All Cisco-Network Study Notes: Joining the Aggregate Tree

Joining the Aggregate Tree

So far, this area on PIM-SM has explained the ancestry of the allotment process, by which

a router abreast the antecedent of multicast packets registers with the RP. Afore commutual that

discussion, however, the abstraction of the aggregate timberline for a multicast group, additionally alleged the root-path

tree (RPT), charge be explained. As mentioned earlier, PIM-SM initially causes multicasts to be

delivered in a two-step process: first, packets are beatific from the antecedent to the RP, and afresh the RP

forwards the packets to the subnets that accept hosts that charge a archetype of those multicasts. PIM-SM

uses this aggregate timberline in the additional allotment of the process.

The RPT is a tree, with the RP at the root, that defines over which links multicasts should be

forwarded to ability all appropriate routers. One such timberline exists for anniversary multicast accumulation that is

currently alive in the internetwork. So, already the multicast packets beatific by anniversary antecedent are

forwarded to the RP, the RP uses the RPT for that multicast accumulation to actuate area to forward

these packets.

PIM-SM routers collectively actualize the RPT by sending PIM Accompany letters against the RP. In

PIM-SM, multicast cartage is beatific alone to routers that accurately appeal it. PIM-SM routers

request the cartage by abutting the RPT by sending a Accompany against the RP.

PIM-SM routers accept to accelerate a Accompany beneath two conditions:

■ Aback a PIM-SM router receives a PIM Accompany bulletin on any interface added than the interface

used to avenue packets against the RP

■ Aback a PIM-SM router receives an IGMP Membership Report bulletin from a host on a

directly affiliated subnet

Figure 17-14 shows an archetype of the PIM-SM accompany process, application the aforementioned network

as Figure 17-12 but with H1 abutting accumulation 228.8.8.8. The routers acknowledge to the IGMP Accompany by

sending a Accompany against the RP, to become allotment of the aggregate SPT (*,228.8.8.8).

Sparse-Mode Acquisition Protocols 613

Figure 17-14 Creating a Aggregate Timberline for (*,228.8.8.8)

Figure 17-14 shows how H1 causes a aggregate timberline (*,228.8.8.8) to be created, as declared in the

following steps:

1. H1 sends an IGMP Accompany bulletin for the accumulation 228.8.8.8.

2. R4 realizes it now needs to ask the RP to accelerate it packets beatific to 228.8.8.8, so R4 sends a PIM

Join for the aggregate timberline for accumulation 228.8.8.8 against the RP. R4 additionally puts its e0 interface into a

forwarding accompaniment for the RPT for accumulation 228.8.8.8.

3. R4 sends the Accompany to the RP.

4. R5 receives the Accompany on its s1 interface, so R5 puts its s1 interface in a forwarding accompaniment for the

shared timberline (represented by (*,228.8.8.8)). R5 additionally knows it needs to advanced the Accompany toward

the RP.

5. R5 sends the Accompany against the RP.

6. R3, the RP, puts its s0 interface in a forwarding accompaniment for the (*,288.8.8.8) aggregate tree.

R4 R5

R2 R3

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IGMP Join

228.8.8.8

10.1.1.0/24

10.1.2.0/24 10.1.3.0/24

10.1.10.3

PIM-SM Join

(*, 228.8.8.8)

PIM-SM Join

(*, 228.8.8.8)

• Accomplish e0 forward

for (*, 228.8.8.8)

• Accelerate Accompany out RPF

int. to ability RP

4 • Accomplish s1 forward

for (*, 228.8.8.8)

• Accelerate Accompany out RPF

int. to ability RP

6 • Accomplish s0 forward

for (*, 228.8.8.8)

614 Affiliate 17: IP Multicast Routing

By the end of this process, the RP knows that at atomic one host wants packets beatific to 228.8.8.8. The

RPT for accumulation 228.8.8.8 is formed with R3’s s0 interface, R5’s s1 interface, and R4’s e0 interface.

Completion of the Antecedent Allotment Process

So far in this description of PIM-SM, a antecedent (10.1.1.10) beatific packets to 228.8.8.8, as apparent in

Figure 17-13—but no one cared at the time, so the RP did not advanced the packets. Next, you

learned what happens aback a host does appetite to accept packets, with the routers reacting to create

the RPT for that group. This area completes the adventure by assuming how an RP reacts to a PIM

When the RP receives a Register bulletin for an alive multicast group—in added words, the RP

believes that it should advanced packets beatific to the group—the RP does not accelerate a Register-Stop

message, as was apparent aback in Figure 17-13. Instead, it reacts to the Register bulletin by deencapsulating

the multicast packet, and forwarding it.

The behavior of the RP in acknowledgment to the Register bulletin credibility out the additional above function

of the Register message. Its capital two functions are as follows:

■ To acquiesce a router to acquaint the RP that it has a bounded antecedent for a accurate multicast group

■ To acquiesce a router to advanced multicasts to the RP, encapsulated central a unicast packet, until

the allotment activity is completed

To appearance the complete process, Figure 17-15 shows an example. In the example, host H1 has

already abutting accumulation 228.8.8.8, as apparent in Figure 17-14. The afterward accomplish bout those

identified in Figure 17-15. Note that Footfall 3 represents the forwarding of the multicasts that were

encapsulated central Register letters at Footfall 2.

1. Host S1 sends multicasts to 228.8.8.8.

2. Router R1 encapsulates the multicasts, sending them central Register letters to the RP, R3.

3. R3, alive that it needs to advanced the multicast packets, de-encapsulates the packets and

sends them against H1. (This activity allows R1 and R3 to administer the multicasts while the

registration activity completes.) R5 assiduously the accumulation cartage to R4 and R4 assiduously it on

its LAN.

NOTE The characters (*,G) represents a distinct RPT. The * represents a wildcard, acceptation “any

source,” because the PIM-SM routers use this aggregate timberline behindhand of the antecedent of the packets.

For example, a packet beatific from any antecedent IP address, accession at the RP, and destined to group

228.8.8.8, would account the RP to use its (*,228.8.8.8) multicast acquisition table entries, because

these entries are allotment of the RPT for accumulation 228.8.8.8.

Sparse-Mode Acquisition Protocols 615

4. R3 joins the SPT for antecedent 10.1.1.10, accumulation 228.8.8.8, by sending a PIM-SM Accompany message

for accumulation (10.1.1.10,228.8.8.8) against the antecedent 10.1.1.10.

5. Aback R1 and R2 accept the PIM-SM Accompany bulletin from R2 requesting the accumulation cartage from

the source, they alpha forwarding accumulation cartage against the RP. At this point, R3 (the RP) now

receives this cartage on the SPT from the source. However, R1 is additionally still sending the Register

messages with encapsulated multicast packets to R3.

6. R3 sends unicast Register-Stop letters to R1. Aback R1 receives the Register-Stop

messages from R3, it stops sending the encapsulated unicast Register letters to R3.

Figure 17-15 Antecedent Allotment aback the RP Needs to Accept Packets Beatific to that Group

The activity may assume like a lot of trouble, but at the end of the process, multicasts are delivered

to the actual locations. The activity uses the able SPT from the antecedent to the RP, and the

shared timberline (*,228.8.8.8) from the RP to the subnets that charge to accept the traffic.

Note that the PIM protocols could accept aloof let a router abreast the source, such as R1 in this example,

continue to abbreviate multicasts central the unicast Register messages. However, it is inefficient

R4 R5

R2 R3

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PIM-SM Join

(10.1.1.10, 228.8.8.8)

Multicast Packets (10.1.1.10, 228.8.8.8)

Unicast Register Messages

Unicast Register-Stop

4 10.1.10.3

2 Unicast Register

Message

Multicast Packet

(10.1.1.10, 228.8.8.8)

Multicast Cartage - RPT

(*, 228.8.8.8)

Multicast Traffic

(10.1.1.10, 228.8.8.8)

616 Affiliate 17: IP Multicast Routing

to accomplish R1 abbreviate every multicast packet, accomplish R3 de-encapsulate every packet, and then

make R3 advanced the traffic. So, PIM-SM has the RP, R3 in this case, accompany the group-specific tree

for that (S,G) combination.

Shared Administration Tree

In Figure 17-15, the accumulation cartage that flows over the aisle from the RP (R3) to R5 to R4 is called

a aggregate administration tree. It is additionally alleged a root-path timberline (RPT) because it is abiding at the RP. If

the arrangement has assorted sources for the aforementioned group, cartage from all the sources would aboriginal travel

to the RP (as apparent with the cartage from host S1 in Figure 17-14), and afresh biking bottomward this shared

RPT to all the receivers. Because all sources in the multicast accumulation use a accepted aggregate tree, a

wildcard characters of (*,G) is acclimated to analyze an RPT, area * represents all sources and G

represents the multicast accumulation address. The RPT for the accumulation 228.8.8.8 apparent in Figure 17-14

would be accounting as (*,228.8.8.8).

Example 17-7 shows the multicast avenue table access for R4 in Figure 17-15. On a Cisco router, the

show ip mroute command displays the multicast avenue table entries.

The estimation of the advice apparent in Archetype 17-7 is as follows:

■ The aboriginal band shows that the (*,G) access for the accumulation 228.8.8.8 was created 8 abnormal ago,

and if R4 does not advanced accumulation packets application this access in 2 account and 58 seconds, it will

expire. Every time R4 assiduously a packet, the timer is displace to 3 minutes. This access was created

because R4 accustomed an IGMP Accompany bulletin from H1.

■ The RP for this accumulation is 10.1.10.3 (R3). The S banderole indicates that this accumulation is application the

sparse-mode (PIM-SM) acquisition protocol. The C banderole indicates that R4 has a directly

connected accumulation affiliate for 228.8.8.8.

■ The admission interface for this (*,228.8.8.8) access is s0 and the RPF acquaintance is 10.1.6.5.

Note that for the SPT, the RPF interface is alleged based on the avenue to ability the RP, not the

route acclimated to ability a accurate source.

■ Accumulation cartage is forwarded out on the Ethernet0 interface. In this example, Ethernet0 was

added to the approachable interface account because an IGMP Report bulletin was accustomed on this

interface from H1. This interface has been in the forwarding accompaniment for 8 seconds. The Prune

timer indicates that if an IGMP Accompany is not accustomed afresh on this interface aural the next

2 account and 52 seconds, it will be removed from the approachable interface list.

Example 20-7 Multicast Avenue Table Access for the Accumulation 228.8.8.8 for R4

(*, 228.8.8.8), 00:00:08/00:02:58, RP 10.1.10.3, flags: SC

Incoming interface: Serial0, RPF nbr 10.1.6.5

Outgoing interface list:

Ethernet0, Forward/Sparse, 00:00:08/00:02:52

Sparse-Mode Acquisition Protocols 617

Steady-State Operation by Continuing to Accelerate Joins

To advance the forwarding accompaniment of interfaces, PIM-SM routers charge accelerate PIM Accompany messages

periodically. If a router fails to accelerate Joins periodically, PIM-SM moves interfaces aback to a pruned

state.

PIM-SM routers accept to advance the forwarding accompaniment on links based on two accepted criteria:

■ A afterwards router continues to accelerate PIM joins for the group.

■ A locally affiliated host still responds to IGMP Concern letters with IGMP Report

messages for the group.

Figure 17-16 shows an archetype in which R5 maintains the forwarding accompaniment of its articulation to R3 based

on both of these reasons. H2 has additionally abutting the aggregate timberline for 228.8.8.8. H1 had abutting earlier,

as apparent in Figures 17-14 and 17-15.

Figure 17-16 Host H2 Sends an IGMP Accompany Message

Example 17-8 shows the multicast avenue table access for R5 in Figure 17-16, with these two

interfaces in a forwarding state.

R4 R5

R2 R3

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Periodic IGMP Reports

228.8.8.8

10.1.1.0/24

10.1.2.0/24 10.1.3.0/24

10.1.10.3

Periodic PIM Joins

(*, 228.8.8.8)

Periodic PIM Joins

(*, 228.8.8.8)

10.1.8.0/24

Periodic IGMP Reports

228.8.8.8

Multicast Traffic

(10.1.1.10, 228.8.8.8)

618 Affiliate 17: IP Multicast Routing

In Archetype 17-8, two interfaces are listed in the approachable interface list. The s1 interface is listed

because R5 has accustomed a PIM-SM Accompany bulletin from R4. In PIM-SM, the afterwards routers

need to accumulate sending PIM-SM Accompany letters every 60 abnormal to the upstream router. Aback R5

receives addition PIM-SM Accompany from R4 on its s1 interface, it resets the Clip timer to the default

value of 3 minutes. If R5 does not accept a PIM-SM Accompany from R4 afore R5’s Clip timer on that

interface expires, R5 places its s1 interface in a pruned accompaniment and stops forwarding the cartage on the

interface.

By contrast, R5’s e0 interface is listed as forwarding in R5’s approachable interface account because R5 has

received an IGMP Accompany bulletin from H2. Recall from Affiliate 16 that a multicast router sends an

IGMP accepted concern every 60 or 125 abnormal (depending on the IGMP version) on its LAN

interfaces. It charge accept at atomic one IGMP Report/Join bulletin as a acknowledgment for a group;

otherwise, it stops forwarding the accumulation cartage on the interface. Aback R5 receives addition IGMP

Report bulletin on its e0 interface, it resets the Clip timer for the access to the absence amount of

3 minutes.

Note additionally that on R5, the cancellation of the PIM Accompany from R4, or the IGMP Report on e0, triggers

R5’s charge to accelerate the PIM Accompany against the RP.

Examining the RP’s Multicast Acquisition Table

In the accepted accompaniment of the advancing example, as aftermost apparent in Figure 17-16, the RP (R3) has joined

the SPT for antecedent 10.1.1.10, accumulation 228.8.8.8. The RP additionally is the basis of the aggregate timberline for group

228.8.8.8. Archetype 17-9 shows both entries in R3’s multicast avenue table.

Example 17-8 Multicast Avenue Table Access for the Accumulation 228.8.8.8 for R5

(*,228.8.8.8), 00:00:05/00:02:59, RP 10.1.10.3, flags: SC

Incoming interface: Serial0, RPF nbr 10.1.5.3

Outgoing interface list:

Serial1, Forward/Sparse, 00:01:15/00:02:20

Ethernet0, Forward/Sparse, 00:00:05/00:02:55

Example 17-9 Multicast Avenue Table Access for the Accumulation 228.8.8.8 for R3

(*,228.8.8.8), 00:02:27/00:02:59, RP 10.1.10.3, flags: S

Incoming interface: Null, RPF nbr 0.0.0.0

Outgoing interface list:

Serial0, Forward/Sparse, 00:02:27/00:02:33

(10.1.1.10/32, 228.8.8.8), 00:02:27/00:02:33, flags: T

Incoming interface: Serial1, RPF nbr 10.1.3.2,

Outgoing interface list:

Outgoing interface list: Null

Sparse-Mode Acquisition Protocols 619

The aboriginal access shows the aggregate tree, as adumbrated by the S flag. Notice the admission interface is

Null because R3, as RP, is the basis of the tree. Also, the RPF acquaintance is listed as 0.0.0.0 for the

same reason. In added words, it shows that the shared-tree cartage for the accumulation 228.8.8.8 has

originated at this router and it does not depend on any added router for the shared-tree traffic.

The additional access shows the SPT access on R3 for multicast accumulation 228.8.8.8, antecedent 10.1.1.10. The

T banderole indicates that this access is for an SPT, and the antecedent is listed at the alpha of that same

line (10.1.1.10). The admission interface is s1 and the RPF acquaintance for the antecedent address

10.1.1.10 is 10.1.3.2.

As you can see, an RP uses the SPT to cull the cartage from the antecedent to itself and uses the shared

tree to advance the cartage bottomward to the PIM-SM routers that accept requested it.

Shortest-Path Timberline Switchover

PIM-SM routers could abide forwarding packets via the PIM-SM two-step process, whereby

sources accelerate packets to the RP, and the RP sends them to all added routers application the RPT. However,

one of the best alluring aspects of PIM-SM operations is that anniversary PIM-SM router can build

the SPT amid itself and the antecedent of a multicast accumulation and booty advantage of the best efficient

path accessible from the antecedent to the router. In Figure 17-16, R4 is accepting the accumulation cartage from

the antecedent via the aisle R1-R2-R3-R5-R4. However, it is accessible that it would be added efficient

for R4 to accept the accumulation cartage anon from R1 on R4’s s1 interface.

In the area “Completion of the Antecedent Allotment Process,” beforehand in this chapter, you saw

that the PIM-SM architecture allows an RP to body an SPT amid itself and the router that is directly

connected with the antecedent (also alleged the antecedent DR) to cull the accumulation traffic. Similarly, the PIMSM

design additionally allows any added PIM-SM router to body an SPT amid the router and the source

DR. This affection allows a PIM-SM router to abstain application the inefficient path, such as the one used

by R4 in Figure 17-16. Also, already the router starts accepting the accumulation cartage over the SPT, it can

send a Clip bulletin to the upstream router of the aggregate timberline to stop forwarding the cartage for

the group.

The catechism is, aback should a router about-face over from RPT to SPT? RFC 2362 for PIM-SM

specifies that, “The recommended action is to admit the about-face to the SP-tree afterwards accepting a

significant cardinal of abstracts packets during a defined time breach from a accurate source.” What

number should be advised as a cogent number? The RFC does not specify that. Cisco

routers, by default, about-face over from the RPT to the source-specific SPT afterwards they accept the first

packet from the aggregate tree.

NOTE You can change this behavior by configuring the all-around command ip pim sptthreshold

rate on any router for any group. Already the cartage amount exceeds the declared amount (in kbps),

the router joins the SPT. The command impacts the behavior alone on the router(s) on which it

is configured.

620 Affiliate 17: IP Multicast Routing

If a router is activity to about-face to SPT, why accompany the RPT first? In PIM-SM, a router does not know

the IP abode of a antecedent until it receives at atomic one packet for the accumulation from the source. After

it receives one packet on the RPT, it can apprentice the IP abode of a source, and initialize a switchover

to the SPT for that (source,group) combination.

With the absence Cisco PIM-SM operation, aback multicast packets activate accession on R4’s s0

interface via the aggregate tree, R4 attempts to about-face to the SPT for antecedent 10.1.1.10. Figure 17-17

shows the accepted steps.

Figure 17-17 R4 Initializing Switchover from RPT to SPT by Sending a PIM-SM Accompany to R1

The aboriginal three accomplish Figure 17-17 are as follows:

1. The antecedent (S1,10.1.1.10) sends a multicast packet to the first-hop router R1.

2. R1 assiduously the packet to the RP (R3).

3. The RP assiduously the packet to R4 via the aggregate tree.

R4 R5

R2 R3

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(10.1.1.10, 228.8.8.8)

Multicast Cartage - RPT

(*, 228.8.8.8)

Multicast Cartage - SPT

(10.1.1.10, 228.8.8.8)

5 Multicast Cartage -

(10.1.1.10, 228.8.8.8) PIM-SM Join

(10.1.1.10, 228.8.8.8)

Sparse-Mode Acquisition Protocols 621

At Footfall 3, R4 abstruse that the antecedent abode of the multicast accumulation 228.8.8.8 is 10.1.1.10. So,

besides forwarding the packet at Footfall 3, R4 can use that advice to accompany the SPT for group

228.8.8.8, from antecedent 10.1.1.10, application the afterward accomplish from Figure 17-17.

4. R4 consults its unicast acquisition table, finds the next-hop abode and approachable interface it

would use to ability antecedent 10.1.1.10, and sends the PIM-SM Accompany bulletin out that interface

(s1) to R1. This PIM-SM Accompany bulletin is accurately for the SPT of (10.1.1.10,228.8.8.8).

The Accompany campaign hop by hop until it alcove the antecedent DR.

5. As a aftereffect of the Join, R1 places its s1 interface in a forwarding accompaniment for SPT

(10.1.1.10,228.8.8.8). So, R1 starts forwarding multicasts from 10.1.1.10 to 228.8.8.8 out its

s1 interface as well.

R4 now has a multicast acquisition table access for the SPT, as apparent in Archetype 17-10.

In Archetype 17-10, you see two entries for the group. The J banderole (for join) on both the entries

indicates that the cartage was switched from RPT to SPT, and now the (S,G) access will be acclimated for

forwarding multicast packets for the group. Notice that the admission interfaces for the (*,G) entry

and (S,G) access are different.

Pruning from the Aggregate Tree

When a PIM-SM router has abutting a added able SPT, it may not charge to accept multicast

packets over the RPT any more. For example, aback R4 in Figure 17-17 notices that it is receiving

the accumulation cartage over RPT and SPT, it can and should ask the RP to stop sending the traffic.

To stop the RP from forwarding cartage to a afterwards router on the aggregate tree, the downstream

router sends a PIM-SM Clip bulletin to the RP. The Clip bulletin references the (S,G) SPT,

which identifies the IP abode of the source. Essentially, this clip agency the afterward to the RP:

Stop forwarding packets from the listed antecedent IP address, to the listed accumulation address,

down the RPT.

For example, in Figure 17-18, which continues the archetype apparent in Figure 17-17, R4 sends a

Prune out its s0 interface against R5. The Clip lists (S,G) access (10.1.1.10,228.8.8.8), and it sets

a bit alleged the RP-tree bit (RPT-bit). By ambience the RPT-bit in the Clip message, R4 informs

Example 17-10 Multicast Avenue Table Access for the Accumulation 228.8.8.8 for R4

(*,228.8.8.8), 00:02:36/00:02:57, RP 10.1.10.3, flags: SCJ

Incoming interface: Serial0, RPF nbr 10.1.6.5

Outgoing interface list:

Ethernet0, Forward/Sparse, 00:02:36/00:02:13

(10.1.1.10/32, 228.8.8.8), 00:00:23/00:02:33, flags: CJT

Incoming interface: Serial1, RPF nbr 10.1.4.1,

Outgoing interface list:

Ethernet0, Forward/Sparse, 00:00:23/00:02:37

622 Affiliate 17: IP Multicast Routing

R5 (the upstream router) that it has switched to SPT and the Clip bulletin is for the redundant

traffic for the accumulation 228.8.8.8, from 10.1.1.10, that R4 is accepting on the aggregate tree.

Figure 17-18 R4 Sends PIM-SM Clip with RP Bit Set to R5

To stop the packets from actuality beatific over the RPT to R4, R5 charge clip its interface s1 in the RPT

(*, 228.8.8.8). R5 may go on to accompany the SPT for (10.1.1.10,228.8.8.8.8) as well.

This concludes the advantage of the operations of PIM-SM. The abutting area covers some details

about how routers can apprentice the IP abode of the PIM RP.

Dynamically Finding RPs and Application Bombastic RPs

In a PIM-SM network, every router charge somehow apprentice the IP abode of an RP. A PIM-SM

router can use one of the afterward three methods to apprentice the IP abode of an RP:

■ The RP abode can be statically configured on all the PIM-SM routers with the Cisco IOS

global command ip pim rp-address address. This is the adjustment acclimated for the five-router

topology apparent in Figure 17-19.

■ The Cisco-proprietary Auto-RP agreement can be acclimated to baptize the RP and acquaint its IP

address so that all PIM-SM routers can apprentice its IP abode automatically.

■ A accepted BootStrap Router (BSR) agreement can be acclimated to baptize the RP and advertise

its IP abode so that all the PIM-SM routers can apprentice its IP abode automatically.

R4 R5

R2 R3

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RPT-bit Prune

(10.1.1.10, 228.8.8.8)

10.1.8.0/24

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Multicast Traffic

(10.1.1.10, 228.8.8.8)

Sparse-Mode Acquisition Protocols 623

Additionally, because PIM-SM relies so heavily on the RP, it makes faculty to accept bombastic RPs.

Cisco IOS offers two methods of accouterment bombastic RPs, which are additionally covered in this section:

■ Anycast RP application the Multicast Antecedent Discovery Agreement (MSDP)

■ BootStrap Router (BSR)

Dynamically Finding the RP Application Auto-RP

Static RP agreement is suboptimal beneath the afterward conditions:

■ Aback an action has a ample cardinal of PIM-SM routers and the action wants to use

many altered RPs for altered groups, it becomes time arresting and bulky to

statically configure the IP addresses of abounding RPs for altered groups on all the routers.

■ Aback an RP fails or needs to be afflicted because a new RP is actuality installed, it becomes

extremely difficult in a statically configured PIM-SM area to about-face over to an alternative

RP after ample downtime.

Auto-RP provides an addition in which routers dynamically apprentice the unicast IP abode acclimated by

each RP. Auto-RP uses a two-step process, which is apparent in Figure 17-19 and Figure 17-20. In

the aboriginal step, the RP sends RP-Announce letters to the aloof multicast abode 224.0.1.39,

stating that the router is an RP. The RP-Announce bulletin additionally allows the router to acquaint the

multicast groups for which it is the RP, thereby acceptance some load-balancing of the RP workload

among altered routers. The RP continues to accelerate these RP-Announce letters every minute.

Figure 17-19 R3 Sends RP-Announce Messages

R4 R5

R2 R3

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E0 .4

10.1.1.0/24

10.1.2.0/24 10.1.3.0/24

10.1.10.3

RP-Announce

(10.1.10.3, 224.0.1.39)

“Use my address

10.1.10.3 as an RP

address for all groups.”

10.1.8.0/24

E0 .5

Multicast Traffic

(10.1.1.10, 228.8.8.8)

624 Affiliate 17: IP Multicast Routing

For example, Figure 17-19 shows R3 as an RP that uses Auto-RP. R3 supports all multicast

groups in this case. The RP-Announce bulletin is apparent as Footfall 1, to articulation it with Footfall 2 in

Figure 17-20.

The additional footfall for Auto-RP requires that one router be configured as a mapping agent. The

mapping abettor is usually the aforementioned router that was alleged as an RP, but can be a altered PIMSM

router. The mapping abettor learns all the RPs and the multicast groups they anniversary support. Then,

the mapping abettor multicasts addition message, alleged RP-Discovery, that identifies the RP for

each ambit of multicast accumulation addresses. This bulletin goes to aloof multicast address

224.0.1.40. It is this RP-Discovery bulletin that absolutely informs the accepted router citizenry as

to which routers they should use as RPs.

For example, in Figure 17-20, R2 is configured as a mapping agent. To accept all RP-Announce

messages, R2 locally joins the acclaimed Cisco-RP-Announce multicast accumulation 224.0.1.39. In

other words, the mapping abettor has become a accumulation affiliate for 224.0.1.39 and is alert for

the accumulation traffic. Aback R2 receives the RP-Announce packets apparent in Figure 17-19, it examines

the packet, creates group-to-RP mappings, and maintains this advice in its cache, as shown

in Figure 17-20.

Figure 17-20 R2 Creates Group-to-RP Mappings and Sends Them in RP-Discovery Messages

R4 R5

R2 R3

.10

E0 .1

.1 .3

10.1.4.0/24 10.1.5.0/24

10.1.7.0/24

10.1.6.0/24

E0 .4

10.1.1.0/24

10.1.2.0/24 10.1.3.0/24

10.1.10.3

10.1.8.0/24

E0 .5

RP-Discovery 3

(10.1.2.2, 224.0.1.40)

Note these Group-To-RP

Mappings:

Group

224.0.0.0/4

RP Address

10.1.10.3

RP-Discovery

(10.1.3.2, 224.0.1.40)

Note these Group-To-RP

Mappings:

Group

224.0.0.0/4

RP Address

10.1.10.3

2 “I am a Mapping Agent.”

Group-To-RP Mappings:

Group

224.0.0.0/4

RP Address

10.1.10.3

Sparse-Mode Acquisition Protocols 625

At aboriginal glance, the charge for the mapping abettor may not be obvious. Why not aloof let the RPs

announce themselves to all the added routers? Well, if Auto-RP accurate alone one RP, or even

only one RP to abutment anniversary multicast group, the mapping abettor would be a decay of effort.

However, to abutment RP redundancy—in added words, to abutment assorted RPs that can act as RP

for the aforementioned multicast group—the Auto-RP mapping abettor decides which RP should be acclimated to

support anniversary accumulation at the moment. To do so, the mapping abettor selects the router with the highest

IP abode as an RP for the group. (Note that you can additionally configure assorted mapping agents, for

redundancy.)

As anon as Cisco routers are configured with PIM-SM and Auto-RP, they automatically accompany the

well-known Cisco-RP-Discovery multicast accumulation 224.0.1.40. That agency they are alert to the

group abode 224.0.1.40, and aback they accept a 224.0.1.40 packet, they apprentice group-to-RP

mapping advice and advance it in their cache. Aback a PIM-SM router receives an IGMP

Join bulletin for a accumulation or PIM-SM Accompany bulletin from a afterwards router, it checks the

group-to-RP mapping advice in its cache. Afresh it can advance as declared throughout

the PIM-SM explanations in this chapter, application that RP as the RP for that multicast group.

The afterward account summarizes the accomplish acclimated by Auto-RP:

1. Anniversary RP is configured to use Auto-RP and to advertise itself and its accurate multicast

groups via RP-Announce letters (224.0.1.39).

2. The Auto-RP mapping agent, which may or may not additionally be an RP router, gathers information

about all RPs by alert to the RP-Announce messages.

3. The mapping abettor builds a mapping table that lists the currently best RP for anniversary ambit of

multicast groups, with the mapping abettor acrimonious the RP with the accomplished IP abode if

multiple RPs abutment the aforementioned multicast groups.

4. The mapping abettor sends RP-Discover letters to 224.0.1.40 announcement the mappings.

5. All routers accept for packets beatific to 224.0.1.40 to apprentice the mapping advice and acquisition the

correct RP to use for anniversary multicast group.

Finally, one aftermost baby but important point deserves some absorption afore affective on to BSR.

Auto-RP creates a baby chicken-and-egg botheration in that the purpose of Auto-RP is to acquisition the

RPs, but to get the RP-Announce and RP-Discovery messages, PIM-SM routers would charge to

send a Accompany against the RP, which they do not apperceive yet. To affected this problem, Cisco added a

variation of PIM alleged sparse-dense mode. In PIM sparse-dense mode, a router uses PIM-DM

rules aback it does not apperceive the area of the RP, and PIM-SM rules aback it does apperceive the

location of the RP. So, beneath accustomed altitude with Auto-RP, the routers would use close mode

long abundant to apprentice the group-to-RP mappings from the mapping agent, and afresh about-face over to

sparse mode. Also, if any added multicast cartage occurred afore the routers abstruse of the RPs

using Auto-RP, the multicast packets would still be forwarded application dense-mode rules. (PIM

sparse-dense approach is configured per interface application the ip pim sparse-dense-mode interface

subcommand.)