20111110_cht_tl_教育訓練 day5
TRANSCRIPT
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Day 5
MPLS Foundamentals
Johnson Liu
[email protected] Nov. 10, 2011
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MPLS Fundamentals
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IGP Metric-Based Forwarding
IGP ForwardingTraffic is routed based on the IGPs best path selection
Traffic that is destined for networks attached to R6 and R7
uses the same path
R1 R2
R3
R4
R5
R6
1
1
1
21
1
R7
1
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Downsides of IGP Metric-Based Forwarding
Redirecting traffic from R1, destined for R7, totraverse R4 causes traffic destined to R6 to use R4
also
This redirecting of traffic causes some of your links to be
underutilized, while others are overutilized
R1 R2
R3
R4
R5
R6
1
1
3
21
1
R7
1
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Additional Drawbacks of IGP Metrics
Adjusting the IGP metric might destabilize the networkMoves the problem to another section of the network
Some of the links will be underutilized
Some of the links will be congested and overutilized
Lacks control All traffic flows over the IGP shortest path
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Internet Growth Required Changes
ATM switched networks also known as an OverlayNetwork
Benefits of using ATM
ATM switches offered performance and predictable behavior
Virtual circuits (VCs) could be reengineered without physicalnetwork changes.
Traffic statistics on a per-VC basis
Downsides of ATM
Maintain separate infrastructure
ATM cell overhead (1 cell = 5 bytes
Header + 48 bytes payload)
Scalability issues
Not well integrated
ATM
IP
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Additional Options for Connecting Sites
Frame Relay networksBenefits of using Frame Relay
Uses virtual circuits (VCs) to move traffic to its destination
Uses Data Link Connection Identifier (DLCI) number to separate
VCs
Built in Congestion Control (DE, FECN, BECN)
Downsides of Frame Relay
Maintain separate infrastructure
FR
IP
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Traditional IP Routing
Destination-based routing lookup is needed on every hop
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MPLS Example
MPLS core routers swap labels and forward packets based on simple
label lookups
MPLS edge routers also perform a routing table lookup, and add or
remove labels
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MPLS Benefits
MPLS support multiple applications
Fixed label lookup faster than IP routing
MPLS support forwarding non-IP protocols
MPLS VPN solve overlapping intranet IP problem
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Benefits of MPLS (1 of 2)
Some benefits of MPLS include:Improved route lookup time by using labels to forward traffic
Increased scalability
Additional control over how traffic moves through the
network usingtraffic engineering (TE)
R1R2
R3
R4
R5
R6
1
1
3
21
1
R7
1
LSP A
LSP B
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Benefits of MPLS (2 of 2)
Service Providers can offer different technologies likeATM, Frame Relay, Ethernet, and IPsec over the same
infrastructure
R1
R2
R3
R4
R5
R6
ATM-LSP
Site 1
Site 2
CoreInfrastructure
ATM
ATM
Site 3Site 4
Frame Relay LSP
Site 5
Site 6
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Labeled Packets
MPLS is often referred to as a "Layer 2.5" protocol.
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MPLS Header information
The MPLS packet headerMPLS header is prepended to packet with a pushoperation
at ingress node
Label is added immediately after Layer 2 encapsulation
header
Packet is restored at the end of the LSP with a pop
operation
Normally the label stack is popped at the penultimate()router
32-Bit
MPLS shim Header
DataL2 Header MPLS Header
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MPLS Shim Header Structure
MPLS shim header consist of four fields
Labelused to associate packet with an LSP (Label SwitchedPath)
Experimental(EXP) bitscarry packet queuing priority (Class of
Service, CoS)
Bottom ofStacking bit
Time to live (TTL)limits packet lifetime within LSP In most cases, the IP TTL is copied into the MPLS TTL
TTLLabel (20 bits) CoS S
Data
32 bits
L2 Header MPLS Header
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MPLS Label Stack
The PID(0x8847=MPLS unicast packets) in a layer 2 headerspecifies that the payload starts with a label followed by an IPheader
The bottom-of-stack bit(S bit) indicates whether the label isthe last label in the stack
The receiving router uses the top label only
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MPLS Labels
Key things to remember about labels:Labels can be assigned manually or by a signaling protocol
in each LSR during path setup
Label values will change at each segment in the path
The LSR(Label Switch Router) will swap incoming label withnew unique outgoing label
MPLS Labels only have local significance
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Reserved Labels
Label values 0 through 15 are currently reserved0 = IPv4Explicit NULL
1 = Router Alert Label
2 = IPv6 Explicit NULL
3 = Implicit NULL (default)
4 through 15 = for future use
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user@R3> show route table mpls.0
mpls.0: 4 destinations, 4 routes (4 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both
0 *[MPLS/0] 01:13:17, metric 1
Receive
1 *[MPLS/0] 01:13:17, metric 1
Receive
2 *[MPLS/0] 01:13:17, metric 1
Receive
1000050 *[MPLS/6] 01:13:16, metric 1
> to 172.20.100.14 via ge-1/0/6.0, Swap 1000515
Label Information Base (LIB)
Label Information BaseThe LIB is stored in the mpls.0 table
The mpls.0 table is automatically created, with label values for 0,
1, and 2, when you configure the MPLS protocol
This table is used by transit routers to make forwarding decisions
The mpls.0 table maps the incoming labels with the outgoing
label and next hop to forward the packets
Incoming Label
Outgoing Label
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Label-Switching Routers
Label-switching router (LSR) performs:- MPLS packet forwarding
- LSP setup
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Label-Switching Routers (LSR)
The LSR performs:MPLS packet forwardingLSP setup
All M Series Routers, T Series Routers, and MX Series
Ethernet Services Routers support LSR capabilities
Simply called routers in this material
R1 R2
R3
R4
R5
R6
R7
LSRLSP A
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Label-Switched Path (LSP)
LSPUnidirectional path through network
Generally within a single MPLS domain
LSP
R1 R2
R3
R4
R5
R6
R7
LSP A
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1000050 COS TTLS
MPLS Router Functions: Ingress
Ingress router
Packets enter LSP at ingress
Also called a head-end router
Upstream from other routers
Performs label push operation
Ingress
R1 R2
R3
R4
R5
R6
R7
Layer 2
LSP A
Data FCS
label push
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1000515 COS TTLS
MPLS Router Functions: Transit
Transit routerThere can be zero or more transit routers
Perform label swap operations
Forward traffic to next hop in LSP
Transit
R1
R3
R4
R5
R6
R7
LSP A
label swap
Layer 2 Data FCS
R2
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MPLS Router Functions: Penultimate
Penultimate router
Second-to-last router
Normally pops the label stack
Unlabeled packets sent to egress
Penultimate
R1 R2
R3
R4
R5
R6
R7
LSP A
Layer 2 Data FCS
label pop
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MPLS Router Functions: Egress
Egress router
Packets exit LSP at egress
Also called tail-end router
Downstream from other routers
Forwards packets based on IP address Egress
R1 R2
R3
R4
R5
R6
R7
LSP A
Layer 2 FCSData
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Label Stacking
PE 2
PE 1
X
P P
Inner Tunnel(LDP-signaled LSP)
Outer Tunnel(RSVP-signaled LSP)
PPY
1) Packet enters
LDP tunnel with
LDP label push
4) Packet
restored with LDP
label pop
2) Packet enters
RSVP engineered
core with RSVP label
push
3) Packet leaves
outer tunnel with
RSVP label pop
5) Packet leavesMPLS domain
Label stacking improves scalability- Similar to ATMs VP and VC hierarchy
http://www.iconbazaar.com/symbols/symbols/home02.gifhttp://www.iconbazaar.com/symbols/symbols/home02.gifhttp://www.iconbazaar.com/symbols/symbols/home02.gifhttp://www.iconbazaar.com/symbols/symbols/home02.gifhttp://www.iconbazaar.com/symbols/symbols/home02.gifhttp://www.iconbazaar.com/symbols/symbols/home02.gif -
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MPLS SIGNALING(LDP vs RSVP)
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Label Distribution Protocol(LDP)
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LDP Signaling Overview
LDP messages types
Discovery: Locate potential LDP peers Session: Manage peer-to-peer TCP sessions
Advertisement: Create, change, or delete label mappings
Notification: Provide advisory information
Upstream
LDP peer
Downstream
LDP peer
TCP Session Establishment
Initialization Messages
Label Request Messages
Hello messages
Label Mapping Messages
Session
Advertisement
Discovery
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Label Assignment
LSR FEC: 10.0.0.1/32
Label: 52
FEC: 10.0.0.1/32
Label: 29
MPLS Table
In Out
(so-0/0/1, 17)
Receive
Outgoing
Label
FEC: 10.0.0.1/32
Label: 17
Advertise
Incoming
Label
(fe-0/0/2, 35)
LDP label mapping:
Downstream peer assigns labels
Benefits:
Traffic engineering information is not piggybacked on routing protocols
Limitations: LSPs follow the conventional IGP path
Does not support explicit routing
fe-0/0/2 so-0/0/1 so-0/0/1 so-0/0/3 so-0/0/3 at-0/2/0
MPLS Table
In Out
(so-0/0/3, 52)(so-0/0/1, 17)
MPLS Table
In Out
(at-0/2/0, 29)(so-0/0/3, 52)
FEC: 10.0.0.1/32
Label: 35
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Hello-Based Neighbor Discovery
Neighbor discovery is asymmetric process
Respond only if LDP session is desired
Active node has the higher IP address
Transport address takes precedence over source address
Basic Discovery
224.0.0.2, UDP port 646Router A Router B
Extended Discovery
Specific Address, UDP port 646
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LDP Session Establishment
Active Node initiates TCP session
LDP Session initiated after TCP session established
Router A
(Passive)
Router B
(Active)TCP 3-way Handshake
10.0.1.210.0.1.1
Session Initialization
(Version, Label modes, Timer Values)Session Initialization
(Version, Label modes, Timer Values)
Keepalives
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LDP Session Maintenance
LDP session requires at least 1 hello adjacency
Hello interval: 5-second default
Hold timer: 15-second default
If hold timer expires, LSR deletes hello adjacency
Can be asymmetric
Transport address selection:
Interface address
Router ID
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[edit]
lab@r7# show protocols mpls
label-switched-path test {
to 10.0.6.1;
ldp-tunneling;
no-cspf;
}
interface all;
LDP Tunneling Through RSVP-TE LSP
Router A Router B
RSVP
LDP LDP
R7 R8
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MPLS Terminology
Label Swapping Connection table maintains mappings
Exact match lookup
Input (port, label) determines:
Label operation
Output (port, label)
Port 1
Port 3
Port 2
Port 4
Connection TableIn
(port, label)Out
(port, label)(1, 22)(1, 24)(1, 25)(2, 23)
(2, 17)(3, 17)(4, 19)(3, 12)
LabelOperation
SwapSwapSwapSwap
25P
19P
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MPLS Forwarding Example
134.5.1.5
200.3.2.7
1 2
200.3.2.1
134.5.6.1
Ingress Routing TableDestination Next Hop134.5/16
200.3.2/24(2, 84)(3, 99)
MPLS TableIn Out
(1, 99) (2, 56)
MPLS TableIn Out
(3, 56) (5, 0)
DestinationEgress Routing Table
Next Hop134.5/16
200.3.2/24134.5.6.1200.3.2.1
200.3.2.7
MPLS TableIn Out(2, 84) (6, 0)
200.3.2.73 5
23
2 6
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LDP over RSVP LSP LDP neighbor relationships between:
A and B via the physical interface
B and E via bi-directional RSVP LSPs
E and F via the physical interface RTR-B performs a swap and push operation
Swap label 583 for label 101 (advertised by E)
Push label 106 (advertised by C)
A B C D E FLDP LDPSVP
PE1 P1 P2 P3 P4 PE2
LDP over RSVP(T-LDP)
IP583
IP101106
IP101150
IP101
IP
*33500683
101
*Note: Label 3 = implicit null
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Resource Reservation Protocol(RSVP)
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Basic RSVP Path/Resv signaling
UnidirectionalflowsIngress router initiates connectionSoft state
Path and resources are maintained dynamicallyCan change during the life of the RSVP sessionPath message (containing label-request object) sent downstreamResv message (containing label object) sent upstream
PATH PATH PATH PATH PATH
Label requestobject
Label requestobject
Label requestobject
Label requestobject
Label requestobject
R2 R1R3R4 R5 R6RESV RESV RESV RESV RESV
1004
Labelobject
5341
Labelobject
4711
Labelobject
2477
Labelobject
0
Labelobject
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RSVP Signaling Example: Path
Miami
Seattle
San
Francisco
(Ingress)
New York(Egress)
RSVP sets up path from San Francisco to New York
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RSVP Signaling Example: Reservation
San
Francisco
(Ingress)
New York
(Egress)
Miami
Seattle
3
LSP Established!
The RESV message visits each router on the path in reverseorder labels assigned hop to hop in the upstream direction
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Traffic Engineering(TE)
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What is traffic engineering?
A major goal of Internet Traffic Engineering is to facilitateefficient and reliable network operations while
simultaneously optimizing network resource utilization
and performance
RFC 2702, Requirements for Traffic Engineering over MPLS
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IGP Metric-Based Forwarding
1 1
1 2
A B
C
Traffic sent to A or B follows path with lowest metrics
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Drawbacks of IGP Metric Forwarding
1 4
1 2
A B
C
Redirecting traffic flow to A via C causes traffic for B to move
also!
- Some links become underutilized or overutilized
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Additional Drawbacks of IGP Metrics
Adjusting an IGP metric might destabilize the network- Moves problem around
Some links underutilized
Some links overutilized
- Lacks control
All traffic flows via the IGP shortest path
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Constraint-Based Path Computation
Constraint-basedrouting is demand-driven. Resource-reservation-aware routing paradigm:
- Based on criteria including, but not limited to, network topology- Calculated at the edge of a network:
Modified Dijkstras algorithm at tunnel headend (CSPF [Constraint-based SPF]).
Output is a sequence of IP interface addresses (next-hop routers)between tunnel endpoints.
Constraint-based routing takes into account:- Using the least-cost path computed by OSPF or IS-IS- Policy constraints associated with the tunnel and physical links- Physical resource availability- Network topology state
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IGP-Based Traffic Engineering
San
Francisco
New York
192.168.1/24
134.112/16
Standard IGP routing IP prefixes are bound to a physical next hop
- Typically based on IGP view of shortest path
30M
50M
30M
30M
50M30M
30M
30M
30M
IGP Shortest Path
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MPLS-Based Traffic Engineering
San
Francisco
IGP Shortest Path
RSVP Traffic Engineered Path
(Need 20M)
New York
Engineer unidirectional paths through your networkwithout using the IGPs shortest path calculation
30M(5M available)
50M
(10M available)
30M
(20M available)30M
(20M available)
30M
(25M available)
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Fast Reroute schemes supported in JUNOS
1. One-to-one backupSeparate backup LSP (called Detour LSP) for each LSP thatrequires protection
2. Facility backupBypass Tunnel created to protect a given facility (a link or anode). Multiple LSPs can share the same bypass tunnel.Two variants:
Link protection Node protection
The schemes are described in RFC 4090 Fast Reroute Extensions to RSVP-TE for LSPTunnels http://www.ietf.org/rfc/rfc4090.txt
http://www.ietf.org/rfc/rfc4090.txthttp://www.ietf.org/rfc/rfc4090.txt -
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Comparison of FRR schemes
R11
R13
R1 R2
LSP X
R3 R4 R5 R6
R7
R8 R9
LSP Y
R10
LSP Z
R14
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Facility protection, Link protectionvariant
R1 R2
LSP X
R3 R4 R5 R6
R7
R8 R9
LSP Y
Shared bypass
tunnel
R10
LSP Z
R11
R13
R14
R1 computes and signals a single bypass tunnel to the next-hop node, R2, that avoids the
R1-R2 link. The bypass tunnel can be shared between all LSPs using R1->R2 link, if
desired. Merge point is R2.
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Facility protection, Node protectionvariant
R1 R2
LSP X
R3 R4 R5 R6
R7
R8 R9
LSP Y
Shared bypass
tunnel
R10
LSP Z
R11
R13
R14
R1 computes and signals bypass tunnel(s) to the next-next-hop node(s) which avoids R1-R2
link and R2 itself. Same bypasstunnel can be shared between all LSPs that pass from R1
to R2 and have the same next-next-hop. In the example, the 3 LSPs have the same next-
next-hop so only one bypass was needed to protect R2.
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FRR 1:1 protection
R1 R2
LSP X
R3 R4 R5 R6
R7
R8 R9
LSP Y
Detour LSP for
LSP Y
Detour LSP for
LSP X
R10
LSP Z
R11
R13
R14
Detour LSP for
LSP Z
Separate detour LSP for each protected LSP.
1:1 protection always protects downstream link and downstream node.
Case study 1 Facility protection:
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BR
BR
CR
CR
CR
CR
CR CR
BR
BRMPLS Core
Link Protection
When one backbone link disconnect, it will compute and signal a single bypass tunnel tothe next-hop node. The bypass tunnel can be shared between all LSPs. One of the CR will
need to carry all traffic of all LSP when failure occurs.
BR: Border Router
CR: Core Router
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BR
BR
CR
CR
CR
CR
CR CR
BR
BRMPLS Core
Case study 2 1:1 FRR
Separate detour LSP for each protected LSP. 1:1 protection can help network admin to load sharebetween different path and node
resource when detouring LSP generated.
BR: Border Router
CR: Core Router
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