mpls architecture overview v1.1. course objectives mpls overview mpls concepts lsrs and labels label...
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MPLS Architecture Overview
V1.1
Course Objectives
MPLS overview MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths Loops and TTL LDP overview Day in the Life of a Packet
MPLS Concepts
MPLS is a layer 2+ switching Developed to integrate IP and ATM MPLS forwarding is done in the same way as in
ATM switches Packet forwarding is done based on Labels
MPLS Origin
MPLS is developed by integrating IP switching protocols of different manufactures IP Switching (Ipsilon) Tag Switching (Cisco) Aggregate RouteBased IP Switching (IBM) Cell Switch Router (Toshiba) IP Navigator (Ascend)
MPLS Origin
MPLS is developed by the combination of ATM and IP at 1990s. In 1996 , Ipsilon proposed IP switching protocol which solve the problem of better supporting IP on ATM switch, this makes an ATM switch a router and at the same time having the high performance of an ATM switch, breaking the performance bottleneck on the router (can not achieve wire speed on POS3,POS12)
MPLS History
When OC48,OC192 interface achieves wire speed, MPLS research switched to these application:TE (Traffic engineering), VPN.
MPLS TE becomes an important method to manage network traffic、 reduce congestion and ensure QOS on IP network.
VPN is regarded as an important method to provide value-added service and explore new service by carriers.
Disadvantages: Change the current network topology, increase the network complexity, VPN security factors.
MPLS Overview
Multi-protocol label switch Label is at the layer 2.5, between link layer and network l
ayer. This technology have the advantage of layer 2 switching and layer 3 routing.
The successful point of MPLS technology is that it introduced connection-oriented mechanism into IP network;
Core idea of MPLS is: routing on the edge and switching on the core part
Establishing of LSP( label switch path): LSP is established by topology not by data. The establishing of LSP can be done by LDP, CR-LDP, RSVP protocols.
MPLS Developing Motion
Current core network is made up of router and ATM switch
Core network based on MPLS is superior than core network based on router and ATM
This is the developing motion of MPLS.
Comparison between MPLS and routing
MPLS simplifies the process of packet forwarding
MPLS supports valid explicit routing MPLS can implement TE best MPLS can support QoS routing Mapping from IP packet to FEC MPLS support dividing network by function MPLS implements single forwarding rule no matt
er how many levels of Qos are there
Advantage of MPLS
Comparison of label switching and hop-by-hop forwarding based on destination IP address: MPLS is implemented on switch, thus taking full advan
tage of high speed of switch. Traditional switch can search label and replace them, but can not analyze network header or at least not fast enough to analyze it, so it is difficult to implement forwarding based on network header information on traditional switch
When packet comes into MPLS network, ingress router will decide which FEC it belongs to, it can use any information about the packet not only the network layer information.( for example the port information)
Advantage of MPLS (Continued.)
MPLS can assign different labels to packet coming from different routers, thus making it easy to implement routing based on ingress routers. Because ingress router id can not be sent with the packet , so traditional routing patterns do not support this function.
Sometimes it is reasonable to designate a predefined route for the packets than hop-by-hop routing.
Course Objectives
MPLS overview MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths Loops and TTL LDP overview Day in the Life of a Packet
MPLS Concepts
Unlike IP, classification/label can be based on: Destination Unicast address Traffic Engineering VPN QoS
FEC: Forwarding Equivalence Class A FEC can represent a: Destination address
prefix, VPN, Traffic Engineering tunnel, Class of Service.
Key words in MPLS
Label FEC (Forwarding equivalence class) LSR (Label switching router) LER (Label edge router) LSP (Label switching path) LDP (Label distribution path)
Label
Label is a key point in MPLS, it is a short fixed-length and locally used identifier which is used to identify FEC
Label processing is done by high speed ASIC chip thus making the delay of packet processing and queuing decrease to a great extent
Why fixed-length label? Balancing between forwarding efficiency and switching
performance. Although fixed-length making the forwarding efficiency low but it can highly improve switching performance.
Label is locally used identifier. It is only meaningful between the sending out port of upstream router and receiving port of downstream router.
Label Format
MPLS label is an integer ranged between 0 to 1048575,it is used to identify specific FEC.
This label is encapsulated in layer 2 header.
Label stack
Two or more MPLS labels, encapsulated after link layer header and before network header.
Top label in the stack appear earliest. Network layer header follow the last label in the
stack
Label stack
Forwarding of packets is based on the top label in the stack, when LSR receives a packet, checks the top label and decides the next hop
The operation about the label stack: replace: Use a new label to replace the top label in t
he stack Pop (delete): pop the top label in the label stack and
delete it push (add): replace the top label and add a new lab
el into the stack
FEC
Forwarding Equivalence Class (FEC): A group of packets that have common attributes. These packets will be forwarded in the same way by LSR in the MPLS network, just because they are forwarded in the same way they are equivalence.
FEC
FEC – packets forwarded in the same way Same destination prefix unicast packet Same destination address multicast packet Same Qos packets
MPLSDomain
LERa
LSRy
LERb
LERc
LERd
LERe
LERf
LSRx LSRz
LSP
Ingress
Egress
FECp
FECq
FECp+q
MPLS key points
LSR –responsible for establishing LSP for FEC LER –responsible for FEC classification, TE, begin the
process of establishing LSP, IP packet forwarding LSP –used for IP packet forwarding LDP –responsible for assigning labels
MPLSDomain
LERa
LSRy
LERb
LERc
LERd
LERe
LERf
LSRx LSRz
LSP
Ingress
Egress
FECp
FECq
FECp+q
MPLS key points
Label switch router (LSR): one node in MPLS network, it is located in the middle of MPLS network, it runs MPLS control protocol and layer 3 routing protocol, its responsibility lies in: exchanging routing information with other LSR to form
route table, implementing the mapping from FEC to IP packets,
redistributing label binding information, establishing label forwarding table and maintaining it.
MPLS key points
Label edge router (LER): Responsible for connecting MPLS domain and non-MPLS domain. Implementing the function of classifying service、assigning labels、 taking off labels etc. LER is the key device in implementing MPLS.
MPLS Key Points
Label switch path (LSP): The forwarding path made up of the LSRs along the way to forward packets which belong to the same FEC.
Label distributing protocol (LDP): Responsible for controlling the label binding information exchanging process between LSRs, LSR establish and maintain the LIB (Label information base) according to the binding information between label and FEC.
MPLS Key points
Upstream router Downstream router
Course Objectives
MPLS overview MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths Loops and TTL LDP overview Day in the Life of a Packet
LSRs and Labels
LSR: Label Switch Router Edge-LSR: LSRs that do label imposition and
disposition ATM-LSR: An ATM switch with Label Switch
Controller
LSRs and Labels
An IP routing protocol is used within the routing domain (e.g.:OSPF, i-ISIS)
A label distribution protocol is used to distribute address/label mappings between adjacent neighbors
The ingress LSR receives IP packets, performs packet classification, assign a label, and forward the labelled packet into the MPLS network
Core LSRs switch packets/cells based on the label value The egress LSR removes the label before forwarding the IP packet
outside the MPLS network
IGP domain with a label distribution protocol
LSRs and Labels
PPP HeaderPPP Header Layer 3 HeaderLayer 3 HeaderShim HeaderPPP Header(Packet over SONET/SDH)
Ethernet HdrEthernet Hdr Layer 3 HeaderLayer 3 HeaderShim HeaderEthernet
FR HdrFR Hdr Layer 3 HeaderLayer 3 HeaderShim HeaderFrame Relay
ATM Cell Header HECHEC DATADATACLPCLPPTIPTIVCIVCIGFCGFC VPIVPI
Label
HECHEC DATADATACLPCLPPTIPTIVCIVCIGFCGFC VPIVPI
Label
Subsequent cells
Course Objectives
MPLS overview MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths Loops and TTL LDP overview Day in the Life of a Packet
Label Assignment and Distribution
Labels have link-local significance Each LSR binds his own label mappings
Each LSR assign labels to his FECs Labels are assigned and exchanged between
adjacent neighboring LSR Applications may require non-adjacent
neighbors
Label Assignment and Distribution
Rtr-C is the downstream neighbor of Rtr-B for destination 171.68.10/24 Rtr-B is the downstream neighbor of Rtr-A for destination 171.68.10/24 LSRs know their downstream neighbors through the IP routing protocol
Next-hop address is the downstream neighbor
171.68.10/24
Rtr-BRtr-A Rtr-C
171.68.40/24
Upstream and Downstream LSRs
Label Assignment and Distribution
LSRs distribute labels to the upstream neighbors
171.68.10/24
Rtr-BRtr-A Rtr-C
171.68.40/24
Next-HopNext-Hop
In In LabLab
--
......
Address Address PrefixPrefix
171.68.10171.68.10
......
OutOutI/FI/F
11
......
Out Out LabLab
3030......
In In I/FI/F
00
...... Next-HopNext-Hop
In In LabLab
3030
......
Address Address PrefixPrefix
171.68.10171.68.10
......
OutOutI/FI/F
11
......
Out Out LabLab
4040......
In In I/FI/F
00
......
Next-HopNext-Hop
In In LabLab
4040
......
Address Address PrefixPrefix
171.68.10171.68.10
......
OutOutI/FI/F
11
......
Out Out LabLab
--......
In In I/FI/F
00
......
Use label 40 for destination 171.68.10/24
Use label 30 for destination 171.68.10/24
IGP derived routes
Unsolicited Downstream Distribution
Label Assignment and Distribution
Upstream LSRs request labels to downstream neighbors Downstream LSRs distribute labels upon request
171.68.10/24
Rtr-BRtr-A Rtr-C
171.68.40/24
Use label 30 for destination 171.68.10/24
Use label 40 for destination 171.68.10/24
Request label for destination 171.68.10/24
Request label for destination 171.68.10/24
On-Demand Downstream Distribution
Label Assignment and Distribution
Label Retention Modes Liberal retention mode
LSR retains labels from all neighbors– Improve convergence time, when next-hop is again available
after IP convergence
– Require more memory and label space
Conservative retention mode LSR retains labels only from next-hops neighbors
– LSR discards all labels for FECs without next-hop
– Free memory and label space
Label Assignment and Distribution
Label Distribution Modes Independent LSP control
LSR binds a Label to a FEC independently, whether or not the LSR has received a Label the next-hop for the FEC
The LSR then advertises the Label to its neighbor Ordered LSP control
LSR only binds and advertise a label for a particular FEC if:– it is the egress LSR for that FEC or
– it has already received a label binding from its next-hop
Label Assignment and Distribution
Several protocols for label exchange LDP
Maps unicast IP destinations into labels RSVP, CR-LDP
Used in traffic engineering BGP
External labels (VPN) PIM
For multicast states label mapping
Course Objectives
MPLS overview MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths Loops and TTL LDP overview Day in the Life of a Packet
Label Switch Path (LSP)
LSPs are derived from IGP routing information LSPs may diverge from IGP shortest path
LSP tunnels (explicit routing) with TE LSPs are unidirectional
Return traffic takes another LSP
LSP follows IGP shortest path LSP diverges from IGP shortest path
IGP domain with a label distribution protocol
IGP domain with a label distribution protocol
LSP establishing process
The establishing of LSP in MPLS network including these three steps: Every node run routing protocols such as BGP、 OS
PF、 IS-IS to form its own route table According to the route table, every node establish labe
l information base under the control of LDP From the ingress LSR 、middle LSR and egress LSR,
the ingress lable and outgress lable together form a LSP.
1st step: form of route table
Dynamic routing protocols help each router form route table.
47.1
47.247.3
Dest Out
47.1 147.2 2
47.3 3
1
23
Dest Out
47.1 147.2 2
47.3 3
Dest Out
47.1 147.2 2
47.3 3
1
23
1
2
3
Traditional ‘hop by hop’ forwarding
47.1
47.247.3
IP 47.1.1.1
Dest Out
47.1 147.2 2
47.3 3
1
23
Dest Out
47.1 147.2 2
47.3 3
1
2
1
2
3
IP 47.1.1.1
IP 47.1.1.1IP 47.1.1.1
Dest Out
47.1 147.2 2
47.3 3
2nd step: form of LIB
IntfIn
LabelIn
Dest IntfOut
3 0.40 47.1 1
IntfIn
LabelIn
Dest IntfOut
LabelOut
3 0.50 47.1 1 0.40
47.1
47.247.3
12
3
1
2
1
2
3
3IntfIn
Dest IntfOut
LabelOut
3 47.1 1 0.50 Mapping: 0.40
Request: 47.1
Mapping: 0.50
Request: 47.1
3rd step: form of LSP
IntfIn
LabelIn
Dest IntfOut
3 0.40 47.1 1
IntfIn
LabelIn
Dest IntfOut
LabelOut
3 0.50 47.1 1 0.40
47.1
47.247.3
1
2
31
2
1
2
3
3IntfIn
Dest IntfOut
LabelOut
3 47.1 1 0.50
IP 47.1.1.1
IP 47.1.1.1
Routing method in MPLS
hop-by-hop routing This method allows each node to select the next hop
for each FEC independently This kind of routing method is commonly used in IP
network
Routing method in MPLS
Explicit Routing In this kind of routing method, each LSR can not selec
t next hop independently, on the contrary, path selection is done under the network management policy, for example, the ingress or egress LSR define the way of the LSP.
When the ingress of egress LSR define every hop along the LSP , we call it “strict explicit routing”, if it only define part of the nodes along the way, we call it “loose explicit routing”
“strict explicit routing” is also called “source routing” in IP network, but compared to IP source routing, strict explicit routing has higher efficiency.
47.1
47.247.3
IP 47.1.1.1
Dest Out
47.1 147.2 2
47.3 3
1
23
Dest Out
47.1 147.2 2
47.3 3
1
2
1
2
3
IP 47.1.1.1
IP 47.1.1.1IP 47.1.1.1
Dest Out
47.1 147.2 2
47.3 3
hop-by-hop routing
#216
#14
#462
#972
#14 #972
A
B
C
Route={A,B,C}
Explicit Routing
IntfIn
LabelIn
Dest IntfOut
3 0.40 47.1 1
IntfIn
LabelIn
Dest IntfOut
LabelOut
3 0.50 47.1 1 0.40
47.1
47.247.3
1
2
3
1
2
1
2
3
3
IntfIn
Dest IntfOut
LabelOut
3 47.1.1 2 1.333 47.1 1 0.50
IP 47.1.1.1
IP 47.1.1.1
ER-LSP
Label Switch Path (LSP) Penultimate Hop Popping
The label at the top of the stack is removed (popped) by the upstream neighbor of the egress LSR
The egress LSR requests the “popping” through the label distribution protocol Egress LSR advertises implicit-null label
The egress LSR will not have to do a lookup and remove itself the label One lookup is saved in the egress LSR
Label Switch Path (LSP) Penultimate Hop Popping
Egress LSR needs to do an IP lookup for finding more specific route
Egress LSR need NOT receive a labelled packet
0 1
Summary route for 171.68/16
01
171.68.10/24
Next-HopNext-Hop
In In LabLab
Address Address PrefixPrefix
OutOutI/FI/F
Out Out LabLab
In In I/FI/F
44 171.68/16171.68/16 22 poppop00
...... ...... ...... ............Next-HopNext-Hop
In In LabLab
Address Address PrefixPrefix
OutOutI/FI/F
Out Out LabLab
In In I/FI/F
-- 171.68/16171.68/16 11 4400
...... ...... ...... ............
Egress LSR summarises morespecific routes and advertises a label for the new FEC
Summary route is propagate through the IGP and label is assigned by each LSR
Use label “implicit-null” for FEC 171.68/16
Summary route for 171.68/16
Use label 4 for FEC 171.68/16
171.68.44/24
Address Address Prefix and maskPrefix and mask
171.68.10/24171.68.10/24
Next-HopNext-Hop
171.68.9.1171.68.9.1
InterfaceInterface
Serial1Serial1
171.68.44/24171.68.44/24 171.68.12.1171.68.12.1 Serial2Serial2
171.68/16171.68/16 ...... NullNull
Course Objectives
MPLS overview MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths Loops and TTL LDP overview Day in the Life of a Packet
Loops and TTL
In IP networks TTL is used to prevent packets to travel indefinitely in the network
MPLS may use same mechanism as IP, but not on all encapsulations TTL is present in the label header for PPP and LAN
headers (shim headers) ATM cell header does not have TTL
Loops and TTL
LSRs using ATM do not have TTL capability Some suggested options:
hop-count object in LDP Path Vector object in LDP
Loops and TTL
TTL is decremented prior to enter the non-TTL capable LSP
If TTL is 0 the packet is discarded at the ingress point TTL is examined at the LSP exit
IGP domain with a label distribution protocol
LSR-1
LSR-2
LSR-4 LSR-5
LSR-3
LSR-6
Egress
IP packetTTL = 6
Label = 25
IP packetTTL = 6
IP packetTTL = 10
LSR-6 --> 25Hops=4
IP packetTTL = 6
Label = 39
IP packetTTL = 6
Label = 21
Course Objectives
MPLS overview MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths Loops and TTL LDP overview Day in the Life of a Packet
LDP Concepts
Label Distribution Protocol Labels map to FECs for Unicast Destination
Prefix LDP works between adjacent/non-adjacent peers LDP sessions are established between peers
LDP
LDP Messages
Discovery messages Used to discover and maintain the presence of new
peers Hello packets (UDP) sent to all-routers multicast
address Once neighbor is discovered, the LDP session is
established over TCP
LDP Messages
Session messages Establish, maintain and terminate LDP sessions
Advertisement messages Create, modify, delete label mappings
Notification messages Error signalling
Course Objectives
MPLS overview MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths Loops and TTL LDP overview Day in the Life of a Packet
Day in the life of a Packet
Egress LSR needs to do an IP lookup for finding more specific route
0
Summary route for 171.68/16
01
171.68.10/24
Next-HopNext-Hop
In In LabLab
Address Address PrefixPrefix
OutOutI/FI/F
Out Out LabLab
In In I/FI/F
77 171.68/16171.68/16 22 poppop00
...... ...... ...... ............Next-HopNext-Hop
In In LabLab
Address Address PrefixPrefix
OutOutI/FI/F
Out Out LabLab
In In I/FI/F
-- 171.68/16171.68/16 11 4400
...... ...... ...... ............
Egress LSR summarises morespecific routes and advertises a label for the new FEC
Summary route is propagate through the IGP and label is assigned by each LSR
Use label “implicit-null” for FEC 171.68/16
Summary route for 171.68/16
Use label 4 for FEC 171.68/16
171.68.44/24
Address Address Prefix and maskPrefix and mask
171.68.10/24171.68.10/24
Next-HopNext-Hop
171.68.9.1171.68.9.1
InterfaceInterface
Serial1Serial1
171.68.44/24171.68.44/24 171.68.12.1171.68.12.1 Serial2Serial2
171.68/16171.68/16 ...... NullNull
Use label 7 for FEC 171.68/16
Next-HopNext-Hop
In In LabLab
Address Address PrefixPrefix
OutOutI/FI/F
Out Out LabLab
In In I/FI/F
44 171.68/16171.68/16 11 7700
...... ...... ...... ............
CE
PEP P PE
0
0
21
Day in the life of a Packet Basic Layout
IP Routing Protocols
Label Distribution Protocol
IP Routing Table
Forward Information Block (FIB)
Label Forward Information Block(LFIB)
Control Plane
Forwarding Plane
Incoming IP PacketsOutgoing IP Packets
Outgoing Labelled PacketsIncoming LabelledPackets
Routing Exchange
Label Binding Exchange
Label RemovedL3 lookup
Day in the life of a Packet Database LayoutOS P F IS IS BGP
fas t-ad jacency
fas ttag-rew ritetag_info
FIB
TFIB
tfib_entrytag_rew rite
Routing Table
LDP
TIB
tfib_entry
tfib_entryloadinfotag_info
output-ifencaps
incom ing-tag
outgoing-tag
tfib_entry
tag_rew rite
Incom ing tag
Dest. IP address
incom ing-tag
tag_rew rite [ ]
tag_hash
IDB v ectors
ip_turbo_fs
tag_optim um _fsip2_tag_optim um _fs
Summary
MPLS basic conception MPLS working process LSP FEC label distribution LDP