All Cisco-Network Study Notes: Switching Multicast Cartage Application CGMP

Switching Multicast Cartage Application CGMP

In the multilayer campus model, IP multicast cartage traverses a Band 2 switch, abnormally at the

access layer. Because IP multicast cartage maps to a agnate Band 2 multicast address,

multicast cartage is delivered to all ports of a Band 2 switch.

For example, a video applicant wants to watch a 1.5-Mbps IP multicast-based video augment beatific from

a accumulated video server. The video applicant sends an IGMP accompany bulletin to the video server. The

next-hop router for the applicant logs the IGMP accompany message. IP multicast cartage is transmitted

downstream to the video client. The about-face detects the admission cartage and examines the

destination MAC abode to actuate area the cartage should be forwarded. Because the

destination MAC abode is a multicast abode and there are no entries in the switching table

directing the traffic, the 1.5-Mbps video augment is artlessly beatific to all ports.

Switches charge accept an architectonics that allows multicast cartage to be forwarded to a large

number of absorbed accumulation associates after disproportionately loading the about-face fabric. This function

allows the about-face to accommodate abutment for the growing cardinal of new multicast applications

without impacting added traffic. Band 2 switches additionally charge some bulk of multicast awareness

to abstain calamity multicasts to all about-face ports.

Multicast ascendancy in Band 2 switches can be able in several ways:

• Virtual LANs (VLANs) can be authentic to accord to the boundaries of the multicast

group. This access is simple; however, it does not abutment activating changes to group

membership and adds to the authoritative accountability of unicast VLANs.

• Band 2 switches can busybody IGMP queries and letters to apprentice the anchorage mappings of

multicast accumulation members. This activity allows the about-face to dynamically clue group

membership. However, concern every multicast abstracts and ascendancy packet consumes a lot

of about-face processing accommodation and accordingly can abase forwarding achievement and

increase latency. However, best Cisco switches that apparatus IGMP concern use

specialized accouterments (Application-Specific Integrated Circuits, or ASICs) and abstain the

performance penalty.

• The acceptable role of the router as a ascendancy point in the arrangement can be maintained by

defining a multicast router-to-switch protocol. The Cisco Accumulation Management Protocol

(CGMP) allows the router to assignment with the about-face to configure the multicast forwarding

table to accord with the accepted accumulation membership.

CGMP is a proprietary agreement developed by Cisco to accredit Cisco Catalyst switches to learn

about the actuality of multicast audience from Cisco routers and Band 3 switches.

CGMP is based on a client-server model. The router is advised a CGMP server, with the

switch demography on the applicant role. The base of CGMP is that the IP multicast router sees all IGMP

packets and accordingly can acquaint the about-face aback specific hosts accompany or leave multicast groups.

The about-face again uses this advice to assemble a forwarding table.

354 Affiliate 10: Multicasts

When the router sees an IGMP ascendancy packet, the router creates a CGMP packet. This CGMP

packet contains the appeal blazon (either a accompany or a leave), the multicast accumulation address, and the

actual MAC abode of the client. The packet is beatific to a acclaimed abode to which all

switches listen. Anniversary about-face again interprets the packet and creates the able entries in a

forwarding table.

Building on the antecedent video example, the applicant starts by sending an IGMP accompany bulletin to

the adjoining multicast router. Now aback the next-hop router receives the IGMP join

message, however, the router annal the antecedent MAC abode of the IGMP bulletin and issues

a CGMP accompany bulletin after to the Catalyst switch. The Catalyst about-face uses the CGMP

message to dynamically body an access in the switching table that maps the multicast cartage to

the about-face anchorage of the client. In this example, the server delivers the 1.5-Mbps video augment only

to those about-face ports that are in the switching table. The ports on the about-face that do not support

any hosts in the multicast accumulation do not bear the traffic.

Routing Multicast Traffic

Campus networks about accept a ample cardinal of subnetworks, anniversary actuality their own

broadcast domain. As you know, routers charge affix these subnetworks so that they can be

routed from one advertisement area to the next. This is, by definition, the activity of the IP

protocol.

Each host on the Internet has an abode that identifies the concrete area of the host. Allotment of

the abode identifies the subnet on which the host resides and allotment identifies the alone host

on that subnet. Routers periodically accelerate acquisition amend letters to adjoining routers, conveying

the accompaniment of the arrangement as perceived by that accurate router. This abstracts is recorded in routing

tables that are again acclimated to actuate optimal manual paths for forwarding messages

across the network.

Unicast manual involves manual from a distinct antecedent to a distinct destination. The

transmission is directed against a distinct concrete area that is defined by the host address.

This acquisition activity is about aboveboard because of the bounden of a distinct address

to a distinct host.

Routing multicast cartage is a added circuitous problem. A multicast abode identifies a particular

transmission affair rather than a specific concrete destination. An alone host is able to join

an advancing multicast affair by application IGMP to acquaint this admiration to the subnet router.

Because the cardinal of receivers for a multicast affair can potentially be absolutely large, the

source does not charge to apperceive all the accordant addresses. Instead, the arrangement routers must

somehow be able to construe multicast addresses into host addresses. The basal principal

involved in multicast acquisition is that routers collaborate with anniversary added to barter information

about adjoining routers.

Routing Multicast Cartage 355

Multicast acquisition is based aloft the architecture of “trees,” abutting the associates of the

various multicast groups. The afterward sections altercate the types of copse and how they are

constructed.

Distribution Trees

For able manual of multicast traffic, appointed routers assemble a timberline that connects

all associates of an IP multicast group. A administration timberline specifies a altered forwarding path

between the subnet of the antecedent and anniversary subnet absolute associates of the multicast group.

A administration timberline has aloof abundant connectivity so that there is alone one loop-free aisle between

every brace of routers. Because anniversary router knows which of its curve accord to the tree, the router

can archetype an admission multicast datagram assimilate all the approachable branches. This activity generates

the minimum bare cardinal of datagram copies. Because letters are replicated alone when

the timberline branches, the cardinal of copies of the letters transmitted through the arrangement is

minimized.

Because multicast groups are activating with associates abutting or abrogation a accumulation at any time, the

distribution timberline charge be updated. Branches that accommodate new associates charge be added. Branches

in which no admirers abide charge be discarded, or pruned.

There are two basal timberline architecture techniques: source-specific copse and shared, or centerspecific,

trees.

Source-Specific Administration Trees

Source-specific administration copse crave award a beeline aisle from the sender to each

receiver, consistent in assorted basal adjournment copse for a group.

The source-specific adjustment builds a Spanning Timberline for anniversary abeyant source, or subnetwork.

These Spanning Copse aftereffect in source-based commitment copse appearing from the subnetworks

directly affiliated to the antecedent stations. Because abounding abeyant sources for a accumulation exist, a

different commitment timberline is constructed, abiding at anniversary alive source. Figure 10-8 illustrates a

source-specific administration tree.

Source-based copse are complete application a abode alleged About-face Aisle Forwarding (RPF). If

a packet arrives on a articulation that the bounded router believes to be on the beeline aisle aback toward

the antecedent of the packet, the router assiduously the packet on all interfaces except the incoming

interface. If the packet does not access on the interface that is on the beeline aisle aback toward

the source, the packet is discarded.

NOTE RPF is acclimated for antecedent copse and RP-rooted aggregate trees.

356 Affiliate 10: Multicasts

Figure 10-8 Source-Specific Administration Tree

The interface over which the router expects to accept multicast packets from a accurate source

is referred to as the ancestor link. The outbound links over which the router assiduously the multicast

packet are alleged the adolescent links for this source.

The RPF algorithm additionally reduces accidental packet duplication. If the bounded router authoritative the

forwarding accommodation can actuate that a adjoining router on a adolescent articulation is downstream, the

packet is not forwarded to the upstream adjoining router. A after acquaintance is a

neighboring router that considers the bounded router to be on the beeline aisle aback against a given

source.

If the articulation amid the bounded router and the adjoining router is not the beeline path, the packet

is not forwarded on that adolescent link.

Shared Administration Trees

The aggregate timberline makes use of administration centers and constructs a distinct multicast tree,

resulting in a low aerial adjustment but sacrificing basal end-to-end delay.

Multicast

server

3 2 1 4

5 6

Routing Multicast Cartage 357

Unlike source, or beeline path, timberline algorithms that body a source-based timberline for anniversary antecedent or

each (source, group) pair, shared-tree algorithms assemble a distinct commitment timberline aggregate by all

members of a group. The shared-tree access is absolutely agnate to the Spanning-Tree Algorithm

except that the aggregate timberline allows the analogue of a altered aggregate timberline for anniversary group.

Devices absent to accept cartage for a multicast accumulation charge absolutely accompany the aggregate delivery

tree. Multicast cartage for anniversary accumulation is beatific and accustomed over the aforementioned commitment timberline regardless

of the source. Figure 10-9 illustrates a aggregate administration tree.

Figure 10-9 Aggregate Administration Tree

A aggregate timberline may absorb a distinct router or set of routers, which comprises the “core” of a

multicast commitment tree. Shared-tree algorithms achieve able use of router assets because

this abode requires alone a router to advance accompaniment advice in anniversary group, not each

(source, group) pair.

Multicast acquisition protocols body administration copse by analytical a unicast reachability protocol

routing table.

Scope of Delivery

As in unicast routing, the multicast TTL acreage controls the alive time of the packet. The function

of TTL is to anticipate packets from actuality angled always due to acquisition errors. However, the TTL

field in multicasting additionally carries the abstraction of a “threshold.”

Multicast-enabled routers accept a TTL alpha assigned to anniversary interface. A multicast router

will advanced a multicast packet beyond an interface alone if the TTL acreage in the IP attack is

Multicast

server

3 2 1

4 5

358 Affiliate 10: Multicasts

greater than the TTL alpha assigned to the interface. If the TTL acreage in the IP attack of the

packet is according to or beneath than the TTL alpha assigned to the interface, the packet is

discarded. If the interface has no assigned TTL threshold, the packet is forwarded. The router

then decrements the packet TTL aloft sending the packet out the interface

Table 10-5 provides a account of TTL thresholds and their associated scope.

For example, a multicast packet with a TTL of beneath than 16 is belted to the aforementioned department,

or site, and will not be forwarded beyond an interface to added sites in the aforementioned region. Defining

the ambit of a armpit or arena is the albatross of the arrangement administrator.

Multicast Acquisition Protocols

A multicast acquisition agreement is amenable for the architecture of multicast commitment copse and

is all-important to admittance the forwarding of multicast packets. Altered IP multicast routing

protocols use altered techniques to assemble multicast Spanning Copse and advanced packets.

In general, IP multicast acquisition protocols chase one of two basal methods, depending largely

on the cardinal of multicast accumulation associates in the network.

Dense Access Acquisition Protocols

The aboriginal adjustment for multicast acquisition is based on the acceptance that the multicast group

members are densely broadcast throughout the arrangement and bandwidth is plentiful, meaning

that about all hosts on the arrangement accord to the group. These abutting access multicast routing

protocols await on alternate calamity of the arrangement with multicast cartage to set up and maintain

the administration tree.

Table 10-5 TTL Thresholds

Value Action

0 Belted to the aforementioned host; not achievement by any interface

1 Belted to the aforementioned subnet; not forwarded by a router

15 Belted to the aforementioned site, organization, or department

63 Belted to the aforementioned region

127 Worldwide

191 Worldwide; bound bandwidth

255 Unrestricted in scope; global

Multicast Acquisition Protocols 359

Dense access acquisition protocols accommodate the following:

• Distance Vector Multicast Acquisition Agreement (DVMRP)

• Multicast Open Beeline Aisle Aboriginal (MOSPF)

• Agreement Independent Multicast Abutting Access (PIMDM)

Dense access acquisition agreement operations accept that about all routers in the arrangement will need

to administer multicast cartage for anniversary multicast group. The abutting access protocols are most

appropriate in environments with densely amassed receivers and the accessible bandwidth to

tolerate flooding.

DVMRP

DVMRP is declared in RFC 1075. DVMRP is broadly acclimated on the Internet multicast backbone

(MBONE).

DVMRP uses a action alleged about-face aisle flooding. This action is agnate to the breach horizon

process on a Cisco router.

When a router receives a packet, it floods the packet out all paths except the one that leads back

to the packet source. This abode allows a abstracts beck to ability all LANs. If a router is

attached to a set of LANs that do not appetite to accept a accurate multicast group, the router can

send a clip bulletin aback up the administration timberline to stop consecutive packets from traveling

where there are no members.

DVMRP periodically floods packets in adjustment to ability any new hosts that appetite to accept a

particular group. A absolute accord exists amid the time it takes for a new receiver to get

the abstracts beck and the abundance of flooding.

DVMRP accouterments its own unicast acquisition agreement in adjustment to actuate which interface

leads aback to the antecedent of the abstracts stream. This unicast acquisition agreement is agnate to RIP and

is based absolutely on hop counts. As a result, the aisle that the multicast cartage follows may not be

the aforementioned as the aisle that the unicast cartage follows.

NOTE PIM is accurate on Cisco routers and these routers are able abundant to collaborate with

external DVMRP neighbors, but do not apparatus DVMRP natively.

MOSPF

MOSPF is a link-state multicast acquisition agreement declared in RFC 1584, Multicast Extensions

to OSPF. MOSPF is a agreement that should be acclimated in a distinct area or organization. MOSPF

is abased on the use of OSPF as the accompanying unicast acquisition protocol. In an OSPF/

MOSPF network, anniversary router maintains an abreast angel of the cartography of the entire

360 Affiliate 10: Multicasts

network. MOSPF, as an addendum to the OSPF protocol, includes multicast advice within

a accepted OSPF link-state advertisement (LSA). By application this, an MOSPF router finds out

about multicast groups that are alive and what router they are alive on.

The link-state advice is activated to body multicast administration trees. The administration tree

has anniversary (source, group) brace listed aural and again creates a timberline to accelerate to alive sources. As

with OSPF, any link-state change requires a re-computation of the tree.

NOTE MOSPF is not accurate on Cisco routers.

PIMDM

PIMDM is agnate to DVMRP. This agreement is best ill-fitted aback there are “dense” associates of

multicast groups. PIM uses calamity as a apparatus to ability all routers in the arrangement and then

prunes those routers that do not abutment associates of that accurate multicast group.

The altitude beneath which PIMDM is best advantageous are

• Senders and receivers are in abutting adjacency to one another.

• There are few senders and abounding receivers.

• The aggregate of multicast cartage is high.

• The beck of multicast cartage is constant.

Two Internet standards-track drafts call PIM, a multicast agreement that can be acclimated in

conjunction with all unicast IP acquisition protocols. These abstracts are draft-ietf-idmr-pimarch-

05.txt, Protocol-Independent Multicast (PIM): Motivation and Architectonics and Protocol-

Independent Multicast (PIM): Agreement Specification. PIM is additionally discussed in RFC 2362.

NOTE PIMDM is accurate on Cisco routers.

Sparse Access Acquisition Protocols

The added adjustment of multicast acquisition is based on a dispersed administration of multicast group

members. Because the multicast accumulation associates are amid sparsely throughout the network,

taking the antecedent access of calamity would be a decay of bandwidth. Therefore, employing

a added able adjustment to achieve multicast acquisition becomes necessary.

Sparse access multicast acquisition protocols use the acceptance that absolute requests are acclimated to

join a multicast distribution.

Multicast Acquisition Protocols 361

Sparse access acquisition protocols accommodate the following:

• Core-Based Copse (CBTs)

• Agreement Independent Multicast Dispersed Access (PIMSM)

Sparse access protocols are broadly acclimated in WAN environments, as against to the campus,

largely because of the angle that alone a few routers are involved.

CBT

CBT was originally declared in RFC 2201. Since then, the CBT agreement has been adapted in

RFC 2189. Unfortunately, CBTv2 isn’t astern compatible. In general, neither CBTv1 nor

CBTv2 has been broadly implemented. The CBT agreement constructs a distinct timberline that is shared

by all associates of the group. Multicast cartage for the absolute accumulation is beatific and accustomed over the

same tree, behindhand of the source. The use of a aggregate timberline can lighten the bulk on individual

routers about to the bulk of multicast acquisition advice stored.

CBT has a amount router that is acclimated to assemble the tree. Aback routers are accessible to accompany the tree,

they accelerate a accompany bulletin to the amount router. Aback the amount router sends a reply, it campaign the

reverse path, thereby basic a annex of the tree. Because the CBT accompany appeal has a TTL of

1 set, CBT routers in the arrangement advanced the bulletin hop by hop until the amount is accomplished or

until a CBT router that is already on the aggregate timberline is reached.

PIMSM

PIMSM is acclimated in those environments area the cardinal of receivers are “sparse,” appropriately the

name. PIMSM can additionally be acclimated aback multicast cartage is sporadic.

Because of this, a altered agency of free the cachet of multicast associates is used.

Because the cardinal is about small, or “sparse,” it makes added faculty to convention a proxy of

sorts, frequently alleged the affair point (RP). So instead of boundless flooding, the host

receiver (or sender) charge annals with the RP. In short, to achieve any multicast cartage breeze under

PIMSM, the host charge annals with the RP.

PIM is a adjustable agreement in that some multicast groups can be abutting access and can coexist

together with added groups that ability be dispersed mode.

362 Affiliate 10: Multicasts

Foundation Summary

The Foundation Arbitrary is a accumulating of tables and abstracts that provides a acceptable review

of abounding key concepts in this chapter. For those of you already adequate with the capacity in

this chapter, this arbitrary could advice you anamnesis a few details. For those of you who aloof read

this chapter, this analysis should advice coalesce some key facts. For any of you accomplishing your final

preparation afore the exam, these tables and abstracts will hopefully be a acceptable way to

review the day afore the exam.

Table 10-6 Multimedia Cartage Types

Traffic Blazon Characteristic

Unicast One-to-one, agitated out assorted times.

Broadcast One-to-many, you get it whether you appetite it or not.

Multicast One-to-subscribed group, best efficient.

Table 10-7 Well-Known Class D Addresses

Well-Known Class D Abode Purpose

224.0.0.1 All hosts on a subnet

224.0.0.2 All routers on a subnet

224.0.0.4 All Distance Vector Multicast Acquisition Protocol

(DVMRP) routers

224.0.0.5 All Open Beeline Aisle Aboriginal (OSPF) routers

224.0.0.6 All OSPF appointed routers

224.0.0.9 All Acquisition Advice Protocol, adaptation 2 (RIP-

2) routers

224.0.0.13 All Agreement Independent Multicast (PIM) routers

Table 10-8 Multicast Routing: Types of Administration Trees

Type of Administration Timberline Characteristic

Source-specific Best efficient—most absolute aisle from the sender to anniversary receiver

Shared Distinct multicast tree—one aisle for all

Foundation Arbitrary 363

Table 10-9 Multicast Acquisition Protocols

Protocol Characteristic

DVMRP About-face Aisle Flooding.

MOSPF Multicast OSPF. Link-state acquisition protocol.

PIM Agreement Independent Multicast. Two types: Dispersed Access and Abutting Mode.

CBT Amount router acclimated to assemble a tree.

364 Affiliate 10: Multicasts

Q&A

The questions and scenarios in this book are added difficult than what you should acquaintance on

the absolute exam. The questions do not attack to awning added across or abyss than the exam;

they are designed, however, to achieve abiding that you apperceive the answer. Rather than acceptance you

to acquire the acknowledgment from clues hidden central the catechism itself, the questions claiming your

understanding and anamnesis of the subject. Questions from the “Do I Apperceive This Already?” quiz

from the alpha of the affiliate are again actuality to ensure that you accept baffled the

chapter’s affair areas. Hopefully, these questions will advice absolute the cardinal of assay questions

on which you attenuated your choices to two options and again guess.

The answers to the questions can be begin in Appendix A, on folio 477.

1 Name the three types of cartage accessible in today’s multimedia environment.

______________________________________________________________________

2 What Band 4 agreement is acclimated to backpack multicast traffic?

______________________________________________________________________

3 What Class of IP abode is acclimated in a multicast environment?

______________________________________________________________________

4 Call the architecture of the Class D multicast abode by octet or bits.

______________________________________________________________________

5 What is the name of the agreement acclimated to abode their multicast accumulation associates with

neighboring multicast routers?

______________________________________________________________________

Q&A 365

6 What is the appropriate name assigned to the one multicast router that performs host

membership queries to actuate which groups accept members?

_______________________________________________________________________

7 What does a host accelerate to the all-router accumulation abode of 224.1.1.1 to accompany a group?

_______________________________________________________________________

8 Which blazon of acquisition involves transmitting packets from one antecedent to one source?

_______________________________________________________________________

9 Define a administration tree.

_______________________________________________________________________

10 Name the two types of administration trees.

_______________________________________________________________________

11 Name the three types of abutting access acquisition protocols.

_______________________________________________________________________

12 Name the two types of dispersed access acquisition protocols.

_______________________________________________________________________

366 Affiliate 10: Multicasts

13 Which multicast acquisition agreement is broadly acclimated on the MBONE?

______________________________________________________________________

14 Name three characteristics of IP multicasting.

______________________________________________________________________

15 Certain acceptable acquisition protocols use multicasts to backpack acquisition information. Name

one acquisition agreement and the multicast abode it uses.

______________________________________________________________________

16 What is the name of the Cisco-specific agreement that is acclimated with routers and switches to

configure the multicast forwarding table to represent accumulation membership?

______________________________________________________________________

17 What is the algorithm acclimated in a source-specific administration tree?

______________________________________________________________________

18 What is acclimated to administer the ambit of multicast delivery?

______________________________________________________________________

19 What two characteristics call aback PIMSM is best useful?

______________________________________________________________________

Q&A 367

20 MOSPF is best ill-fitted to which blazon of environment?

_______________________________________________________________________

This affiliate covers the afterward capacity that you will charge to adept for the CCNP

Switching Exam:

• Planning for Multicast Casework in a Network—This area outlines the planning

process bare above-mentioned to enabling and configuring multicast casework in a network.

• Configuring IP Multicast—This area discusses agreement capacity bare to

establish multicast services.

• Configuring Internet Accumulation Management Agreement (IGMP)—This section

discusses agreement of IGMP.

• Configuring Cisco Accumulation Management Agreement (CGMP)—The area discusses

the agreement of CGMP.