MPLS Stands for Multi Protocol Label Switching

8/14/2019 MPLS Stands for Multi Protocol Label Switching.

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Lecture#1

MPLS stands for Multi Protocol Label Switching.

Up till now we have IP as a best protocol. All the technologies are coming on IP.e.g. cameras,

SS7 signaling, telephony, video etc.

But IP has its own drawbacks.

So we need a technology which can overcome these drawbacks and it can run the networks in

more OPTIMUM and FLEXIBLE way.

If someone ask u that what is the advantage of MPLS over IP then its answer is

OPTIMIZATION and FLEXIBILITY.

So MPLS is a technology which remove the flaws of IP and improve the functionalities ofIP.

How IP operate? What is the flow of IP? How traffic flows in actual?

1st of all we know about the networks with the help of any routing protocol, static route or

directly connected.

2nd information of these networks is taken into the routing table.

3rd the usual traffic coming to the router is routed by using the information in the routing table.

So it is a two step process

1st gather the information about networks in the Routing table

2nd forward the IP packets towards the destination using that information.

1st process is done by Routing Protocols and the 2nd process is done by routed

protocols.

So routing protocols like RIP, EIGRP, OSPF, and BGP gather the routing information and IP

use this information to send the packets towards destination. Routers are the devices which do

IP FORWARING.

In MPLS environment the forwarding decision is not based on the IP. That means there is no IP

FORWARING. In this environment router read the label attached with the packet and doforwarding on the base of this Label. So here the decision is not based on destination IP

address but it is on the base of label value and this Label is just like a VLAN tag which is

providing an identity to the frame. So the basic difference is that here forwarding is based on

Label not on IP. That is why it is called LABEL SWITCHING.

MPLS is not limited to IP traffic. It can forward the traffic of any protocol like IPX, APPLE TALK

etc. So that is why it is called multiprotocol.


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Concept of CONTROL PLANE and DATA PLANE:-

Remember these are only the imaginary planes.

CONTROL PLANFunction:-Take care of routing information exchange and label exchange between adjacent

Devices.

1.

Routing information is exchanged with routing protocols.

Label information is exchanged with Label Distribution protocol (LDP).

Another protocol responsible for exchanging the Label Distribution is Tag Distribution

Protocol (TDP).

TDP is cisco properietry and is not very popular.LDP is more popular than LDP.

Although BGP is used for exchanging routing information but in certain situation it can be

used for exchanging label information. For example in case of MPLS VPNs BGP canwork both as a routing and label protocol.

RSVP which is primarily Qos related protocol but it can also be used for label exchange

in MPLS TE.

2. Control plane is more intelligent but less powerful than the data plane. Because it is

involved in decision making.

3. Here we have two tables

Routing table where the routing protocols gathers the routing information. It is

also called as routing information base (RIB).

Label table where the label protocols gathers the label information. It is called as

label information base (LIB).

DATA PLANE

Function:-Take care of forwarding based on either destination addresses or labels; also known

as forwarding plane.

1. Its function is just to forward the IP packets

2. It is less intelligent but more powerful than control plane.

3. Two tables are built here against the tables built in routing table.

Forwarding information base(FIB)

Label forwarding information base(LFIB)

These four tables in the control plane and data plane are actually controlling the whole

functionality of MPLS.


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Brief Introduction of MPLS process:-

Router A Router B

10.0.0.0 10.0.0.0

10.0.0.0

Label=17 Label=40

label=51

10.0.0.0 10.0.0.0

10.0.0.0

Label=17 Label=40Label=51

1. Routing protocols in the control plane exchange the routing information.

2. After this exchange LDP starts its function. From the left of router A, any router using

LDP advertizes the label number 17 for 10.0.0.0.Now the router A will advertize the label

40 for network 10.0.0.0. Note that label advertisement of both the routers is different

although the network is same. So the label assigned by a router for the same network

can be different because every router is assigning or advertizing the label independently

or independent of other routers .similarly B will advertize the different label 51 and so on.

Actually the label assigning router is instructing the other router that they will only

receive the packet for network 10.0.0.0 with this label. For example router A isinstructing the router B that it will only forward the packet for 10.0.0.0 with label 40 so

whenever router B send the packet to A for 10.0.0.0 it will assign the label 40 to it.

Although every neighbor is dependent on the other router as it is getting the label value

from it while forwarding but remember label assigning process is independent. Each

router assign the label to a network independent to the label value assigned by the other

router to the same network.

3. So from LIB, LFIB is built. In this table there is mapping between the assigned labels and

the advertized labels for a specific network. For example if router A will receive packet

for network 10.0.0.0 with label 40 it will forward it with label value 17 to the left. Similarly

router B will replace the label 51 with for 10.0.0.0 with 40.so remember label values are

changed on every router although ip address address remains the same.So ip is an end-

to-end protocol but label information of ip exchanged locally.

LDP

10.0.0.0

=40

1740

LFIB

LDP

10.1.1.1

=51

4051

LFIB


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MPLS label:-

LABEL=20 bits EXP=3bit *Bos=1bit TTL=8bits

EXP=experimental bit, used to implement QoS.These bits plays the same role as the

precedence bit in the IP header. In IP header there are 8 bits of CoS in which last three bits are

used for precedence.

BoS=Bottom of stack bit

An Routing protocols forwards the routing information.

Where the MPLS packet is inserted?

Once an IP packet is generated by a computer it is forwarded to router. As computers are

unable to create an MPLS labeled packet so MPLS router actually insert or shim the MPLS label

in the IP packet. MPLS label is inserted in between the layer 2 and layer 3.

Layer 2 MPLS Label

Layer 2.5

Layer 3 Payload

Modes of MPLS:-i) Frame mode

ii) Cell Mode

we will discuss only the frame mode as cell mode is not used.


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MPLS Domain

Lecture#2

Now we discuss the MPLS domains or network.

We have mostly discussed the networks with reference to custumer or subscriber or an

enterprise network. We always show the service provider with a cloud. But when we discuss

MPLS it is the technology used largly by SPs.So now we should see the SPs could as our own

cloud and the enterprise cloud as other cloud.

What are the roles of different routers in an MPLS domain?

IP Packet

We can easily explain from above figure that MPLS routers has three function to perform.That is

Inserting, swaping and poping labels on the ip packets.Also that edge LSR has two types i.e.

ingress and egress LSR depending upon the flow of traffic.A simple edge LSR can contain both

the roles depending upon the flow of traffic.So here is the summary of the story

Ed

ge

LS

R

Ed

ge

LS

R

Ed

ge

LSR

Ed

ge

LS

R

Co

re

LS

RR1 R2

IP Packet

Label

INSERTEDPack

et

Label

SWAPEDPack

et

Label POPED Packet

mean simple IP Packet

ngress Edge LSR

Converting IP

packet to

Label

Inserting Label

LSR within the MPLS

domain only do the

swapping

Egress Edge LSR

Converting

Label packet

to IP

Popping Label

R3

IPPa

cket

LabelSWAPED

R2

IP packet


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i) Ordinary or Core LSR(Label Switching Router)ii) Edge LSR

Ingress Edge LSR

Egress Edge LSR

A router can be ingress, egress, Core LSR or an ordinary router at the same time depending

upon the flow of traffic. For example our edge routers in above figure is actually serving all the

roles. So role the router depends upon the flow of traffic.

Architecture of Edge LSR

LSR regardless of its types perform the three functions

i) Exchange routing information. CONTROL PLAN

ii) Exchange Label information.

iii) With help of this information forward the user traffic or data. DATA PLAN

Control Plan

Data Plan

If incoming packet is IP packet then it will pass through the FIB and the forwarding

decision is taken by FIB. If incoming packet is label packet then it will pass through theLFIB and the forwarding decision is taken by LFIB.

FIB always receive IP packet but it can forward both IP and label packets. Similarly LFIB

always receive labeled packet but it can forward both the label and IP packets.

RIB

LIB

FIB

LFIB

FIBIP

Packet

IP

Packet

Routin


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In labeled information it instruct the neighboring router that if you

want to route traffic to 10.0.0.0 through me then I need this (e.g.

93 specified by instructing router) label.

The difference between routing update and labeled update is that in routing update IP will

always remain the same throughout every router but in label update every router will change thelabel independent of the neighboring router. So router updates are end-to-end but label updates

are local.

Ingress router MPLS Domain Egress Router

Every router here is informing the its neighbor that what should be the next hop label if it wana

route the traffic to 10.0.0.0/8.note that in LFIB there is no net.ID entry.It mean here routing

lookup is only on the base of labels.

Now we see this process

This is normal routing without labels.

AB D

E

FIB

10/8NH,

25LFIB

untag25

C

FIB

10/8NH,

44LFIB

2544

FIB

10/8NH

LFIB

44unta

g


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Every router take all entries from its routing table and assign labels to each entry and store this

information into its LIB as local table. Here router B has assigned label 25 to network X in its

LIB.

This label information is send to all neighbors. Now every router will take this entry in its LIB. In

other word router B is informing or updating every neighbor that if they want to route traffic to

network X then they should insert label 25 with this network. Only in that case router B will do

label forwarding for network X.

How E will forward this labeled packet?

As E has not received any label from C so it can forward the label packet to C.So it simply

remove the label and send an IP packet to C.


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Now C will also assign a label 47 for network X independent of the neighbors and take it into the

LIB table and then take it into the LFIB also. Also we can see that in LIB we have two entries for

network X. So we can say that for every entry it has two tables in the LIB. One is local and

second for the labels learned from its neighbors.


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As D is not in our MPLS domain so C forward only an IP packet after popping the label to D.How LDP determine the next hop?

This is done by routing protocols. So LDP is dependent upon the routing protocol to tell the

exact path.

How the labels are assigned?

Labels are assigned in two ways.

On per platform basis

On per interface basis

Per plate form means that a label is assigned to the whole router. That is same label is

advertized to all of its interfaces.

Per interface means that every interface can advertize the different labels. In this case different

labels can be attached to single network. This is only done in cell mode. In frame mode we

assign labels only on per platform basis.


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How LDP works?

It is very similar to OSPF and EIGRP.

LDP like other routing protocols 1st build the neighbor relationship between the MPLS speaking

routers and then it exchange its label information.

LDP use UDP/ TCP port 646. It use UDP for neighbor relationship and TCP for label information

exchange.It multicast on All Router multicast address 224.0.0.2.

Its hello interval is 5 seconds and dead interval is 15 seconds.

Penultimate Hop Popping (PHP)

Here when packet reaches at egress LSR the network 10.0.0.0 is now directly connected so

here the router will not send label rather it will do ARP to resolve its Mac and then send the IP

packet. So to do it it will first remove LFIB and then FIB. So it needs a double lookup here. If the

next router is non MPLS then still it needs a double lookup. So efficiency of the router will

decrease.

So in penultimate Hop Popping the penultimate means second last router will not insert label for

the last ingress edge router. So the last router means egress edge router will not do double

lookup and its performance improves.

PHP optimizes the performance of the network (One less LFIB lookup).


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How the second last router knows, not to send the labeled packet?

The last egress edge LSR will send labeled packet to its penultimate router with label value 3.The label 3 is reserved for pop or Implicit Null value. When it is send to its neighbors then it

mean not to send it labeled packets.

Actually 1-16 label values are reserved. Every value has a specific meaning and purpose. Only

are few values are in use.

KEY ACRONYMS

MPLS Multiple Protocol Label Switching; also, Multiple Protocol Lambda Switching

LER Label Edge Router

LSR Label Switch Router

LIB Label Information Base


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LSP Label Switch Path

FEC Forward Equivalence Class; also, Functional Equivalent Class

MPLS HIGHLIGHTS

MPLS allows for the marriage of IP to layer 2 technologies by overlaying a protocol on top of IPnetworks.

Network routers equipped with special MPLS software process MPLS labels contained in the ShimHeader.Raw IP traffic is presented to the LER, where labels are pushed; these packets are forwarded over LSPto LSR where labels are swapped.At the egress to the network, the LER removes the MPLS labels and marks the IP packets for delivery.

If traffic crosses several networks, it can be tunneled across the networks by using stacked labels.


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R3(config)#int serial 1/0

R3(config-if)#ip add 2.1.1.2 255.0.0.0

R3(config-if)#no sh

R3(config-if)#exit

R3(config)#int s 1/1

R3(config-if)#ip add 3.1.1.1 255.0.0.0R3(config-if)#no shut

R3(config-if)#exit

R3(config)#int ethernet 2/0

R3(config-if)#ip add 30.1.1.1 255.0.0.0

R3(config-if)#no sh

R3(config-if)#exit

R3(config)#router ospf 1

R3(config-router)#net 2.0.0.0 0.255.255.255 a 0




R3(config-router)#exit

Router(config)#hostname R4

R4(config)#int loopback 0

R4(config-if)#ip add 44.4.4.4 255.255.255.255

R4(config-if)#no sh

R4(config)#int s 1/0

R4(config-if)#ip add 3.1.1.1 255.0.0.0

R4(config-if)#no shR4(config)#int serial 1/1

R4(config-if)#ip add 4.1.1.1 255.0.0.0

R4(config-if)#no sh

R4(config-if)#exit

R4(config)#int Ethernet 2/0

R4(config-if)#ip add 40.1.1.1 255.0.0.0

R4(config-if)#no sh

R4(config-if)#exit


R4(config-router)#network 3.0.0.0 0.255.255.255 a 0




Router(config)#hostname R5


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R5(config)#int loopback 0

R5(config-if)#ip add 55.5.5.5 255.255.255.255

R5(config-if)#no sh

R5(config)#int serial 1/0

R5(config-if)#ip add 4.1.1.2 255.0.0.0

R5(config-if)#no shR5(config-if)#exit

R5(config)#int e

R5(config)#int ethernet 2/0

R5(config-if)#ip add 50.1.1.1 255.0.0.0

R5(config-if)#no sh

R5(config-if)#exit





R1#sh ip route ospf 1

O 50.0.0.0/8 [110/266] via 1.1.1.2, 00:04:12, Serial1/0

O 2.0.0.0/8 [110/128] via 1.1.1.2, 00:04:12, Serial1/0

33.0.0.0/32 is subnetted, 1 subnets

O 33.3.3.3 [110/129] via 1.1.1.2, 00:04:12, Serial1/0

O 3.0.0.0/8 [110/192] via 1.1.1.2, 00:04:12, Serial1/0

O 4.0.0.0/8 [110/256] via 1.1.1.2, 00:04:12, Serial1/0


O 55.5.5.5 [110/257] via 1.1.1.2, 00:04:12, Serial1/0

O 20.0.0.0/8 [110/74] via 1.1.1.2, 00:04:12, Serial1/022.0.0.0/32 is subnetted, 1 subnets

O 22.2.2.2 [110/65] via 1.1.1.2, 00:04:12, Serial1/0

O 40.0.0.0/8 [110/202] via 1.1.1.2, 00:04:12, Serial1/0


O 44.4.4.4 [110/193] via 1.1.1.2, 00:04:12, Serial1/0

O 30.0.0.0/8 [110/138] via 1.1.1.2, 00:04:12, Serial1/0

R1#sh ip route connected

C 1.0.0.0/8 is directly connected, Serial1/0

C 10.0.0.0/8 is directly connected, Ethernet2/0


C 11.1.1.1 is directly connected, Loopback0




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44.0.0.0/32 is subnetted, 1 subnetsC 44.4.4.4 is directly connected, Loopback0


O 1.0.0.0/8 [110/192] via 3.1.1.1, 00:15:49, Serial1/0

O 50.0.0.0/8 [110/74] via 4.1.1.2, 00:15:49, Serial1/1

O 2.0.0.0/8 [110/128] via 3.1.1.1, 00:15:49, Serial1/0


O 33.3.3.3 [110/65] via 3.1.1.1, 00:15:49, Serial1/0


O 55.5.5.5 [110/65] via 4.1.1.2, 00:15:49, Serial1/1

O 20.0.0.0/8 [110/138] via 3.1.1.1, 00:15:49, Serial1/0


O 22.2.2.2 [110/129] via 3.1.1.1, 00:15:49, Serial1/0

O 10.0.0.0/8 [110/202] via 3.1.1.1, 00:15:49, Serial1/0


O 11.1.1.1 [110/193] via 3.1.1.1, 00:15:49, Serial1/0

O 30.0.0.0/8 [110/74] via 3.1.1.1, 00:15:49, Serial1/0

R4#sh ip route connR4#sh ip route connected







O 1.0.0.0/8 [110/192] via 3.1.1.1, 00:15:49, Serial1/0

O 50.0.0.0/8 [110/74] via 4.1.1.2, 00:15:49, Serial1/1

O 2.0.0.0/8 [110/128] via 3.1.1.1, 00:15:49, Serial1/0


O 33.3.3.3 [110/65] via 3.1.1.1, 00:15:49, Serial1/0


O 55.5.5.5 [110/65] via 4.1.1.2, 00:15:49, Serial1/1

O 20.0.0.0/8 [110/138] via 3.1.1.1, 00:15:49, Serial1/0



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O 22.2.2.2 [110/129] via 3.1.1.1, 00:15:49, Serial1/0

O 10.0.0.0/8 [110/202] via 3.1.1.1, 00:15:49, Serial1/0


O 11.1.1.1 [110/193] via 3.1.1.1, 00:15:49, Serial1/0

O 30.0.0.0/8 [110/74] via 3.1.1.1, 00:15:49, Serial1/0







O 1.0.0.0/8 [110/256] via 4.1.1.1, 00:17:09, Serial1/0

O 2.0.0.0/8 [110/192] via 4.1.1.1, 00:17:09, Serial1/0


O 33.3.3.3 [110/129] via 4.1.1.1, 00:17:09, Serial1/0

O 3.0.0.0/8 [110/128] via 4.1.1.1, 00:17:09, Serial1/0

O 20.0.0.0/8 [110/202] via 4.1.1.1, 00:17:09, Serial1/0


O 22.2.2.2 [110/193] via 4.1.1.1, 00:17:09, Serial1/0

O 40.0.0.0/8 [110/74] via 4.1.1.1, 00:17:09, Serial1/0

O 10.0.0.0/8 [110/266] via 4.1.1.1, 00:17:09, Serial1/0


O 11.1.1.1 [110/257] via 4.1.1.1, 00:17:09, Serial1/0


O 44.4.4.4 [110/65] via 4.1.1.1, 00:17:09, Serial1/0O 30.0.0.0/8 [110/138] via 4.1.1.1, 00:17:09, Serial1/0

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MPLS Stands for Multi Protocol Label Switching