MPLS Fundamentals (Slides)

8/13/2019 MPLS Fundamentals (Slides)

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2009 Cisco Systems, Inc. All rights reserved. 1

MPLS


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Agenda

MPLS Concepts

MPLS Label Distribution Protocol MPLS Forwarding

MPLS Configuration


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MPLS Concepts


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What Is MPLS?

Multi Protocol Label Switching

MPLS is an efficient encapsulation mechanism

Uses Labels appended to packets(IP packets, AAL5 frames) for transport of data

MPLS packets can run on other Layer 2 technologies such as ATM,

FR, PPP, POS, Ethernet Other Layer 2 technologies can be run over an

MPLS network

MPLS is a technology for delivery of IP Services


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MPLS Standard

MPLS is the standardized version of Ciscos proprietary tagswitching

MPLS and tag switching are identical except for the mechanismsused for label distribution

Tag switching uses the proprietary Tag Distribution Protocol (TDP)

MPLS uses the IETF standard Label Distribution Protocol (LDP)

TDP and LDP are not compatible but can coexist


7/59 2009 Cisco Systems, Inc. All rights reserved. 7

Control Plane

Every MPLS node must run an IGP

Class flows into FEC(Forwarding Equivalent Class)Group of packets with the same forwarding treatment

Each FEC corresponds to an IP destination prefix (default)

Associate Labels to FEC (label bindings)Labels mapping is distributed via LDP(label distribution protocol)

Data Planeforwarding of packets based on labels

uses a label forwarding database LFIB

CEF is the switching mecanism

MPLS Switching structure



Unit Components

Edge Label Switching Routers

label previously unlabeled packets- at the beginning of a Label Switched Path

strip labels from labeled packets

- at the end of a Label Switched Path

Label Switching Routersforward labeled packets based on the information carried by labels - not IPaddresses



Label Switching Devices

Label Switching Routers

Edge Label Switching

Routers



Forwarding Components

Label Forwarding Information Base (LFIB)- each entry consists of incoming label, outgoing label, outgoinginterface, outgoing MAC address

- LFIB requires CEF to be enabled globally within the router (ip cef

global command)

Forwarding algorithm:- extract label from a packet

- find an entry in the LFIB with the incoming label equal to the label inthe packet

- replace the label in the packet with the outgoing label (from thefound entry)

- send the packet on the outgoing interface (from the found entry)

Carrying label information:- as part of the MAC header

- via a shim between the MAC and the Network Layer header



MPLS: Forwarding



MPLS: Forwarding

1. Existing rout ing protocols (e.g. OSPF, IGRP) establish

routes.



MPLS: Forwarding

2a. Label Distribution Protocol (e.g.,

LDP) establishes label to routes

mappings


routes.



MPLS: Forwarding



mappings

2b. Label Distribution

Protocol (e.g., LDP)

creates LFIB entries on

LSRs


routes.



MPLS: Forwarding



mappings

3.Ingress edge LSR receives

packet, performs Layer 3 value-

added services, and label

packets

2b. Label Distribution

Protocol (e.g., LDP)

creates LFIB entries on

LSRs


routes.



MPLS: Forwarding



mappings

4.LSRs forward

labelled packets

using label swapping




packets

2b. Label

Distribut ion Protocol

(e.g., LDP) creates LFIB

entries on LSRs


routes.



MPLS: Forwarding

5.Edge LSR at

egress removes

label and delivers

packet



mappings

4.LSRs forward

labelled packets

using label swapping




packets

2b. Label

Distribut ion Protocol

(e.g., LDP) creates LFIB

entries on LSRs


routes.



RIB/FIB/LIB/LFIB

RIB is the Routing Information Base that is analogous to the iprouting table

FIB aka CEF is Forwarding information base that is derived fromthe ip routing table

LIB is Label Information Base that contains all the label bindingslearned via LDP

LFIB is Label Forwarding Information Base that is derived from FIBentries and corresponding LIB entries

Lets go through the pictorial view



Control-plane to Data-plane

Data plane in a node

IP Routing Protocol

IP Routing TableRouter

IGPRouting information

exchange withother routers

IP FIB

Incoming IP

packetsOutgoing IP

packets



Control-plane to Data-planeMPLS / LSR


IP Routing Protocol

IP Routing Table

LSRLabel

Switch

Router

Incoming labelled

packets Outgoing labelledpacketsMPLS LFIB

MPLS LIB



(Link-staterecommended)

Label Distribution

ProtocolLabel binding

exchange with otherrouters



Control-plane to Data-planeMPLS / E-LSR


IP Routing Protocol

IP Routing Table

MPLS LIB

MPLS LFIB

E-LSR

EdgeLabel

Switch

Router



(Link-staterecommended)

Label Distribution

ProtocolLabel binding

exchange with otherrouters

Incoming labelledpackets

Outgoing IP

packets

Outgoing labelledpackets

Incoming IP

packets IP FIB



Summary of Packet-mode MPLS LabelAllocation and Distribution

Label allocation and distribution in packet mode MPLS environment followsthese steps:

IP routing protocols build the IP routing table.

Each LSR assigns a label to every destination in the IP routing tableindependently.

Labels are assigned once per LSR (per-platform).

LSRs announce their assigned labels to all its neighbor LSRs.

Every LSR builds its LIB, LFIB and FIB data structures based on receivedlabels.

Every LSR stores all advertised labels in the LIB.

Labels received from next-hop LSRs are used to populate label information inFIB and the outgoing label in LFIB.



10.89

172.69

1

01

0

You can reach 172.69

through me

You can reach 10.89 and

172.69 through me

Routing Updates

(OSPF, EIGRP, )

You can reach 10.89

through me

Address

Prefix

10.89

172.69

...

Out

Iface

0

1

...

Address

Prefix

10.89

172.69

...

Out

Iface

1

1

...

Address

Prefix

10.89

...

Out

Iface

0

...

MPLS switching :Routing Information



1

01

0

Use label 7 for 172.69

Use label 4 for 10.89 and


Label DistributionProtocol


10.89

172.69

In

Lbl

Address

Prefix

10.89

172.69

...

Out

Iface

0

1

...

Out

Lbl

In

Lbl

Address

Prefix

10.89

172.69

...

Out

Iface

1

1

...

Out

Lbl

In

Lbl

Address

Prefix

10.89

...

Out

Iface

0

...

Out

Lbl

-

-

4

5

4

5

9

7

9 -

MPLS switching:Assigning Labels



10.89

172.69

1

01

Data10.89.25.4410.89.25.4 Data

10.89.25.4 Data

10.89.25.4 Data9

0

Label Switch Forwards

Based on Label

In

Lbl

Address

Prefix

10.89

172.69

...

Out

Iface

0

1

...

Out

Lbl

In

Lbl

Address

Prefix

172.69

...

Out

Iface

1

1

...

Out

Lbl

In

Lbl

Address

Prefix

10.89

...

Out

Iface

0

...

-

-

4

5 5 7

-4 910.89

Out

Lbl

9

MPLS switching :Forwarding Packets


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MPLS Forwarding Plane: Ethertype

Ethertype 0x0800 refers to IP

Ethertype 0x8847 refers to MPLS Based on the Ethertype, the packet is handed over to

the appropriate processing engine in the router


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What Is a Label

A LABEL is 4 bytes identifier, which is carried by the packet andused to identify a prefix

Label = Actual Label, = 20 bitsEXP/QoS = Experimental bits, = 3 bits

S = End of Stack, = 1 bit

TTL = Time to Live, = 8 bits

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

Label EXP S TTL


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Packet-over-SONET/SDH

Ethernet: similar

Frame Relay PVCs: similar

Label over ATM PVCs

PPP Header Label Layer 3 Header Data

Ethernet Hdr Label Layer 3 Header Data

Frame Rly Hdr Label Layer 3 Header Data

ATM Header Label Layer 3 Header Data

ATM Header Data(subsequent cells)

Label Encapsulation


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172.68.10/24

Rtr-A

Next-Hop

In

Lab

5

...

Address

Prefix

172.68.10

...

Out

I/F

1

...

Out

Lab

7

...

In

I/F

0

...

IP packetD=172.68.10.12

Label = 5

Label = 21

IP packetD=172.68.10.12

Label = 7

Label = 21

Rtr-A forwards the labelled packet based on the label at the top ofthe label stack

The Label StackMPLS is recursive


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London Brussels Paris

192.26.15.1/32

In Label FEC Out Label

- 192.26.15.1/32 28


28 192.26.15.1/32 POP


- 192.26.15.1/32 -

Use label 28 for destination

192.26.15.1/32

Use label implicit-null for

destination 192.26.15.1/32

Penultimate Hop Popping

May be disabled using explicit-null option(Usefull to maintain end-to-end label for QoS classification)


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Label DistributionProtocol


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Cisco IOS implements two label binding protocols for destination-based unicastrouting

TDP (Tag Distribution Protocol)proprietary protocol - TCP port 711

LDP (Label Distribution Protocol)IETF standards based protocol - TCP port 646

Both protocols are functionally equivalent and can be used concurrently

Labels are exchanged between adjacent LSRs

Applications may require non-adjacent neighbors

- LDP/TDP Directed Peers-AToM

Label Distribution Protocols


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MPLS Control Plane: LDP vs. TDP

LDP is quite similar to TDP

LDP is standardized by IETF

LDP has more features such as abort,MD5 authentication, notification,

backoff logic, etc. TDP is the default on Cisco routers

LDP is the default with this global config

mpls label protocol ldp


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MPLS Control Plane: LDP/TDP

LDP/TDP operates in three steps

Neighbor Discovery

Session establishment

Label Distribution/exchange

Once labels are exchanged, LIB is built

LIB and FIB together helps to build LFIB


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MPLS Control Plane: TDP

TDP Neighbors are discovered via TDP Hellos (likemost of the routing protocols)

TDP Hellos are sent to 255.255.255.255

TDP hellos are sent to UDP port = 711

TDP hellos are sent only after mpls ip is configured onan interface

PE1 PE2

Tx Hello (PE1:0)

Rx Hello (PE2:0)


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MPLS Control Plane: LDP

LDP Neighbors are discovered via LDP Hellos (likemost of the routing protocols)

LDP Hellos are sent to 224.0.0.2

LDP hellos are sent to UDP port = 646

LDP hellos are sent only after both mpls ip and mplslabel protocol ldp are configured on an interface **

LDP_ID should be hardcoded via mpls ldp router-ID

** If LDP is the global default, then interface-level LDP is not needed.

PE1 PE2

Tx Hello (PE1:0)

Rx Hello (PE2:0)


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MPLS Control Plane: LDP (1)

Use the same Loopback0 as the router-ID for LDP,IGP, BGP, etc.

Assign an IP address to the Loopback0 from theseparate IP address subnet (or space)

Avoid the IGP summarization of prefixes thatcorrespond to the router-ids


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sh mpls ldp discovery [detail]

Must show xmit/recv on LDP enabled interface

PE1#sh mpls ldp discovery

Local LDP Identifier:

10.13.1.61:0

Discovery Sources:

Interfaces:Ethernet0/0 (ldp): xmit/recv

LDP Id: 10.13.1.101:0

Ethernet1/0 (ldp): xmit/recv

LDP Id: 10.13.1.101:0

PE1#

debug mpls ldp transport connections

Should give information regarding whether the HELLOS are

advertised/received

Local LDP_ID

Discovered Neighbors

LDP_ID

Xmited and Recvd Hellos

on that interface

Eth0/0 is

configured

with LDP


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LDP session is a TCP session (port = 646)

Multiple links between two routers still mean single LDP session

PE1#sh mpls ldp neighbor

Peer LDP Ident: 10.13.1.101:0; Local LDP Ident 10.13.1.61:0

TCP connection: 10.13.1.101.11031 - 10.13.1.61.646

State: Oper; Msgs sent/rcvd: 58/60; DownstreamUp time: 00:39:27

LDP discovery sources:

Ethernet0/0, Src IP addr: 10.13.1.5

Ethernet1/0, Src IP addr: 10.13.1.9

Addresses bound to peer LDP Ident:

10.13.1.9 10.13.1.5 10.13.2.5 10.13.1.101

PE1#

LDP_ID

Unsolicited Label

Distribution*

Interfaces on Which

Peer Is Discovered

PeersConnected Int

PE1#sh tcp brief| i 646

43ABB020 10.13.1.101.11031 10.13.1.61.646 ESTAB

PE1#

* Unsolicited downstream label Distribution mode is the default in Frame-mode MPLS. In cell-mode MPLS,distribution on demand (DoD) is the default distribution mode. They are not configurable.


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LIB entry can be verified with the following

PE1#sh mpls ip bindings 10.13.1.62 32

10.13.1.62/32

in label: 20

out label: 2001 lsr: 10.13.1.101:0

PE1#

PE1 P110.13.1.61/32 10.13.1.101/32

E0/0E0/1 10.13.1.62/32

Ok. I hear you 10.13.1.101:0.

I have the binding from you in

my LIB now

This is 10.13.1.101:0.

Use label 2001 to reach 10.13.1.62/32

PE1#sh mpls forwarding 10.13.1.62

Local Outgoing Prefix Bytes tag Outgoing Next Hop

tag tag or VC or Tunnel Id switched interface

20 2001 10.13.1.62/32 0 Et0/0 10.13.1.52001 10.13.1.62/32 0 Et1/0 10.13.1.9

PE1#

But whether I use

your binding or notwill be dictated by

RIB entry

Oh ok. Per RIB,

10.13.1.101 is the

next-hop for

10.13.1.62/32.

I have to uselabel 2001 in

LFIB.

Local binding

Remote binding


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sh mpls ip binding detail

Lists all prefixes with labels and LDP neighbors

sh mpls ip binding det

Lists ACLs (if any), prefix bindings, and LDP neighbors NoticeAdvertised to: field

sh mpls ip binding advertisement-acls

Lists LDP filter, if there is any, on the first line. Prefixes followedby Advert acl(s): are advertised via LDP, others are not


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MPLS Forwarding

MPLS F di Pl O t i


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RSP-PE-SOUTH-5#sh mpls forwarding 10.13.1.11



59 46 10.13.1.11/32 0 Se10/0/0 point2point

RSP-PE-SOUTH-5#

MPLS Forwarding Plane: OutgoingLabels

Outgoing label also conveys what treatment the packet is going to

get; it could also bePop Pops the topmost label

Untagged Untag the incoming MPLS packet

Aggregate Untag and then do a FIB lookup

0 Nullify the top label (first 20bits)

Label values 0-15 are reserved

MPLS Forwarding Plane: Outgoing


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MPLS Forwarding Plane: OutgoingLabels

Untagged

Convert the incoming MPLS packet to an IP packet and forward it

PopPop the top label from the label stack present in an incoming MPLS packetand forward it as an MPLS packet.; if there was only one label in the stack,then forward it as an IP packet; SAME as imp-null label

Aggregate

Convert the incoming MPLS packet to an IP packet and then do a FIBlookup for it to find out the outgoing interface

0 (zero)

Same as exp-null label; simplify fills 0 in the first 20 bits of label; helps to

preserve the EXP value of the top label


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MPLS Forwarding Plane: Outgoing Labels

PE1#sh mpls forwarding-table



16 2002 10.13.1.22/32 0 Et0/0 10.13.1.5

2002 10.13.1.22/32 0 Et1/0 10.13.1.917 2001 10.13.1.62/32 0 Et0/0 10.13.1.5

2001 10.13.1.62/32 0 Et1/0 10.13.1.9

18 Pop tag 10.13.1.101/32 0 Et1/0 10.13.1.9

Pop tag 10.13.1.101/32 0 Et0/0 10.13.1.5

19 Pop tag 10.13.2.4/30 0 Et1/0 10.13.1.9

Pop tag 10.13.2.4/30 0 Et0/0 10.13.1.520 Untagged 5.5.5.5/32[V] 0 Se2/0 point2point

21 Pop tag 10.13.21.4/30 0 Et1/0 10.13.1.9

Pop tag 10.13.21.4/30 0 Et0/0 10.13.1.5

22 Pop tag 10.13.22.4/30 0 Et1/0 10.13.1.9

Pop tag 10.13.22.4/30 0 Et0/0 10.13.1.5

23 Aggregate 0.0.0.0/0[V] 0

24 Aggregate 200.1.61.4/30[V] 0

26 Untagged 30.30.30.1/32[V] 0 Se2/0 point2point

PE1#

V Means It Is a VPN Prefix

Connected

VPN Prefix

No Outgoing Interface for theAggregate Entries; an

Addit ional FIB Lookup Is Done


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MPLS Forwarding Plane

Three cases in the MPLS forwarding

1. Label ImpositionIP to MPLS conversion

2. Label swappingMPLS to MPLS

3. Label dispositionMPLS to IP conversion

So, depending upon the case, we need to check1. FIB For IP packets that get forwarded as MPLS

2. LFIB For MPLS packets that get fwded as MPLS

3. LFIB For MPLS packets that get fwded as IP


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PE1 does a FIB lookup forthe incoming IP packet

It imposes the label

For troubleshooting, look atthe FIB (not LFIB)

PE1PE2

P1 1.1.1.0/30

Case 1: IP Packets Get Forwarded as MPLS

PE1#sh ip cef 1.1.1.0

1.1.1.0/30, version 25, epoch 0, cached adjacency 10.13.1.5

0 packets, 0 bytes

tag information set

local tag: 20

fast tag rewrite with Et0/0, 10.13.1.5, tags imposed: {2001}

via 10.13.1.5, Ethernet0/0, 0 dependencies

next hop 10.13.1.5, Ethernet0/0

valid cached adjacencytag rewrite with Et0/0, 10.13.1.5, tags imposed: {2001}

PE1#

IP Packet

IP Packet2001


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P1 does the LFIB lookup forincoming MPLS packets

P1 could swap (or dispose)

the label For troubleshooting, look at

the LFIB (not FIB)

PE1

Case 2: MPLS Packets Get Forwarded as MPLS

P1#sh mpls for 10.13.1.62Local Outgoing Prefix Bytes tag Outgoing Next Hop

tag tag or VC or Tunnel Id swi tched interface

2002 Pop tag 10.13.1.62/32 0 Se2/0 point2point

P1#

P1#sh mpls for 1.1.1.0


tag tag or VC or Tunnel Id swi tched interface

2001 20 1.1.1.1.0/30 0 Se2/0 point2point

P1#

IP Packet2001

PE2

P1 1.1.1.0/30

IP Packet20


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Typically happen at theedge

Could also happen at the

PHP router

For troubleshooting, look atthe LFIB (not FIB)

PE1

Case 3: MPLS Packets Get Forwarded as IP

PE2#sh mpls for 1.1.1.0



20 Untagged 1.1.1.1.0/30 0 Se2/0 point2point

PE2#

PE2

P1 1.1.1.0/30

IP Packet20 IP Packet


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MPLS Fwd Plane: Fragmentation

After the Layer 2 header is added to the IP packet, theresulting packet size shouldnt exceed the max packet

size (MTU size) applicable; otherwise, packet will befragmented

MTU size needs to be tuned to avoid fragmentation in

MPLS network MTU could be increased only for MPLS packets =>

MPLS MTU

Fragmentation


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FragmentationMTU Setting in MPLS

If the label imposition makes the packet bigger than the MPLSMTU size of an outgoing interface, then

If the DF bit set, then discard the packet and send ICMP reply back(with code=4)

If the DF bit is not set, then fragment the IP packet (say, into 2packets), and then impose the same label(s) on both the packets, andthen transmit MPLS packets

Fragmentation should be done at the edge itself


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MPLS Fwd Plane: Show Commands

sh mpls forwarding

Shows all LFIB entries (vpn, non-vpn, TE, etc.)

sh mpls forwarding

LFIB lookup based on a prefix

sh mpls forwarding label LFIB lookup based on an incoming label

sh mpls forwarding detail

Shows detailed info such as L2 encap, etc.


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RSP-PE-WEST-4#sh mpls for 10.13.1.11 detail



45 51 10.13.1.11/32 0 Fa1/1/1 10.13.7.33

MAC/Encaps=14/18, MRU=1500, Tag Stack{51}0003FD1C828100044E7548298847 00033000

No output feature configured

Per-packet load-sharing

RSP-PE-WEST-4#

MPLS Fwd Plane: Show Command

Detail Is Optional

MAC header =0003FD1C828100044E754829

MPLS Ethtype= 0x8847

Label = 0x00033000=3x16+3=51

14/18 means that the L2 header is of 14 bytes, butL2+label header is 18 bytes (one label is 4 bytes)

MRU Max Receivable Unit. The

received packet wil l be transmitted

unfragmented on Fa1/1/1, if its size

is not more than 1500B.

Only One Outgoing

Label in the Label

Stack


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MPLS Fwd Plane: Debugs

Debug mpls lfib cefUseful for seeing FIB and LFIB interaction when a label ismissing for a prefix

debug mpls lfib structShows changes in the LFIB structures when label isallocated/deallocated

Be Careful on the Production Routers


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MPLS Configuration


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Configuration Example

Ip cef need to be enabled

If cef is not enabled by default:

r out er # conf tEnt er conf i gur at i on commands,one per l i ne. End wi t h CNTL/ Z.r out er ( conf i g) #i p cef


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Configuration Example

mpl s i p

mpl s l abel pr ot ocol l dpmpl s l dp nei ghbor 10. 0. 0. 3 passwor d no mpl s l dp adver t i se- t agsmpl s l dp adver t i se- t ags f or 1

mpl s l dp r out er - i d Loopback0

i nt er f ace Gi gabi t Et her net 2/ 1i p addr ess 10. 0. 1. 38 255. 255. 255. 252mpl s mt u 1512

mpl s i p

access- l i st 1 per mi t 10. 0. 0. 0 0. 0. 0. 255


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Documents

MPLS Fundamentals (Slides)