CS计算机代考程序代写 algorithm scheme flex Computer Simulation of Computer Networks

Computer Simulation of Computer Networks

CE321

Network Engineering

“beyond Cisco CCNA” material Part 2
v 2.2

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Part 2: Multiprotocol Label Switching (MPLS)

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IP uses hop-by-hop forwarding paradigm
IP is connectionless
Routers forward packets hop-by-hop using destination address according to routing table in each router
Source and destination addresses in packet header are unchanged by routers
Packet is of variable length

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IP routing table example
Routers perform longest prefix match to determine next hop for packet
Packet destined for 197.8.3.54 matches both routers T and Y in the Z’s routing table
Y is chosen as it has the longest match
Routing table at Z
Network Gateway
197.8.0.0/22 T
197.8.3.0/24 Y
197.8.2.0/24 T

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IP routing protocols
Routing table in each IP router updated by routing protocols:
Interior gateway protocols
Disseminate topology or routing information among the routers
From this, each router determines its routing table
Open Shortest Path First (OSPF) or Routing Information Protocol (RIP)
Used within an Autonomous System (AS)
Exterior gateway protocols
Allow an AS to advertise its reachable networks to another AS
Topology of AS not advertised
Border Gateway Protocol (BGP)
Used between ASs
IP routing protocols periodically update the routing tables
Truly distributed route determination algorithms
Nodes calculate next hop for packets, based on information from neighbours

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Routing loops and failures
Distributed algorithm does not necessarily provide up-to-date information
Routing errors may occur
Can result in packets looping endlessly
Avoided in IP by routers decrementing the time to live field in the IP header
When it reaches zero, packet is discarded
When a failure is discovered, the algorithm converges to suit the new network topology
As the algorithm is truly distributed it is resilient to node failure
This is part of the driving ethos behind the evolution of IP networks

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ATM networks
Different information types require different qualities of services from network
In terms of bandwidth, delay, loss
Real-time traffic
Video, telephony
Non real-time traffic
Email, file transfer
Telephone networks support a single quality of service
Expensive to implement and run
In its original form, Internet supports no quality of service
Flexible and cheap
ATM networks were meant to support a range of service qualities at a reasonable cost
ATM was connection-oriented
ATM transported fixed-size packets (cells) over virtual circuits

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Label switching: ATM
ATM is connection oriented, providing virtual circuits
Header defines virtual path (VPI) and virtual channel (VCI)
Labels, swapped at each switch according to switching table
Not necessary to carry entire destination address
Cells are of fixed length (53 bytes)

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Worked Example – labels
ATM has a 12-bit label field for implementing “virtual” paths
How many possible labels are there?
Suppose that labels cannot be re-used, and that each bi-directional circuit in the network must have a unique label assigned to it along its entire length
In a complete mesh of paths among N nodes, what is the maximum possible number of nodes?
Now suppose that label swapping takes place at each node, and that a label can hence be re-used in different parts of the network
Assume that the minimum capacity of a circuit (in each direction) is 30 Kbyte/s and that the maximum capacity of a link in each direction is 622 Mbit/s
How many labels would be required to ensure unique identification of each path?

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Answers
212 = 4096 possible labels
N(N – 1)/2 ≤ 4096 so solve N(N – 1) = 8192
Hence N2 – N – 8192 = 0
So N = (1 + √(1 + 32768))/2 = 91.011
Therefore the maximum number of nodes is 91
Maximum number of paths on link =
622 × 106/(30000 × 8) = 2591.67
Therefore a maximum of 2591 labels will be required

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Virtual circuits
Label saves on header space
Needs to be pre-established
Needs to be swapped at intermediate points
Need translation table and connection setup
All packets must follow the same path
Switches store per-VCI state
Can store QoS information
Separation of forwarding and control (route determination)
Virtual circuits do not automatically guarantee reliability
Small IDs can be looked up quickly in hardware
Harder to do this with IP addresses

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Label swapping vs hop-by-hop forwarding
Label swapping needs simple one-to-one mapping
Input label/port to output label/port
Use content addressable memory
IP forwarding uses longest prefix match
Requires search through routing table
May be of the order of 100,000 entries
More complex algorithm
More software intensive
Switches generally have a better price/performance ratio
To achieve fast line rates with IP forwarding requires very fast processors and/or parallel architectures
Growth of IP requires increase in line rate over time
Would rather use switching than IP forwarding if only considering price/performance of transport components
One solution runs IP over a switched network using an overlay

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IP overlay network
IP routers with ATM cards are connected at the IP layer through ATM virtual channels
Switching is performed in the high speed core
IP forwarding is performed at the slower network edge

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Problems with overlay network 1
Requires management of both an IP and ATM network
IP and ATM have evolved as isolated systems
Integration of IP and ATM is problematic
There are many different systems for achieving IP over ATM (e.g. LANE, CLIP, MPOA, …)
Although IP over ATM gives more cost effective switching it has lower bandwidth utilisation
IP must be encapsulated in ATM cells
Many IP over ATM solutions require complex servers to achieve integration
Gives a single point of failure and can limit scalability

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Problems with overlay network: example
Ingress and egress are through the same ATM switch
However the IP datagrams have to pass through the IP forwarding of router B
IP and ATM do not use information from each other
IGP thinks shortest route is through B
Some IP over ATM techniques supply a cut-through mechanism to get around this problem
These have increased complexity

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Problems with overlay network 2
Another problem is the number of router adjacencies
Consider N routers
Require full mesh to reduce bottlenecks
Single router hops between every ingress/egress
Each router has N – 1 adjacencies
N(N – 1) ≈ N2 (for large N) virtual circuits required
If there is a failure, or change in the network, routing algorithms send information to other nodes to converge
Each router sends an LSA for all the other N – 1 routers
Each other routers re-floods this to N – 2 neighbours
Update information can hence grow as a factor of N4
Generic problem with, for example, OSPF
This solution has scalability problems

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Additional new requirements for IP routing
High speed and cost effective IP forwarding is required
However there are also other new requirements:
End-to-end quality of service guarantees
Moving away from the current best effort IP quality of service
Traffic engineering facilities
Such as load balancing across routers, rate limiting via policing, ….
Support for virtual private networks
Traditionally, IP route determination (or control) and forwarding components are closely integrated
Changing functionality of one always affects the other
Adding this new functionality requires both the forwarding and control components to be updated in all routers
These added facilities place extra burden on forwarding component
A considerably more complex algorithm is required than just longest prefix match
Want to address new requirements without complicating forwarding

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Problem with tight IP forwarding/control integration: example
Traffic from A and B is destined for F
If forwarding decision is only based upon destination all traffic for F passes either all through D or all through E
Conventional destination based IP routing does not allow load balancing across D and E
More advanced forms of certain routing protocols and router implementations, e.g. OSPF, allow a limited form of load balancing
Not adequate for more complex traffic engineering

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Worked Example – capacity allocation
All traffic in this (highly artificial) network is from A to G (500 Mb/s), A to H (40 Mb/s), B to G (400 Mb/s) and B to H (50 Mb/s)
Without MPLS, OSPF at router C sends all traffic destined for G via E, and all traffic for H via D
If all links have capacity 622 Mb/s, what happens?
With MPLS, each demand can be routed via either D or E
How many ways in total are there of routing all demands, assuming unlimited link capacity?
State at least one way of routing label switched paths which is compatible with the constraint of 622 Mb/s for each link
A
B
C
D
E
F
G
H

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Answers
The traffic over C-E-F is 500 Mb/s + 400 Mb/s > 622 Mb/s
The links are overloaded and congestion occurs
The traffic over C-D-F is 40 Mb/s + 50 Mb/s << 622 Mb/s The links are underutilised There are 4 demands therefore 24 = 16 ways of routing all of them If demands A-G and A-H are routed via D, 540 Mb/s is carried over C-D-F Also, demands B-G and B-H are routed via E, so 450 Mb/s is carried over C-E-F This avoids overloading or underutilisation of links CE321 – beyond Cisco notes ‹#› 20 Problems to be solved Need to evolve the routing architecture of IP networks There is a need to add new routing functionality (QoS, load balancing etc) IP is the dominant end-to-end protocol It makes sense to optimise the network structure to support it Need better performance or price/performance ratio in routers Mapping IP onto ATM or Frame Relay is problematic Overlay networks can introduce routing scaling problems (N4) Need to have a highly scalable solution Many see the price/performance issue and overlay N4 problem as mostly historical Modern IP routing systems have greatly improved IP is carried over a transport network (typically SDH/SONET) that is unaware of QoS, traffic engineering requirements or routing Adding new services and simplifying management is now seen as the main driving force for change The suggested solution is Multiprotocol Label Switching (MPLS) The control function of IP routing is now separated from the forwarding function CE321 – beyond Cisco notes ‹#› 21 Previously considered overlay architecture This has problems as discussed before CE321 – beyond Cisco notes ‹#› 22 New architecture with MPLS MPLS nodes perform switching Under control of normal IP routing protocols CE321 – beyond Cisco notes ‹#› 23 History behind MPLS A number of bodies realised the problems of IP over ATM They all came up with comparable schemes: Use ATM switching hardware as the core forwarding component Discard ATM signalling It is difficult to map IP control onto ATM signalling Replace it with control protocols that map into the IP domain The schemes include: Toshiba's cell switching router – prototype demonstrated in 1995 Ipsilon's IP switching – set out as open standards Cisco's tag switching – standards as part of the IETF Supports switching over several link layer technologies (not just ATM) IBM's aggregate route-based IP switching (ARIS) – similar to tag switching These proprietary solutions started a new IETF working group that has developed into MPLS as we know it today CE321 – beyond Cisco notes ‹#› 24 MPLS network architecture CE321 – beyond Cisco notes ‹#› 25 Network components Label switch router (LSR) A node, forwarding labeled datagrams according to a table Table maps incoming labels to a specific outgoing interface Table defines the new label for the packet when it leaves the node MPLS uses label swapping as in ATM Edge label switch router (ELSR) A node device, accepting incoming unlabelled packets Places suitable labels onto the packets So that they can be forwarded through the MPLS network At the egress it strips off the label and forwards the packet using conventional IP routing All packets that belong to a forwarding equivalence class (FEC) are mapped to the same label CE321 – beyond Cisco notes ‹#› 26 Forwarding Equivalence Classes (FECs) An FEC defines a set of packets that have attributes that can be uniquely determined These attributes are common to all packets in the FEC All packets that share the same destination address prefix could be part of the same FEC A single application flow could form an FEC Taken from source address, destination address and destination port Could involve inspection of headers for UDP, TCP and/or RTP Even application header fields in theory Examples above demonstrate extremes of FEC granularity, choice is up to system designer Using application flow could exhaust the label space in a core network CE321 – beyond Cisco notes ‹#› 27 Worked Example – FECs A network has N ELSRs, each with L local area networks attached to it via IP routers Each LAN supports U users running A applications each How many LSPs must exist in the network if they exist between: All pairs of ELSRs? All pairs of LANs? All pairs of users? All applications of the same type between each pair of users? Pick representative values for N, L, U and A, and work out the number of LSPs in each case Comment on the feasibility of finer FEC granularities CE321 – beyond Cisco notes ‹#› 28 Answers Choose N = 50, L = 4, U = 32 and A = 8 Mesh between ELSRs: N(N – 1)/2 = 1,225 Mesh between LANs: LN(LN – 1)/2 = 19,900 Mesh between users: LNU(LNU – 1)/2 = 20,476,800 Mesh between similar applications: LNUA(LNU – 1)/2 = 163,814,400 Finer FEC granularity results in a very large number of LSPs and may overload the label space 220 labels = 1,048,576 CE321 – beyond Cisco notes ‹#› 29 MPLS and the ISO OSI reference model Mapping MPLS onto the ISO OSI protocol stack model is problematic The OSI model does have limits of usefulness Not layer 2 – independent of layer 2 technology Not layer 3 – has no routing and addressing of its own Not really a “layer” – no single format for transporting data from layer above CE321 – beyond Cisco notes ‹#› 30 MPLS control architecture MPLS control architecture defines how label switching forwarding tables are updated in each LSR Binding represented by ordered pair: (label, FEC) CE321 – beyond Cisco notes ‹#› 31 MPLS forwarding component MPLS changes the conventional IP forwarding model In IP the control component (e.g routing algorithm) and forwarding component are closely integrated Leads to problems of network evolution (discussed before) MPLS separates the forwarding from the control component Label switching forwarding component uses a single simple forwarding algorithm based on label swapping The label carried in a packet is short, has a fixed length, and has no structure; it uniquely specifies forwarding and resource attributes Label switching forwarding component does not place any restrictions on the granularity associated with a label Label switching forwarding component can support multiple network layer protocols as well as link layer protocols All this has been abstract so far we can now ask: what is a label? CE321 – beyond Cisco notes ‹#› 32 MPLS label An MPLS label only has local significance Significance only between two neighbouring LSRs It can be anything that can be used in a label swapping switch architecture Examples of systems that can support a label in existing link layer headers: ATM VCI field ATM VCI and VPI fields Frame Relay Data Link Circuit Identifier (DLCI) For other link layer protocols that cannot support labels in the link layer headers (e.g. Ethernet) MPLS defines a shim label This is inserted between the link layer header and the IP packet:                                                                                           Successive LSRs may use different label encodings For example, LSR with both ATM and Ethernet interfaces CE321 – beyond Cisco notes ‹#› 33 Label switch forwarding table The label switch forwarding table maps incoming labelled packets by label value to the appropriate output interface and outgoing label The table is effectively a pool of available labels, some of which are assigned to FECs and provide forwarding Some labels may be unassigned, awaiting a new FEC definition that requires a label The table is updated by the control components using a label distribution protocol (LDP) Incoming label Outgoing label Next hop Outgoing interface 1 3 197.0.3.24 if0 2 4 192.2.6.32 if1 3 2367 192.0.3.24 if0 4 - - - 5 63 192.2.5.56 if2 etc etc etc etc CE321 – beyond Cisco notes ‹#› 34 Label switch forwarding table example Consider table entries on each router for FEC representing destination 192.6/16 Table entries use the default hop-by-hop IP route Assume that conventional IP routing protocols have updated the routing table in each LSR These reflect the network topology Thus, each LSR has a routing table entry for the network 192.6/16 which is used by MPLS to identify a FEC CE321 – beyond Cisco notes ‹#› 35 Label switch forwarding table example (continued) Initially each LSR knows nothing of labels at other LSRs Each LSR locally binds an available label to the FEC by putting it in its table Outgoing label is not yet known Router Incoming label Outgoing label Next hop Outgoing int A 100 ? B if1 B 6 ? E if1 C 17 ? D if2 D 5 ? E if0 E 22 ? E if0 Each line represents one entry from each of the five label tables CE321 – beyond Cisco notes ‹#› 36 Label switch forwarding table example (continued) Each LSR distributes local bindings to adjacent LSRs Ordered pair (FEC, label) Consider A sending bindings to B and C B and C know from IP routing tables that A is not the next hop to the destination so no table updates are made Router Incoming label Outgoing label Next hop Outgoing int A 100 ? B if1 B 6 ? E if1 C 17 ? D if2 D 5 ? E if0 E 22 ? E if0 Each line represents one entry from each of the five label tables CE321 – beyond Cisco notes ‹#› 37 Label switch forwarding table example (continued) Consider B sending bindings to A, D and E For D and E, B is not the next hop to the destination so the information is ignored by these LSRs But, B is the next hop for A so A takes this remote binding and adds it to its table (see below) Router Incoming label Outgoing label Next hop Outgoing int A 100 6 B if1 B 6 ? E if1 C 17 ? D if2 D 5 ? E if0 E 22 ? E if0 Each line represents one entry from each of the five label tables CE321 – beyond Cisco notes ‹#› 38 Label switch forwarding table example (continued) Distribution of label information continues between adjacent nodes Final table entries shown below Node E is an edge LSR and thus there is no outgoing label entry for this node It strips off the label and uses conventional IP forwarding Router Incoming label Outgoing label Next hop Outgoing int A 100 6 B if1 B 6 22 E if1 C 17 5 D if2 D 5 22 E if0 E 22 ? E if0 Each line represents one entry from each of the five label tables CE321 – beyond Cisco notes ‹#› 39 Label switch forwarding table example (continued) Once the bindings are complete a label switched path (LSP) is said to be established Packet transport may start using label swapping as shown If the FEC is no longer required, label bindings are deleted For example, routing change due to a fault CE321 – beyond Cisco notes ‹#› 40 Note, it is possible for labels to be reused Labels 100, 6 and 22 are rearranged on C-D-E Label 6 is assigned to part of the path on D-E This is possible if router B has a per-interface label space No ambiguity between label 6 on if0 and on if2 Ambiguity results if router B has a per-platform label space CE321 – beyond Cisco notes ‹#› 41 Label distribution protocols MPLS requires a mechanism to transport the label binding information A label distribution protocol (LDP) is required MPLS may be used in various network scenarios and with different forwarding functionality requirements Hence there is not one but many different LDPs specified Default LDP is the new protocol defined in MPLS for disseminating labels for the conventional hop-by-hop destination based IP routing Label distribution on BGP (BGP-LDP) “piggybacks” label bindings as an extension of the existing BGP protocol An exterior gateway IP routing protocol RSVP-LDP adds label bindings to the existing RSVP protocol, which is used for signalling QoS reservations in IP routers Constraint routed LDP (CR-LDP) is a new protocol for disseminating label bindings in a network that requires QoS Alternative to RSVP-LDP Only LDP is considered in detail in these notes CE321 – beyond Cisco notes ‹#› 42 The MPLS label distribution protocol MPLS-LDP will be used to refer to the new default label distribution protocol defined within MPLS Must be distinguished from the generic term LDP In some texts the term LDP is used ambiguously MPLS-LDP has several components These implement LSR peer discovery Configure communication between neighbours Four classes of messages: DISCOVERY messages – learn of existence of neighbours ADJACENCY messages provide initialisation, keepalive and shutdown sessions between adjacent LSRs LABEL ADVERTISEMENT messages are used to send label binding advertisements, requests, withdrawal and release NOTIFICATION for advisory and error information DISCOVERY messages transported over UDP All other messages transported over reliable TCP Like most IP-based protocols it is designed to be extensible Through type, length, value (TLV) encoded objects CE321 – beyond Cisco notes ‹#› 43 MPLS-LDP (continued) DISCOVERY message: HELLO message is sent to well-known UDP port on “all routers on this subnet” multicast group All LSRs listen and thus learn about existence of neighbours ADJACENCY messages: INITIALIZATION messages allow LSRs to agree on and define: unsolicited downstream vs downstream on demand label assignment ordered vs independent LSP control liberal vs conservative label retention KEEPALIVE messages sent periodically to show that LSR is still active and that parameters are acceptable LABEL ADVERTISEMENT messages: LABEL MAPPING messages send information about a label binding LABEL WITHDRAWAL messages remove a label binding e.g when routing table entry removed due to change in route LABEL REQUEST message is used in downstream on demand label assignment to demand a label binding CE321 – beyond Cisco notes ‹#› 44 Concept of label stack CE321 – beyond Cisco notes ‹#› 45 Concept of label stack (continued) MPLS supports multiple labels organised as a stack This allows the creation of a LSP tunnel In previous slide there is one tunnel with two levels of hierarchy E1, B1, B2 and E2 are peers on one level of the hierarchy B1, X, Y, Z, B2 are peers on the second level of the hierarchy B1 and B2 are gateways between the two routing hierarchies B1 has a FEC to destination address beyond E2 and B2 supplies a label binding for this route (L3) However, B2 is not the next hop in the internal network Hence B1 must push L3 onto the label stack and request a new label binding for the FEC that represents the address of B2 A key applications of a LSP tunnel is for transit networks (and VPNs) For example, OSPF sets up tunnels between BGP speakers iBGP peers use the tunnels via label stacks Internal routers need not be aware of external BGP routes Significant performance advantages Amount of IP forwarding is reduced Separates routing inside and outside the AS Operator can control routing independently CE321 – beyond Cisco notes ‹#› 46 Worked Example – LSP tunnels Two LSP tunnels share a number of consecutive links along their respective routes They both start and end at different LSRs Draw a diagram to illustrate this What happens if two LSPs each pass through one of the tunnels using the same label? If these links carry two tunnels, how many LSPs in total can exist on each, including those inside the tunnels? CE321 – beyond Cisco notes ‹#› 47 Answer Even although both LSPs use label 567, they are not confused along C-D-E since these routers only look at the outer label – one for tunnel A and one for tunnel B All the other labels apart from those two can be used 220 – 2 = 1,048,574 in total There are 220 possible LSPs within each tunnel, so total is 1,048,574 + 1,048,576 + 1,048,576 = 3,145,726 LSPs tunnel A tunnel B LSP X label 567 LSP Y label 567 A B C D E F G CE321 – beyond Cisco notes ‹#› 48 Providing customer connectivity through MPLS MPLS is widely used in operator networks Traffic can be broken into two categories: Internet access for broadband and corporate customers traffic between customer sites (site-to-site virtual private connections) These notes will concentrate on the second category Aside: in some ways the first case can be considered as a special case of the second where the operator is its own customer and the service is simply Internet access for all its customers. CE321 – beyond Cisco notes ‹#› Site-to-site connections using MPLS Three (common) ways to provide connectivity between customer sites using MPLS MPLS L3 VPN (BGP/MPLS VPNs RFC4364) MPLS L2 virtual private LAN service (VPLS) using BGP (RFC4761) MPLS L2 VPLS using LDP (RFC4762) These notes will consider the first two (the third may be considered similar to the second – as far as a customer is concerned) CE321 – beyond Cisco notes ‹#› General model for site-to-site MPLS CE – Customer Edge router (usually owned an managed by the customer) PE – Provider Edge router, an edge label switch router P – Provider router A label stack two-deep is used at the PE router First, inner, label is only used at the PE routers to identify customers Second, top, label, is used to forward between PE routers via the P routers IP/MPLS PE attachment circuit customer 1 site customer 1 site customer 2 site customer 2 site Packet Switched Network (PSN) IP or MPLS PE CE CE CE CE Demarcation points P P CE321 – beyond Cisco notes ‹#› 51 MPLS L3 VPN (BGP/MPLS VPNs RFC4364) Providers manage the WAN routing and export routes to the customer using some means e.g. using an interior gateway protocol (IGP) between CE and PE routers, or through statically configured routes If using an IGP, this may be OSPF, but an instance only used for this purpose, ie not the OSPF instances used for internal routing in either the customer or the provider networks. A customer’s main IGP in each site is kept separate (ie IGPs in different sites do not peer) Provider uses BGP in the PE routers (but not the P routers) Each customer has a different VPN Routing and Forwarding (VRF) table in the PE routers. BGP has the ability to give each VRF a different route distinguisher (RD), ie typically a unique RD for each customer. This allows different customers to use the same IP address ranges – as long as they do not need to communicate between themselves. Provider manages the IP routing for the customer between the customer sites. This means the customer does not have the effort of managing the WAN routing. CE321 – beyond Cisco notes ‹#› BGP as a label distribution protocol PE routers form internal BGP (iBGP) peers in a full-mesh to share the VRFs VRFs ensure customers CEs have the correct routes to access their other sites (but do not have the whole topology of the remote sites) iBGP is used for PE label distribution (as well as sharing routing information) BGP is a highly flexible protocol so the label information is shared in a BGP extension iBGP peers are only PE routers (not P routers) so the BGP-based label distribution protocol only controls the distribution of the inner, customer, labels LDP (or some other) is used to configure the top, outer, labels Use of label stack (inner for customer, outer for core) has benefit: PE routers have large routing tables (the many VRFs for each customer) P routers only need to have an IGP routing table (e.g. using OSPF) for the core network and do not need to hold the VRF routes CE321 – beyond Cisco notes ‹#› MPLS L2 VPLS using BGP (RFC4761) End-to-end architecture Allows MPLS networks to provide multipoint Ethernet services It is “Virtual” because multiple instances of this service share the same physical infrastructure It is “Private” because each instance of the service is independent Isolated from one another It is “LAN Service” because it emulates Layer 2 multipoint connectivity between subscribers Connections between PE routers are called pseudo-wires (PW) Provider has no L3 routing communication with customer Customer has to manage all the WAN routing between the sites CE321 – beyond Cisco notes ‹#› 54 VC label identifies interface Tunnel label(s) gets to PE router Unidirectional Tunnel LSP between PE routers to transport PW PDU from PE to PE using tunnel label(s) Both LSPs combined to form single bi-directional pseudo wire Directed LDP session between PE routers to exchange VC information, such as VC label and control information VC distribution mechanism using LDP IP/MPLS PE1 LSP created using IGP+LDP or RSVP-TE customer site customer site customer site customer site Label Switched Path iBGP between PE1 and PE2 PE2 CE CE CE CE CE321 – beyond Cisco notes ‹#› 55 A PWES is either: - an Ethernet link or a VLAN link between two ports, or - an ATM VC or VP, or - a Frame Relay VC, or - a TDM circuit, or - an MPLS LSP Note that the PSN tunnel may be MPLS, L2TP, GRE and so on .. UTI is another mechanism to transport the PDUs between ingress and egress PE – in this case the PW is created using a UTI tunnel. Typical VPLS frame DA SA Type= 0x8847 PW CW Top Label FCS Inner Label DA SA Type= 0x8100 VLAN Type= 0x0800 IP Datagram Ethernet MPLS Payload (an Ethernet frame) One possible Ethernet frame PWCW – Pseudo wire control word is 4 bytes (provides sequence number, a type field for some control frames and other features) Note the two MPLS labels (as defined earlier) Large overhead means Ethernet “Jumbo Frames” are used by provider (9000 byte payload. CE321 – beyond Cisco notes ‹#› Summary: MPLS L3VPN and VPLS (using BGP) MPLS L3VPN VPLS (using BGP) Customer does not manage WAN routing tables. IGPs at each customer site are completely separate. Customer has to manage all the WAN routing, the MPLS network appears as a single Ethernet bridge. Customer obtains routing information from provider as static routes, or from an IGP between CE/PE. No routing protocol required between customer and provider Said to scale well as IP routing can scale to a large number of sites Has some scalability problems if not designed well as broadcasts have to go to all customer sites MPLS L3VPN and VPLS similarities MPLS used in the core network with two layer stack (inner identifies customer) BGP used to distribute customer labels in PE routers IGP and LDP (or RSVP-TE) used to manage internal, provider, label distribution Both likely to use Ethernet as access technology for customers Differences CE321 – beyond Cisco notes ‹#› /docProps/thumbnail.jpeg