IP over MPLSread.pudn.com/downloads155/ebook/688769/studa.com/S11 - IP Qo… · Basic MPLS Concepts • Multi-protocol Label Switching (MPLS) is a new forwarding mechanism in which

IP over MPLS

Overview This module focuses on the IP QoS mechanisms available in combination with Multiprotocol Label Switching (MPLS).

Objectives Upon completion of this module, you will be able to perform the following tasks:

n Describe and configure QoS Mechanisms in Frame-mode MPLS networks

n Describe and configure QoS Mechanisms in Cell-mode MPLS networks

23-2 World Wide Training Word Templates v1 Copyright 1999, Cisco Systems, Inc.

MPLS Introduction

Objectives Upon completion of this lesson, you will be able to perform the following tasks:

n Describe basic features of MPLS

n Describe Frame-mode MPLS

n Describe Cell-mode MPLS

Copyright 1999, Cisco Systems, Inc. Release Date: 2/1/99 23-3

© 2001, Cisco Systems, Inc. IP QoS IP over MPLS

Basic MPLS ConceptsBasic MPLS Concepts

• Multi-protocol Label Switching (MPLS) is a new forwarding mechanism in which packets are forwarded based on labels

• Labels may correspond to IP destination networks (equal to traditional IP forwarding)

• Labels can also correspond to other parameters (QoS, source address, ...)

• MPLS was designed to support forwarding of other protocols as well

Multi-protocol Label Switching (MPLS) is a switching mechanism that uses labels (numbers) to forward packets.

Labels usually correspond to layer-3 destination addresses (equal to destination-based routing). Labels can also correspond to other parameters (QoS, source address, etc.).

MPLS was designed to support other protocols as well. Label switching is performed regardless of the layer-3 protocol.



MPLS ExampleMPLS Example

• Only edge routers must perform a routing lookup.

• Core routers switch packets based on simple label lookups and swap labels.

L=5L=3

10.1.1.110.1.1.1

Routing lookup and

label assignment10.0.0.0/8 à L=5

Label swappingL=5 à L=3

Label removal and

routing lookupL=3

The example in the figure illustrates a situation where the intermediary router does not have to perform a time-consuming routing lookup. Instead this router simply swaps a label with another label (5 is replaced by 3) and forwards the packet based on the received label (5).

In larger networks, the result of MPLS labeling is that only the edge routers perform a routing lookup. All the core routers forward packets based on the labels.



MPLS vs. IP-over-ATMMPLS vs. IP-over-ATM

• Layer-2 devices are IP-aware and run a routing protocol

• There is no need to manually establish virtual circuits

• MPLS provides a virtual full-mesh topology

10.1.1.1L=5L=3

L=1710.1.1.1

Layer-2 devices run a layer3 routing protocol

and establish virtual circuits dynamically based

on layer3 information

The example in the figure shows how MPLS is used in ATM networks to provide optimal routing across layer-2 ATM switches. In order for MPLS to work with ATM switches, the switches must be layer-3 aware (ATM switches must run a layer-3 routing protocol).

Another benefit of this setup is that there is no longer a need to manually establish virtual circuits. ATM switches automatically create a full mesh of virtual circuits based on layer-3 routing information.



Traffic Engineering with MPLSTraffic Engineering with MPLS

• Traffic can be forwarded based on other parameters (QoS, source, ...)

• Load sharing across unequal paths can be achieved

Secondary OC48 link

Large site A

Large site B

Small site C

Primary OC192 link

MPLS also supports traffic engineering. Traffic engineered tunnels can be created based on a traffic analysis to provide load balancing across unequal paths.

Multiple traffic engineering tunnels can lead to the same destination but can use different paths. Traditional IP forwarding would force all traffic to use the same path based on the destination-based forwarding decision. Traffic engineering determines the path at the source based on additional parameters (available resources and constraints in the network).



MPLS ArchitectureMPLS Architecture

• MPLS has two major components:• Control plane – exchanges layer-3 routing information and

labels• Data plane – forwards packets based on labels

• Control plane contains complex mechanisms to exchange routing information (OSPF, EIGRP, IS-IS, BGP,...) and labels (TDP, LDP, BGP, RSVP, ...)

• Control plane maintains the contents of the label switching table (label forwarding information base or LFIB)

• Data plane has a simple forwarding engine

To better understand the inner workings of MPLS, its two major components should be clarified:

n Control plane, which takes care of the routing information exchange and the label exchange between adjacent devices

n Data plane, which takes care of forwarding either based on destination addresses or labels.

There is a large number of different routing protocols such as OSPF, IGRP, EIGRP, IS-IS, RIP, BGP, etc. that can be used in the control plane.

The control plane also requires protocols such as TDP (MPLS), LDP (MPLS), BGP (MPLS/VPNs), RSVP (Traffic Engineering), CR-LDP (Traffic Engineering), etc. to exchange labels.

The data plane however, is a simple label-based forwarding engine that is independent of the type of routing protocol or label exchange protocol. A Label Forwarding Information Base (LFIB) is used to forward packets based on labels. The LFIB table is populated by the control plane.



MPLS ArchitectureMPLS Architecture

• Router’s functionality is divided into two major parts: control plane and data plane

Data plane

Control plane

OSPF: 10.0.0.0/8

LDP: 10.0.0.0/8Label 17

OSPF

LDP

LFIB

LDP: 10.0.0.0/8Label 4

OSPF: 10.0.0.0/8

4à17Labeled packet

Label 4Labeled packet

Label 17

A simple MPLS-enabled network implements destination-based forwarding that uses labels to make forwarding decisions.

A layer-3 routing protocol is still needed to propagate layer-3 routing information. A label exchange mechanism is simply an add-on to propagate labels that are used for layer-3 destinations.

The example in the figure illustrates the two components of the control plane:

n OSPF that receives and forwards IP network 10.0.0.0/8, and places that prefix into the routing table.

n LDP that receives label 17 to be used for packets with a destination address 10.x.x.x. A local label 4 is generated and sent to upstream neighbors so these neighbors can label packets with the appropriate label. LDP inserts an entry into the Data Plane’s LFIB table where label 4 is mapped to label 17.

The data plane then forwards all packets with label 4 through the appropriate interfaces and replaces the label with label 17.



MPLS Modes of OperationMPLS Modes of Operation

• MPLS technology is designed to be Layer-1 and Layer-2 independent

• MPLS uses a 32-bit label field which is inserted between Layer-2 and Layer-3 headers (frame mode)

• MPLS over ATM uses the ATM header as the label (cell mode)

MPLS is designed for use on virtually any media and layer-2 encapsulation. Most layer-2 encapsulations are frame-based and MPLS simply inserts a 32-bit label between the layer-2 and layer-3 headers (“frame-mode” MPLS).

ATM is a special case where fixed-length cells are used and a label cannot be inserted on every cell. MPLS uses the VPI/VCI fields in the ATM header as a label (“cell-mode” MPLS).



Label FormatLabel Format

MPLS uses a 32-bit label field that contains the following information:

• 20-bit label• 3-bit experimental field• 1-bit bottom-of-stack indicator• 8-bit time-to-live field (TTL)

LABEL EXP S TTL

0 19 22 23 3120 24

A 32-bit label contains the following fields:

n 20-bit label: the actual label

n 3-bit experimental field: used to define a class of service (i.e. IP precedence)

n Bottom-of-stack bit: MPLS allows multiple labels to be inserted; this bit is used to determine if this is the last label in the packet

n 8-bit time-to-live (TTL) field: has the same purpose as the TTL field in the IP header



Frame Mode MPLSFrame Mode MPLS

Frameheader

IP header Payload

Layer 2 Layer 3

Frameheader

Label IP header Payload

Layer 2 Layer 2½ Layer 3

Routing lookup and

label assignment

The example in the figure shows an edge router that receives a normal IP packet. The router then performs the following actions:

n A routing lookup to determine the outgoing interface

n A label is assigned and inserted between layer-2 frame header and layer-3 packet header if the outgoing interface is enabled for MPLS and a next-hop label for the destination exists

n The labeled packet is sent

Other routers in the core simply forward the packet based on the label.



Cell mode MPLSCell mode MPLS

Frameheader

IP header Payload

Layer 2 Layer 3

Frameheader



AAL5header



ATMheaderCell 1

PayloadATMheaderCell 2

VPI/VCI fields are used for label

switching

Cell-mode MPLS uses the ATM header’s VPI/VCI fields to make forwarding decisions while the 32-bit label is still preserved in the frame but not used in the ATM network. The original label is only present in the first cell of a packet.



Label Switch RouterLabel Switch Router

• Label Switch Router (LSR) primarily forwards labeled packets (label swapping)

• Edge LSR primarily labels IP packets and forwards them into the MPLS domain, or removes labels and forwards IP packets out of the MPLS domain

MPLS Domain

Edge LSR

LSR

10.1.1.1 L=3 L=5

L=43L=3120.1.1.1

10.1.1.1

20.1.1.1

Before proceeding with a detailed description of MPLS, some of the terminology that is used in this course is presented:

n Label Switch Router (LSR): a device that primarily forwards packets based on labels.

n Edge LSR: a device that primarily labels packets or removes labels.

LSRs and Edge LSRs are usually devices that are capable of doing both label switching and IP routing. Their names are based on their position in an MPLS domain. Routers that have all interfaces enabled for MPLS are called LSRs because they mostly forward labeled packets. Routers that have some interfaces that are not enabled for MPLS are usually at the edge of an MPLS domain (autonomous system). These routers also forward packets based on IP destination addresses and label them if the outgoing interface is enabled for MPLS.



ATM Label Switch RouterATM Label Switch Router

• ATM LSR can only forward cells• ATM Edge LSR segments packets into cells and

forwards them into an MPLS ATM domain, or reassembles cells into packets and forwards them out of an MPLS ATM domain

MPLS Domain

ATMEdge LSR

ATMLSR

10.1.1.1 L=1/3

L=1/620.1.1.1

10.1.1.1

20.1.1.1

L=1/3 L=1/3 L=1/5 L=1/5 L=1/5

L=1/6 L=1/6 L=1/9 L=1/9 L=1/9

Label Switch Routers that perform cell-mode MPLS are called:

n ATM LSR if they are ATM switches. All interfaces are enabled for MPLS and forwarding is done based only on labels.

n ATM Edge LSR if they are routers connected to an MPLS-enabled ATM network.



Architecture of LSRsArchitecture of LSRs

LSRs, regardless of the type, perform the following three functions:• Exchange routing information• Exchange labels• Forward packets (LSRs and edge LSRs) or

cells (ATM LSRs and ATM edge LSRs)

The first two functions are part of the control planeThe last function is part of the data plane

LSRs of all types must perform the following functions:

n Exchange layer-3 routing information (ATM LSRs must also exchange layer-3 routing information)

n Exchange labels

n Forward packets or cells

Frame-mode and cell-mode MPLS use a different data plane:

n Frame-mode MPLS forwards packets based on the 32-bit label

n Cell-mode MPLS forwards packets based on labels encoded into the VPI/VCI fields in the ATM header

The control plane performs the following functions:

n Exchange routing information regardless of the type of LSR;

n Exchange labels according to the type of MPLS (frame-mode or cell-mode);



Architecture of LSRsArchitecture of LSRs

LSRs primarily forward labeled packets or cells (ATM LSRs)

LSR

Control plane

Data plane

Routing protocol

Label distribution protocol

Label forwarding table

IP routing table

Exchange ofrouting information

Exchange oflabels

Incoming labeled packets

Outgoing labeled packets

The primary function of an LSR is to forward labeled packets. Therefore, every LSR needs a layer-3 routing protocol (OSPF, EIGRP, IS-IS, etc.) and a label exchange protocol (LDP, TDP, etc.).

The label exchange protocol populates the LFIB table in the data plane that is used to forward labeled packets.

Note LSRs may not be able to forward unlabeled packets either because they are ATM LSRs, or they do not have all the routing information.



Architecture of Edge LSRsArchitecture of Edge LSRs

Note: ATM edge LSRs can only forward cells

Edge LSR

Control plane

Data plane

Routing protocol

Label distribution protocol

Label forwarding table

IP routing table

Exchange ofrouting information

Exchange oflabels

Incoming labeled packets

Outgoing labeled packets

IP forwarding table

Incoming IP packets

Outgoing IP packets

Edge LSRs also forward IP packets based on their IP destination addresses and optionally label them if a label exists.

The following combinations are possible:

n A received IP packet is forwarded based on the IP destination address and sent as an IP packet.

n A received IP packet is forwarded based on the IP destination address and sent as a labeled packet.

n A received labeled packet is forwarded based on the label; the label is changed and the packet is sent.

The following scenarios are possible if the network is misconfigured:

n A received labeled packet is dropped if the label is not found in the LFIB table even if the IP destination exists in the FIB table.

n A received IP packet is dropped if the destination is not found in the FIB table even if there is a label-switched path available for the destination.


Summary MPLS architecture is divided into two parts:

n Control plane that takes care of routing information and label propagation.

n Data plane that takes care of the forwarding of packets.

MPLS has two modes:

n Frame-mode MPLS that is used on all frame-based media.

n Cell-mode MPLS that is used in MPLS-enabled ATM networks.

MPLS networks use the following devices:

n Label Switch Router (LSR) to forward packets based on a 32-bit label

n Edge LSR to forward labeled packets or label IP packets or remove labels.

n ATM LSRs to forward cells based on labels encoded into the VPI/VCI fields in the ATM header.

n ATM Edge LSRs that segment labeled or unlabeled packets into ATM cells where a label is encoded into VPI/VCI fields in the ATM header.

Review Questions 1. What are the main benefits of MPLS?

2. How is an MPLS label encoded into IP packets?

3. How are labels propagated?


Frame-mode MPLS


n Describe the QoS possibilities in networks using Frame-mode MPLS

n Use MQC to implement QoS with Frame-mode MPLS



MPLS QoSMPLS QoS

• MPLS uses labels to make a forwarding decision

• The MPLS label is inserted between Layer-2 (frame) and Layer-3 (IP packet) headers

• All Layer-3 information becomes invisible to routers in an MPLS domain

• Classification in MPLS-enabled networks can be performed on:• MPLS experimental bits• MPLS labels (future enhancement)

Frame-mode MPLS uses 32-bit labels primarily to make a forwarding decision. Three bits in the label are used for experimental purposes.

When an IP packet enters an MPLS domain a label is inserted between the frame and the IP header.

The MPLS experimental bits can be used for classification and marking purposes when implementing QoS in an MPLS domain.

Future enhancements will allow multiple labels to be used to describe the quality of service.



MPLS Label AssignmentMPLS Label Assignment

• An MPLS label has a three-bit experimental field• Cisco routers automatically copy IP precedence bits

into the MPLS experimental bits• The Modular QoS CLI can be used to classify labeled

packets based on their MPLS experimental bits

LABEL IPFrameHeader

FrameHeader

Payload

PayloadIP

IP precedece

MPLS exp

The figure illustrates the default behavior of Cisco routers. IP precedence is automatically copied from the IP header into MPLS label’s experimental bits.

The modular QoS CLI can be used to classify labeled packets based on MPLS experimental bits as well as mark labeled packets with MPLS experimental-bit values.



MPLS-aware QoS MechanismsMPLS-aware QoS Mechanisms

• The following QoS mechanisms are MPLS aware:- Weighted Random Early Detection (WRED): MPLS

experimental bits are used as weight in the same manner as IP precedence

- Committed Access Rate (CAR): marking of MPLS experimental bits

- Class-Based Policing: marking of MPLS experimental bits

- Class-based Marking: marking of MPLS experimental bits

• If classification is performed based on MPLS experimental bits, other MQC QoS mechanisms can also be used

The figure lists the QoS mechanisms that can interact with MPLS-specific information:

n WRED performs random drops based on MPLS experimental values.

n CAR can mark labeled packets with MPLS experimental values. Conforming and exceeding packets can be marked with different MPLS experimental values.

n Class-based Policing can mark labeled packets with MPLS experimental values. Conforming, exceeding and violating packets can be marked with different MPLS experimental values.

n Class-based Marking can statically mark labeled packets with an MPLS experimental value.

Other QoS mechanisms (for example: CB-WFQ, CB-LLQ) can be used in combination with classification that is based on the value of the MPLS experimental bits.



Configuring CB-WFQ for MPLSConfiguring CB-WFQ for MPLS

match mpls experimental expmatch mpls experimental expRouter(config-cmap)#

• Classifies packets based on MPLS experimental bitsclass-map match-any Gold

match ip precedence 3 4match mpls experimental 3 4

!class-map match-any Silver


!policy-map IP+MPLS

class Goldbandwidth 3000

class Silverbandwidth 1000

!Interface Ethernet0/0ip address 10.1.1.1 255.255.255.0mpls ipservice-policy output IP+MPLS

!

class-map match-any Goldmatch ip precedence 3 4match mpls experimental 3 4

!class-map match-any Silver


!policy-map IP+MPLS

class Goldbandwidth 3000

class Silverbandwidth 1000

!Interface Ethernet0/0ip address 10.1.1.1 255.255.255.0mpls ipservice-policy output IP+MPLS

!

Classification based on MPLS experimental bits is performed by using the match mpls experimental command in the class-map configuration mode. Up to eight values can be used within one class map.

The sample configuration shows a generic class map using the match-any classification strategy to classify IP packets and labeled packets with the same IP precedence or MPLS experimental value.



CAR DiagramCAR Diagram

MeterMeter

Conforms?Conforms?

Set IP prec?Set IP prec?

Set DSCP?Set DSCP?

Set MPLS exp?Set MPLS exp?

Set QoS grp?Set QoS grp?

Mark?Mark?

Transmit?Transmit?Conform or exceed

marking value

Set IP PrecedenceSet IP Precedence

Set DSCPSet DSCP

Set MPLS ExperimentalSet MPLS Experimental

Set QoS GroupSet QoS Group

Continue?Continue?

Yes

Yes

No

No

Forwardor

Enqueue

Go toNext

CAR command

• Marking depends on whether the packet conforms to or exceeds the policy

Yes

Yes

Yes

Yes

DropDrop

Committed Access Rate (CAR) can be used to differentially mark packets based on the arrival rate of packets within the selected class. If a packet conforms (is within contract) it is marked with one value, if it exceeds it is marked with a different value.

CAR also supports recursive processing of packets. One packet can be processed by multiple rate-limit commands.



Configuring CAR for MPLSConfiguring CAR for MPLS

rate-limit {input | output} {access-group rate-limit acl} rate BC BEconform-act {set-mpls-exp-transmit exp | set-mpls-exp-continue exp}exceed-act {set-mpls-exp-transmit exp | set-mpls-exp-continue exp}

rate-limit {input | output} {access-group rate-limit acl} rate BC BEconform-act {set-mpls-exp-transmit exp | set-mpls-exp-continue exp}exceed-act {set-mpls-exp-transmit exp | set-mpls-exp-continue exp}

Router(config-if)#

• CAR can mark MPLS packets based on their arrival rate• CAR supports recursive processing of rate-limit commands • CAR supports classification based on MPLS experimental bit values by

using rate-limit access list• Both conform and exceed actions support other actions: transmit,

continue, drop, set-prec-transmit, set-prec-continue, …

interface Serial0/0ip address 10.1.1.1 255.255.255.252rate-limit input 64000 2000 2000 conform set-mpls-exp-tr 5 exceed set-mpls-exp-tr 0rate-limit output 64000 2000 2000 conform set-mpls-exp-tr 5 exceed set-mpls-exp-tr 0!

interface Serial0/0ip address 10.1.1.1 255.255.255.252rate-limit input 64000 2000 2000 conform set-mpls-exp-tr 5 exceed set-

mpls-exp-tr 0rate-limit output 64000 2000 2000 conform set-mpls-exp-tr 5 exceed set-

mpls-exp-tr 0!

CAR also supports a special rate-limit access list that can match labeled packets based on their MPLS experimental values.

The action options include the two that can set MPLS experimental values:

n set-mpls-exp-continue: sets the MPLS experimental bits (0 to 7) and evaluates the next rate-limit command.

n set-mpls-exp-transmit: set the MPLS experimental bits (0 to 7) and transmits the packet.



Configuring CAR for MPLSConfiguring CAR for MPLS

access-list rate-limit acl {exp | mask mask}access-list rate-limit acl {exp | mask mask}

Router(config)#

• The acl index must be between 200 and 299 to select the rate limit access list for MPLS experimental bits

• Rate limit access lists can be used to match on one or more MPLS experimental values

• Set one value (exp) to be matched or use the mask option to match on more values

• Each access list can have only one lineinterface Serial0/0rate-limit output access-group rate-limit 200 64000 2000 2000 conform transmit exceed droprate-limit input access-group rate-limit 201 64000 2000 2000 conform set-mpls-exp-tr 0 exceed set-mpls-exp-tr 0!access-list rate-limit 200 2access-list rate-limit 201 mask FE!

interface Serial0/0rate-limit output access-group rate-limit 200 64000 2000 2000 conform

transmit exceed droprate-limit input access-group rate-limit 201 64000 2000 2000 conform set-

mpls-exp-tr 0 exceed set-mpls-exp-tr 0!access-list rate-limit 200 2access-list rate-limit 201 mask FE!

Special rate-limit access lists allow high-performance classification based on the following parameters:

n IP precedence value if the number of the access list is in the range from 1 to 99

n MAC address if the number of the access list is in the range from 100 to 199

n MPLS experimental bits if the number of the access list is in the range from 200 to 299

A rate limit access list can have only one line. A single MPLS experimental value can be matched by setting the exp value. Multiple values can be matched by using the mask keyword and applying a mask in hex. This mask is an 8 bit value where each bit corresponds to one experimental value 0 through 7. The low order bit corresponds to value 0 and the high-order bit corresponds to value 7. Setting the bit value to 1 indicates that the corresponding experimental value is a match; setting the value to 0 indicates that the corresponding value is not a match. A combination of bits in the mask can be used to match on any number of MPLS experimental values.

For example, to match an experimental value of 0, the mask would be 01 (0000 0001 binary). To match a value of 5, the mask would be 20 (0010 0000 binary). The second rate-limit command in the sample configuration above uses the mask FE (1111 1110 binary) to match all MPLS experimental values except value 0.



CB-PolicingCB-Policing

• CB-Policing is similar to CAR except:- It uses the Modular QoS CLI for classification

- It supports three different actions (conform,exceed and violate)

- It does not support recursive processing of packets

Class-based Policing is used for the same purpose as CAR. CB-Policing differs from CAR in the following ways:

n The Modular QoS CLI is used to classify packets.

n It can use two token buckets to determine whether a packet conforms to, exceeds or violates the policy.

n It does not support recursive processing of packets (the continue option is not available).



Configuring CB-Policing for MPLSConfiguring CB-Policing for MPLS

police avg-rate [BC [BE]] [conform-action [action] [exceed-action [action] [violate-action [action]]]]police avg-rate [BC [BE]] [conform-action [action] [exceed-action [action] [violate-action [action]]]]

Router(config-pmap-c)#

• avg-rate – traffic rate in bps (8.000 to 200.000.000)• BC – normal burst size dimensions the first token bucket in

bytes (default is 1500 or avg-rate/32; whatever is higher)• BE – excess burst size dimensions the second token bucket in

bytes (equals BC if not configured)• action – can be:

- transmit (default conform action)- drop (default exceed and violate action)- set-prec-transmit ip-precedenceset-dscp-transmit dscpset-qos-transmit qos-groupset-mpls-exp-transmit mple-exp- set frde-transmit - set-clp-transmit

The figure shows that one of several actions can be used to mark labeled packets with an MPLS experimental value. Three different values can be used within a single class depending on whether a packet conforms to, exceeds or violates the policy.



CB MarkingCB Marking

• Class-based Marking can be used to mark labeled packets by setting the MPLS experimental bits

• MPLS experimental bits can currently only be set on input

• DSCP should be translated to IP precedence prior to entry into an MPLS domain

Class-based Marking can use the classification options available in the Modular QoS CLI and statically mark classes with the MPLS experimental values.

Implementation limitations should be considered when translating between any pair of parameters on MPLS domain borders (DSCP to MPLS, IP precedence to MPLS). MPLS marking is currently only supported on input. Inbound IP packets can be directly marked with MPLS experimental values. Using the QoS group parameter is necessary when translating MPLS experimental values back to IP precedence or DSCP (for example: MPLS to QoS group translation on input and QoS group to DSCP translation on output). This functionality and these limitations may change with new IOS versions.



Configuring MPLS MarkingConfiguring MPLS Marking

set mpls experimental exp-bitsset mpls experimental exp-bitsRouter(config-pmap-c)#

• Mark labeled packets with the specified value (0 to 7)• MPLS marking can only be used on input

policy-map SetMPLSclass Class1 qos-group 1set mpls experimental 1class Class2 qos-group 2set mpls experimental 2class Class3 qos-group 2set mpls experimental 3

!

policy-map SetMPLSclass Class1 qos-group 1set mpls experimental 1

class Class2 qos-group 2set mpls experimental 2

class Class3 qos-group 2set mpls experimental 3

!

Use the set mpls experimental command in the policy-map class configuration mode to mark inbound packets with MPLS experimental values.

The sample configuration shows how a QoS group parameter can be translated into MPLS experimental bits.



MPLS TranslationCase Study

MPLS TranslationCase Study

• IP domain is using the DiffServ model:- EF – Class Premium- AF1 – Class Gold- AF2 – Class Silver- Default – Best effort class

• Translate IP DSCP values to and from MPLS experimental bits to achieve a similar result in the MPLS domain

MPLS Domain

IP Domain

The QoS design in the case study uses DSCP to mark packets. Four classes must also be managed in the MPLS domain. A translation between DSCP and MPLS is needed to implement a similar QoS solution in the MPLS domain.

Although standard DSCP values for AF classes seamlessly map to IP precedence values for backward compatibility it is sometimes necessary to manually translate markers between DSCP an IP precedence or DSCP and MPLS. For example:

n A QoS design based on IP precedence is using two IP precedence values to mark packets belonging to one class:

- Class Premium is marked with IP precedence 5 and is guaranteed low latency

- Class Gold is using IP precedence 4 for conforming (low-drop) packets and IP precedence 3 for exceeding (high-drop) packets

- Class Silver is using IP precedence 2 for conforming (low-drop) packets and IP precedence 1 for exceeding (high-drop) packets

- Best effort traffic is marked with IP precedence 0

n When migrating to DSCP-based implementation it is necessary to still support the old QoS design until the entire network is migrated to support DSCP.

The case study shows how this translation can be done manually.

If the original IP-precedence-based design did not use multiple IP precedence values per class there should be no need to configure the translation manually. All class-maps, however, should include class selectors in their match options to support backward compatibility with IP precedence:

n Matching packets for AF1 requires af11, af12, af13 and cs1 to be matched





n Matching packets for EF requires ef and cs5 to be matched

The solution shown on the following pages illustrates how default behavior can be changed by manually configuring the translation between IP precedence (MPLS experimental bits) and the DSCP.



MPLS TranslationCase Study DesignMPLS TranslationCase Study Design

IP DSCP MPLEexperimental

EF 5AF1 low-drop 4AF1 medium-drop 4AF1 high-drop 3AF2 low-drop 2AF2 medium-drop 2AF2 high-drop 1Default 0

MPLS DomainIP Domain

DSCP MPLS exp

IP precedenceQoS group

The figure illustrates how DSCP values should be mapped to IP precedence or MPLS experimental values. Some information is lost because low-drop and medium-drop packets of AF1 and AF2 are marked as one low-drop class in the MPLS domain.

The case study shows how some information about the conforming and exceeding packets within one class can be retained when entering a non-DSCP part of the network (either because routers do not support DSCP or because MPLS experimental bits are used to select Class of Service).

The figure illustrates the translation from three drop probability levels on the DSCP layer into two drop probability level in the IP precedence (MPLS experimental) layer. Using this design further limits the network to only use two classes for AF PHB.



MPLS TranslationCase Study Implementation



DSCP MPLS exp

IP precedence

class-map EFmatch ip dscp ef

class-map AF1LDmatch ip dscp af11 af12

class-map AF1HDmatch ip dscp af13

!policy-map DSCP2precclass EFset ip precedence 5

class AF1LDset ip precedence 4

class AF1HDset ip precedence 3

!

class-map EFmatch ip dscp ef

class-map AF1LDmatch ip dscp af11 af12

class-map AF1HDmatch ip dscp af13

!policy-map DSCP2precclass EFset ip precedence 5class AF1LDset ip precedence 4class AF1HDset ip precedence 3

!

interface Serial5/1/0service-policy input DSCP2prec!

interface Serial5/1/0service-policy input DSCP2prec

!

The first part of the configuration shows how DSCP is translated to IP precedence on ingress into the MPLS network. IP precedence is then automatically copied into MPLS experimental bits.

The default DSCP value equals the default IP precedence value and does not need to be translated. The EF class does not need to be translated either because the EF value (101110) is copied as IP precedence into the MPLS experimental field (101), which equals 5. The configuration for AF2 is not shown in the figure.






DSCP MPLS exp

QoS group

class-map match-any MPLS5match mpls exp 5match ip precedence 5class-map match-any MPLS4match mpls exp 4match ip precedence 4class-map match-any MPLS3match mpls exp 3match ip precedence 3!policy-map MPLS2QoSclass MPLS5set qos-group 5

class MPLS4set qos-group 4


class-map match-any MPLS5match mpls exp 5match ip precedence 5



!policy-map MPLS2QoSclass MPLS5set qos-group 5



class-map QoS5match qos-group 5class-map QoS4match qos-group 4class-map QoS3match qos-group 3!policy-map QoS2DSCPclass QoS5set ip dscp ef

class QoS4set ip dscp af12


!

class-map QoS5match qos-group 5



!policy-map QoS2DSCPclass QoS5set ip dscp ef



!

interface Serial5/1/1service-policy input MPLS2QoS

!interface Serial5/1/0service-policy output QoS2DSCP

interface Serial5/1/1service-policy input MPLS2QoS

!interface Serial5/1/0service-policy output QoS2DSCP

The remainder of the configuration is used to translate MPLS experimental values back into DSCP. The class-maps are configured to process IP packets (very likely due to penultimate hop popping) or labeled packets. Low-drop packets are translated into medium-drop packets in the DiffServ domain.


Summary Frame-mode MPLS allows most IP QoS mechanisms to be used. The three MPLS experimental bits are used in the same way as IP precedence. IP precedence is actually copied into MPLS experimental bits.

Review Questions 1. Which MPLS parameter is used for classification and marking?

2. What is the default value of the MPLS experimental bits?

3. Which QoS mechanisms can be used to set MPLS experimental bits?


Cell-mode MPLS


n Describe QoS features available with Cell-mode MPLS

n Implement QoS on interfaces using Cell-mode MPLS



Cell-mode MPLS QoSCell-mode MPLS QoS

• Classes are encoded with MPLS experimental bits

• Cell-mode MPLS uses the VPI/VCI fields as labels for forwarding

• ATM switches are not capable of looking into the frame-mode label where the experimental bits are

• QoS is implemented using up to four parallel virtual circuits (label-switched paths)

ATM is a Layer-2 technology that does not use frames to transmit Layer-3 packets. Packets are fragmented into fixed-length cells. Cell-mode MPLS makes use of the ATM header to encode labels into VPI/VCI fields. These fields are only used to make a forwarding decision. QoS cannot be achieved using MPLS experimental bits because:

n They are only propagated in the first cell of a packet.

n ATM switches do not look into the payload of cells.

QoS is therefore achieved using multiple labels (up to four).



Cell-mode MPLSCell-mode MPLS

• IP precedence used in IP domain is automatically translated into MPLS experimental bits

• MPLS experimental bits are optionally translated into up to four parallel virtual circuits (label-switched paths)

Native IP

Frame-mode MPLS

Cell-mode MPLS

The figure illustrates how IP packets can be propagated over a native IP network (no MPLS and no ATM or with ATM PVCs), a frame-based MPLS network and a cell-based MPLS network.

QoS is retained when IP packets enter a frame-based MPLS network by copying the IP precedence bits into MPLS experimental bits.

When labeled packets enter a cell-based MPLS network, QoS is retained by forwarding the packet through one of four VCs, which are based on the value of MPLS experimental bits.



Configuring Multi-VCConfiguring Multi-VC

mpls atm multi-vcmpls atm multi-vcRouter(config-if)#

• The command enables Multi-VC operation of cell-mode MPLS

• Eight MPLS experimental values are mapped to four virtual circuits

• The class is determined by the two least significant MPLS experimental bits

• Default mapping is similar to classification of distributed ToS-based WFQ

• Default mapping can be replaced usingthe cos-map command

MPLS exp VC

0 Available1 Standard2 Premium3 Control4 Available5 Standard6 Premium7 Control

Cell-mode MPLS uses one single VC for each IP destination. Use the mpls atm multi-vc interface command to enable routers to request up to four VCs for each IP destination. Classification is based on the low-order two bits of the MPLS experimental field (like ToS-based dWFQ).

The table in the figure shows the default mapping of MPLS values into four VCs: available, standard, premium and control.

Default mapping can be changed using the mpls cos-map and mpls prefix-map commands.



Configuring CoS MappingConfiguring CoS Mapping

mpls cos-map numbermpls cos-map numberRouter(config)#

• Create a CoS map• Allowed values are from 1 to 255

class class {available | control | premium | standard}class class {available | control | premium | standard}Router(config-mpls-cos-map)#

• Assigns a class to one of four virtual circuits• Class values can be in the range from 0 to 3

mpls prefix-map pfmap access-list acl cos-map cos-mapmpls prefix-map pfmap access-list acl cos-map cos-mapRouter(config)#

• Uses CoS map cos-map for all destinations permitted by access list acl

A CoS map must be configured to change the default behavior of the translation of MPLS experimental values into one of four virtual circuits (available, standard, premium and control).

Classes are identified by the two low-order bits of the MPLS experimental field.

Use the mpls prefix-map command to bind a cos-map to all destinations permitted by the acl access list.

Note Most MPLS-related commands are available with the starting keyword mpls or the older tag-switching version. Furthermore, using the mpls keyword results in the command being automatically translated into the tag-switching version for compatibility with older IOS versions.



Configuration ExampleConfiguration Example

tag-switching prefix-map 10 access-list 100 cos-map 10tag-switching prefix-map 11 access-list 101 cos-map 10tag-switching prefix-map 21 access-list 32 cos-map 34!tag-switching cos-map 10class 0 availableclass 1 standardclass 2 premiumclass 3 control

!interface ATM1/0.1 mplsip unnumbered Loopback0no ip mroute-cachempls atm multi-vcmpls ip

!access-list 100 permit ip 10.0.0.0 0.255.255.255 10.0.0.0 0.255.255.255

tag-switching prefix-map 10 access-list 100 cos-map 10tag-switching prefix-map 11 access-list 101 cos-map 10tag-switching prefix-map 21 access-list 32 cos-map 34!tag-switching cos-map 10class 0 availableclass 1 standardclass 2 premiumclass 3 control!interface ATM1/0.1 mplsip unnumbered Loopback0no ip mroute-cachempls atm multi-vcmpls ip!access-list 100 permit ip 10.0.0.0 0.255.255.255 10.0.0.0 0.255.255.255

The sample configuration shows that all traffic to network 10.0.0.0/8 uses four parallel VCs. MPLS experimental bits are mapped using cos-map 10.

Note that only prefix map 10 is properly configured. Prefix map 11 does not have the corresponding access list and prefix map 21 is missing the CoS map as well.



Monitoring and Troubleshooting Cell-mode MPLS


show mpls cos-map [cos-map]show mpls cos-map [cos-map]Router#

• Lists all configured CoS maps

Router#show mpls cos-map 10cos-map 10 class tag-VC

3 control2 premium1 standard0 available

Router#

Router#show mpls cos-map 10cos-map 10 class tag-VC

3 control2 premium1 standard0 available

Router#

Use the show mpls cos-map command to verify the parameters assigned to a cos-map.





show mpls prefix-map [prefix-map]show mpls prefix-map [prefix-map]Router#

• Lists all configured prefix maps

Router#show mpls prefix-mapprefix-map 10 access-list 100 cos-map 10prefix-map 11 access-list 101 cos-map 10Warning: In prefix-map 11, acl 101 is not configuredprefix-map 21 access-list 32 cos-map 34Warning: In prefix-map 21, acl 32 and cos-map 34 are not configuredRouter#

Router#show mpls prefix-mapprefix-map 10 access-list 100 cos-map 10prefix-map 11 access-list 101 cos-map 10Warning: In prefix-map 11, acl 101 is not configured

prefix-map 21 access-list 32 cos-map 34Warning: In prefix-map 21, acl 32 and cos-map 34 are not configured

Router#

Use the show mpls prefix-map command to display one or all configured prefix maps with their corresponding access lists and cos-maps.

Using this command helps determine if there is a component missing:

n Access list 101 is not configured for prefix map 11

n Prefix map 21 is missing both the access list and the CoS map


Summary Cell-mode MPLS uses up to four virtual circuits to achieve differentiated quality of service. Packets are classified based on the two low-order bits of the MPLS experimental field.

Review Questions 1. How is differentia ted QoS implemented on MPLS-enabled ATM

interfaces?

2. What information is used for classification in cell-mode MPLS?


Summary After completing this module, you should be able to perform the following tasks:

n Describe and configure QoS Mechanisms in Frame-mode MPLS networks

n Describe and configure QoS Mechanisms in Cell-mode MPLS networks


Review Questions and Answers MPLS Introduction

Question: What are the main benefits of MPLS?

Answer: Simplified BGP designs, support for MPLS-based VPNs.

Question: How is an MPLS label encoded into IP packets?

Answer: A 32-bit label header is inserted in front of the IP header.

Question: How are labels propagated?

Answer: Labels are propagated between adjacent routers using TDP or LDP.

Frame-mode MPLS

Question: Which MPLS parameter is used for classification and marking?

Answer: The MPLS experimental bits are used to classify and mark labeled packets.

Question: What is the default value of the MPLS experimental bits?

Answer: Cisco routers copy the IP precedence bits into MPLS experimental bits.

Question: Which QoS mechanisms can be used to set MPLS experimental bits?

Answer: CAR, Class-based Policing and Class-based Marking.

Cell-mode MPLS

Question: How is differentiated QoS implemented on MPLS-enabled ATM interfaces?

Answers: By using up to 4 VCs (labels) for each destination.

Question: What information is used for classification in cell-mode MPLS?

Answers: Classification is performed based on the two low-order IP precedence bits.

Documents

IP over MPLSread.pudn.com/downloads155/ebook/688769/studa.com/S11 - IP Qo… · Basic MPLS Concepts • Multi-protocol Label Switching (MPLS) is a new forwarding mechanism in which