is dropped on router A. An ICMP TTL exceeded message is sent to the source from router A.,IP TTL,Label TTL,2,traceroute 10.1.1.1 1 10 ms A,A,D,B,C,Traceroute Through ATM LSRs Example (3),The third traceroute pack

34、et that reaches the network is dropped on router A. An ICMP TTL exceeded message is sent to the source from router A.,IP TTL,Label TTL,3,traceroute 10.1.1.1 1 10 ms A 2 10 ms A,TTL is decreased by 3 The new TTL value is 1 The packet is forwarded.,A,D,B,C,Traceroute Through ATM LSRs Example (4),The f

35、ourth traceroute packet that reaches the network is dropped on router D. An ICMP TTL exceeded message is sent to the source from router D.,IP TTL,Label TTL,4,traceroute 10.1.1.1 1 10 ms A 2 10 ms A 3 10 ms A,LDP Path Vector TLV,Path vector TLV is another safeguard that prevents loops in LDP. This TL

36、V is used to carry router IDs of all ATM LSRs in the path. If an LSR receives an LDP update with its own router ID in the path vector TLV, the update is ignored. Path vector TLV is similar to BGPs AS path or cluster list attributes. Path vector TLV is not present in TDP.,A,D,B,C,E,Path Vector Exampl

37、e,The LDP update is dropped because it contains the router ID of router C in the path vector TLV.,10.0.0.0/16 PV=D,10.0.0.0/16 PV=D,C,E,10.0.0.0/16 PV=D,C,E,10.0.0.0/16 PV=D,C,E,B,Loop Detection Summary,MPLS primarily relies on loop detection mechanisms built into IGPs. Hop count TLV is used to simu

38、late TTL functionality on ATM LSRs with the help of ATM edge LSRs. Path vector TLV is used to prevent loops in LDP updates.,Summary,After completing this section, you should be able to perform the following tasks: Explain the challenges of loop detection in cell-mode MPLS Describe how the label-dist

39、ribution procedures enable loop detection in cell-mode MPLS List loop detection mechanisms available during TDP/LDP label distribution,Review Questions,Which mechanisms are used to prevent routing loops in MPLS-enabled networks using cell-mode MPLS? Which TLVs in LDP are used to prevent loops? Descr

40、ibe TTL operation in cell-mode MPLS.,MPLSBGP Interaction, 2001, Cisco Systems, Inc.,MPLS v1.04-47,Objectives,Upon completion of this section, you will be able to perform the following tasks: Describe label allocation procedures for external IP routes Explainlabel sharing between external routes and

41、BGP next hops Describe traditional BGP core design requirements Explain the relaxation of core design requirements made possible by MPLS List BGP design rules applicable in MPLS-based networks,Label Allocation in Unicast IP,Labels are assigned to FECs. FEC in unicast IP routing is equal to a destina

42、tion prefix found in an IP routing table. This is true only for IGP-derived prefixes. BGP-derived prefixes are assigned the label that is used for the BGP next-hop address. Result: all prefixes learned from an external BGP neighbor use a single label.,Transit AS,Traditional BGP AS System Design Requ

43、irements,All core routers are required to run BGP. All core routers require full Internet routing information (more than 100,000 networks) to be able to forward IP packets between ISP1 and ISP2.,Core1,Border1,Border2,Core2,ISP1,ISP2,Transit AS,Core1,Border1,Border2,Core2,ISP2,ISP1,Simplified BGP Net

44、work Design in MPLS-Based Networks,Only border routers are required to run BGP. Core routers run an IGP to learn about BGP next-hop addresses. Core routers run LDP or TDP to learn about labels for next-hop addresses.,MPLS-Based Transit ASBuilding FIB and LFIB,All routers are capable of forwarding pa

45、ckets to external destinations: Border (edge) routers label and forward IP packets. Core routers forward labeled packets.,10.0.0.0/8,1.2.3.4,10.0.0.0/8,10.0.0.0/8,FIB: 1.2.3.4 23 10.0.0.0/8 23,FIB: 1.2.3.4 serial0/0 10.0.0.0/8 1.2.3.4,MPLS-Based Transit ASPacket Propagation,1.2.3.4,FIB: 1.2.3.4 23 1

46、0.0.0.0/8 23,FIB: 1.2.3.4 serial0/0 10.0.0.0/8 1.2.3.4,10.1.1.1,10.1.1.1,10.1.1.1,Benefits of MPLS-Based Transit AS,Simplified BGP topology (only AS edge routers are required to run BGP with full Internet routing). Core routers do not require a lot of memory (100,000 networks may require more than 5

47、0 MB of memory for the BGP table, IP routing table, and CEFs FIB table and distributed FIB tables). Changes in the Internet do not impact core routers. Allows private addresses (RFC 1918) to be used in the core if TTL propagation is disabled (traceroute across the AS will not show any private addresses).,Common Design and Configuration Errors,BGP next-hop addresses should