MPLS cisco QOS VPN full 01 mpls arch overview

MPLS Architecture
Overview
Jay Kumarasamy


Adopted from Stefano Previdi’s presentation

© 2001, Cisco Systems.

1


Agenda
•

MPLS Concepts

•

LSRs and labels

•

Label assignment and distribution

•

Label Switch Paths

•

ATM LSRs

•

Loops and TTL

•

LDP overview

•

Day in the Life of a Packet

2001 Cisco Systems, Inc.

www.cisco.com

2


MPLS Concepts

• MPLS: Multi Protocol Label Switching
• 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

2001 Cisco Systems, Inc.

www.cisco.com

3


MPLS Concepts

• Unlike IP, classification/label can be based
on:
Destination Unicast address
Traffic Engineering
VPN
QoS
• FEC: Forwarding Equivalence Class
2001 Cisco Systems, Inc.

www.cisco.com

4


Agenda
•

MPLS Concepts

•

LSRs and labels

•

Label assignment and distribution

•

Label Switch Paths

•

ATM LSRs

•

Loops and TTL

•

LDP overview

•

Summary

2001 Cisco Systems, Inc.

www.cisco.com

5


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

2001 Cisco Systems, Inc.

www.cisco.com

6


LSRs and Labels

IGP domain with a label
distribution protocol

• 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

2001 Cisco Systems, Inc.

www.cisco.com

7


LSRs and Labels
0
1
2
3
01234567890123456789012345678901
Label

| Exp|S|

TTL

Label = 20 bits
Exp = Experimental, 3 bits
S = Bottom of stack, 1bit
TTL = Time to live, 8 bits

• Uses new Ethertypes/PPP PIDs/SNAP values/etc
• More than one Label is allowed -> Label Stack
• MPLS LSRs always forward packets based on the value
of the label at the top of the stack

2001 Cisco Systems, Inc.

www.cisco.com

8


LSRs and Labels
PPP Header(Packet over
SONET/SDH)
Ethernet
Frame Relay

ATM Cell Header

GFC

PPP Header

Shim Header

Layer 3 Header

Ethernet Hdr

Shim Header

Layer 3 Header

FR Hdr

Shim Header

Layer 3 Header

VPI

VCI

PTI CLP HEC

DATA

VCI

PTI CLP HEC

DATA

Label
Subsequent cells GFC

VPI

Label
2001 Cisco Systems, Inc.

www.cisco.com

9


Agenda

2001 Cisco Systems, Inc.

•

MPLS Concepts

•

LSRs and labels

•

Label assignment and distribution

•

Label Switch Paths

•

ATM LSRs

•

Loops and TTL

•

LDP overview

•

Day in the Life of a Packet

www.cisco.com

10


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

2001 Cisco Systems, Inc.

www.cisco.com

11


Label Assignment and
Distribution
Upstream and Downstream LSRs
171.68.40/24

171.68.10/24
Rtr-A

Rtr-B

Rtr-C

• 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
2001 Cisco Systems, Inc.

www.cisco.com

12


Label Assignment and
Distribution
Unsolicited Downstream Distribution
Use label 30 for destination
171.68.10/24

Use label 40 for destination
171.68.10/24

171.68.40/24

171.68.10/24

Rtr-A
In
I/F

In
Lab

Address
Prefix

0

-

171.68.10

... ...

Out
I/F

Rtr-B
Out
Lab

30
Next-Hop...
...
...
1

Rtr-C

In
I/F

In
Lab

0

30 171.68.10

... ...

Address
Prefix

Out
I/F

Out
Lab

40
Next-Hop...
...
...
1

In
I/F

In
Lab

0

40 171.68.10

... ...

Address
Prefix

Out
I/F

Out
Lab

1

...

Next-Hop...
...

IGP derived routes

• LSRs distribute labels to the upstream neighbors
2001 Cisco Systems, Inc.

www.cisco.com

13


Label Assignment and

Distribution
On-Demand Downstream Distribution

Use label 40 for destination
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Use label 30 for destination
171.68.10/24

171.68.10/24
171.68.40/24 Rtr-A

Rtr-B
Request label for
destination 171.68.10/24

Rtr-C
Request label for
destination 171.68.10/24

• Upstream LSRs request labels to downstream neighbors
• Downstream LSRs distribute labels upon request
2001 Cisco Systems, Inc.

www.cisco.com

14


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
2001 Cisco Systems, Inc.

www.cisco.com

15


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
2001 Cisco Systems, Inc.

www.cisco.com

16


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
2001 Cisco Systems, Inc.

www.cisco.com

17


Agenda

2001 Cisco Systems, Inc.

•

MPLS Concepts

•

LSRs and labels

•

Label assignment and distribution

•

Label Switch Paths

•

ATM LSRs

•

Loops and TTL

•

LDP overview

•

Day in the Life of a Packet

www.cisco.com

18


Label Switch Path (LSP)

IGP domain with a label
distribution protocol

IGP domain with a label
distribution protocol

LSP follows IGP shortest path

LSP diverges from IGP shortest path

• 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
2001 Cisco Systems, Inc.

www.cisco.com

19

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
2001 Cisco Systems, Inc.

www.cisco.com

20


Label Switch Path (LSP)
Penultimate Hop Popping
In
I/F
0

In
Lab
-

...

...

Address
Prefix
171.68/16

Out Out
I/F Lab
1
4

Next-Hop
...
...

...

In
I/F
0

In
Lab
4

...

...

Address
Prefix
171.68/16

Out
I/F
2

Next-Hop
...
...

Summary route
for 171.68/16
0

1

1

Address
Next-Hop
Prefix and mask

Out
Lab
pop
...

Interface

171.68.10/24

171.68.9.1

Serial1

171.68.44/24

171.68.12.1

Serial2

171.68/16

...

Null

Summary route
for 171.68/16

0
171.68.44/24

Use label 4 for
FEC 171.68/16

Use label “implicit-null”
for FEC 171.68/16

171.68.10/24

Egress LSR summarises more
specific routes and advertises
a label for the new FEC

Summary route is propagate through
the IGP and label is assigned by each
LSR

Egress LSR needs to do an IP lookup for finding more
specific route
Egress LSR need NOT receive a labelled packet
2001 Cisco Systems, Inc.

www.cisco.com

21


Agenda

2001 Cisco Systems, Inc.

•

MPLS Concepts

•

LSRs and labels

•

Label assignment and distribution

•

Label Switch Paths

•

ATM LSRs

•

Loops and TTL

•

LDP overview

•

Summary
www.cisco.com

22


ATM LSRs
• ATM switches forward cells, not packets
• Label Dist is Downstream on-demand, Ordered
• IGP label is carried in the VPI/VCI field
• Merging LSR:
Ability to use the same label for different FECs if outgoing
interface is the same
Save label space on ATM-LSRs
Cell interleave problem

• Non Merging LSR:
ATM-LSR requests one label per FEC and per incoming
interface (upstream neighbors)
Downstream LSR may request itself new label to its
downstream neighbors

2001 Cisco Systems, Inc.

www.cisco.com

23


ATM LSRs

Non-Merging
Downstream on Demand

In
In Address
I/F Lab
Prefix

Out Out
I/F Lab

1

5

171.68

0

3

2

8

171.68

0

4

...

...

...

...

...

ATM-LSR requested additional label
for same FEC in order to distinguish
between incoming interfaces
(Downstream on Demand)

5
IP
Packet

ATM
cell

5
ATM
cell

IP
Packet

8
ATM
cell

2001 Cisco Systems, Inc.

8
ATM
cell

8
ATM
cell

4

3

4

3

4

ATM
cell

ATM
cell

ATM
cell

ATM
cell

ATM
cell

www.cisco.com

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24


ATM LSRs
VC-Merging Downstream
on Demand
In
In Address
I/F Lab
Prefix

Out Out
I/F Lab

1

5

171.68

0

3

2

8

171.68

0

3

...

...

...

...

...

ATM-LSR transmitted cells in sequence
in order for the downstream LSR to
re-assembling correctly the cells into
packets

5

IP
Packet

ATM
cell

5
ATM
cell

IP
Packet

8
ATM
cell

2001 Cisco Systems, Inc.

8
ATM
cell

8
ATM
cell

3

3

3

3

3

ATM
cell

ATM
cell

ATM
cell

ATM
cell

ATM
cell

www.cisco.com

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25