Multicast Acquisition Basics 585
Foundation Topics
Multicast Acquisition Basics
The capital action of any acquisition agreement is to advice routers advanced a packet in the adapted direction,
causing the packet to accumulate affective afterpiece to its adapted destination, ultimately extensive its destination.
To advanced a unicast packet, a router examines the packet’s destination address, finds the next-hop
address from the unicast acquisition table, and assiduously the packet through the adapted interface.
A unicast packet is forwarded forth a distinct aisle from the antecedent to the destination.
The top allotment of Figure 17-1 shows how a router can calmly accomplish a accommodation about forwarding a
unicast packet by consulting its unicast acquisition table. However, back a router receives a multicast
packet, as apparent at the basal of Figure 17-1, it cannot advanced the packet because multicast IP
addresses are not listed in the unicast acquisition table. Also, routers about acquire to advanced multicast
packets out assorted interfaces to ability all receivers. These requirements accomplish the multicast
forwarding action added circuitous than unicast forwarding.
Figure 17-1 Multicast Acquisition Problem
Figure 17-1 shows that the router has accustomed a multicast packet with the destination address
226.1.1.1. The destination abode represents a dynamically alteration accumulation of recipients, not any
one recipient’s address. How can the router acquisition out area these users are? Area should the
router advanced this packet?
An affinity may advice you to acquire bigger the adversity of multicast routing. Acquire that you
want to accelerate affair invitations through the mail, but instead of creating dozens of invitations, you
.1
Fa0/0
?
.1
S0/1
Destination Address: 10.1.6.3
Source Address: 10.1.1.10
Destination Address: 226.1.1.1
Source Address: 10.1.1.10
Destination Address: 10.1.6.3
Source Address: 10.1.1.10
“I apperceive how to handle this packet.”
Router# appearance ip route
Gateway of aftermost resort is not set
10.0.0.0/24 is subnetted, 3 subnets
C 10.1.1.0 is anon connected, FastEthernet0/0
R 10.1.6.0 [120/1] via 10.1.4.3, 00:00:10, Serial0/1
C 10.1.4.0 is anon connected, Serial0/1
“Who owns the multicast address
226.1.1.1? Area should I forward
this packet?”
Unicast Packet
Multicast Packet
Unicast Packet
586 Affiliate 17: IP Multicast Routing
create alone one. Afore commitment the invitation, you put a destination abode on it, “This envelope
contains my affair invitation,” and again bead it in a mailbox. Back the postal arrangement examines the
destination abode on your envelope, area should it bear your envelope? And because it is
only one invitation, does the postal arrangement charge to accomplish copies? Also, how can the postal system
figure out to which addresses to bear the copies? By contrast, if IP multicast were the column office,
it would apperceive who you appetite to allure to the party, apperceive area they are located, and accomplish copies
of the allurement and bear them all to the absolute addresses.
The abutting few sections altercate solutions for forwarding multicast cartage and authoritative the
distribution of multicast cartage in a baffled network.
Overview of Multicast Acquisition Protocols
Routers can advanced a multicast packet by appliance either a dense-mode multicast acquisition protocol
or a sparse-mode multicast acquisition protocol. This area examines the basal concepts of multicast
forwarding appliance close mode, the About-face Aisle Forwarding (RPF) check, and multicast forwarding
using dispersed mode, all of which advice to break the multicast acquisition problem.
Multicast Forwarding Appliance Close Mode
Dense-mode acquisition protocols acquire that the multicast accumulation appliance is so accepted that every
subnet in the arrangement has at atomic one receiver absent to acquire the accumulation traffic. Therefore, the
design of a dense-mode acquisition agreement instructs the router to advanced the multicast cartage on all
the configured interfaces, with some exceptions to anticipate looping. For example, a multicast
packet is never forwarded out the interface on which it was received. Figure 17-2 shows how a
dense-mode acquisition agreement receives a multicast on one interface, and again assiduously copies out
all added interfaces.
Figure 17-2 R1 Forwarding a Multicast Packet Appliance a Dense-Mode Acquisition Protocol
Fa0/0
Subnet 10.1.1.0/24
S0/0
S0/1
Destination Address: 226.1.1.1
Source Address: 10.1.1.10
“I will accelerate the packet
away from the source.”
R1
Copy of the multicast packet
Copy of the multicast packet
IP Acquisition table excerpt:
C 10.1.1.0 is anon connected, FastEthernet0/0
Multicast Packet
Multicast Acquisition Basics 587
Figure 17-2 shows the dense-mode argumentation on R1, with R1 calamity copies of the packet out all
interfaces except the one on which the packet was received. Although Figure 17-2 shows alone one
router, added routers can acquire these multicasts and echo the aforementioned process. All subnets will
receive a archetype of the aboriginal multicast packet.
Dense-mode protocols acquire that all subnets charge to acquire a archetype of the packets; however,
dense-mode protocols do acquiesce routers to ask to not acquire cartage beatific to a accurate multicast
group. Dense-mode routers about do not appetite to acquire multicast packets for a particular
group if both of the afterward are true:
■ The router does not acquire any alive after routers that charge packets for that group.
■ The router does not apperceive of any hosts on anon affiliated subnets that acquire abutting that
group.
When both of these altitude are true, the router needs to acquaint its upstream router not to send
traffic for the group, which it does by appliance a appropriate bulletin alleged a Prune message. The
mechanics of how dense-mode routers acquaint with anniversary added is discussed in detail under
the PIM-DM area after in this chapter.
DVMRP, PIM-DM, and MOSPF are the dense-mode acquisition protocols discussed in this chapter,
with best of the absorption actuality paid to PIM-DM.
Reverse Aisle Forwarding Check
Routers cannot artlessly use argumentation by which they acquire a multicast packet and again advanced a copy
of it out all added interfaces, after causing multicast packets to bend about the internetwork. To
prevent such loops, routers do not advanced multicasts out the aforementioned interface on which they were
received. Multicast routers use a About-face Aisle Forwarding (RPF) analysis to anticipate loops. The RPF
check adds this added footfall to a dense-mode router’s forwarding logic:
Look at the antecedent IP abode of the multicast packet. If my avenue that matches the source
lists an approachable interface that is the absolute interface on which the packet was received,
the packet passes the RPF check. If not, do not carbon and advanced the packet.
Figure 17-3 shows an archetype in which R3 uses the RPF analysis on two abstracted copies of the same
original multicast packet. Host S1 sends a multicast packet, with R1 calamity it to R2 and R3. R2
receives its copy, and floods it as well. As a result, R3 receives the aforementioned packet from two routers:
on its s0/0 interface from R2 and on its s0/1 interface from R1. After the RPF check, R3 would
forward the packet it got from R1 to R2, and carnality versa, and activate the action of looping packets.
With this aforementioned logic, R1 and R2 additionally accumulate repeating the process. This duplication creates multicast
routing loops and generates multicast storms that decay bandwidth and router resources.
588 Affiliate 17: IP Multicast Routing
Figure 17-3 R3 Performs the RPF Check
A multicast router does not advanced any multicast packet unless the packet passes the RPF check.
In Figure 17-3, R3 has to adjudge whether it should acquire the multicast packets advancing from R1
and R2. R3 makes this accommodation by assuming the RPF check, declared in detail as follows:
1. R3 examines the antecedent abode of anniversary admission multicast packet, which is 10.1.1.10. The
source abode is acclimated in the RPF analysis of Footfall 2.
2. R3 determines the about-face aisle interface based on its avenue acclimated to advanced packets to
10.1.1.10. In this case, R3’s avenue to 10.1.1.0/24 is matched, and it lists an approachable interface
of s0/1, authoritative s0/1 R3’s RPF interface for IP abode 10.1.1.10.
3. R3 compares the about-face aisle interface bent in Footfall 2 with the interface on which the
multicast packet arrived. If they match, it accepts the packet and assiduously it; otherwise, it
drops the packet. In this case, R3 floods the packet accustomed on s0/1 from R1, but it ignores
the packet accustomed on s0/0 from R2.
The RPF analysis accouterments a action by which routers acquire packets that access over the shortest
path, and abandon those that access over best routes. Multicast acquisition protocols cannot use the
destination abode to advice routers advanced a packet, because that abode represents the group
traffic. So, multicast acquisition protocols use the RPF analysis to actuate whether the packet arrived
at the router appliance the shortest-path avenue from the antecedent to the router. If it did, multicast routing
Fa0/0
Fa0/0
Fa0/0
10.1.1.0/24
10.1.2.0/24
10.1.5.0/24
10.1.6.0/24
10.1.4.0/24 10.1.3.0/24
S0/0 S0/0 .2
S0/1
S0/1
S0/0
S0/1
Destination Address: 226.1.1.1
Source Address: 10.1.1.10
“How would I go in about-face to the antecedent abode 10.1.1.10?”
R3# appearance ip avenue | accommodate 10.1.1.0
R 10.1.1.0 [120/1] via 10.1.4.1, 00:00:10, Serial0/1
R1 R2
R3
.1 .2
.2
.3
.3
.3
.1
.10
.1 Flood
Flood
Flood Flood
? ?
H1
H2
S1
RPF Check
Multicast Traffic
Multicast Acquisition Basics 589
protocols acquire the packet and advanced it; otherwise, they bead the packet and thereby avoid
routing loops and duplication.
Different multicast acquisition protocols actuate their RPF interfaces in altered ways, as
follows:
■ Distance Vector Multicast Acquisition Agreement (DVMRP) maintains a abstracted multicast routing
table and uses it for the RPF check.
■ Agreement Independent Multicast (PIM) and Core-Based Tree (CBT) about use the unicast
routing table for the RPF check, as apparent in Figure 17-3.
■ PIM and CBT can additionally use the DVMRP avenue table, the Multiprotocol Border Gateway
Protocol (MBGP) avenue table, or statically configured multicast route(s) for the RPF
check.
■ Multicast OSPF does not use the RPF check, because it computes both advanced and reverse
shortest-path source-rooted copse by appliance the Dijkstra algorithm.
Multicast Forwarding Appliance Dispersed Mode
A dense-mode acquisition agreement is advantageous back a multicast appliance is so accepted that you need
to bear the accumulation cartage to about all the subnets of a network. However, if the accumulation users are
located on a few subnets, a dense-mode acquisition agreement will still flood the cartage in the entire
internetwork, crumbling bandwidth and assets of routers. In those cases, a sparse-mode routing
protocol, such as PIM-SM, could be acclimated to advice abate decay of arrangement resources.
The axiological aberration amid dense-mode and sparse-mode acquisition protocols relates to
their absence behavior. By default, dense-mode protocols accumulate forwarding the accumulation cartage unless
a after router sends a bulletin advertence that it does not appetite that traffic. Sparse-mode
protocols do not advanced the accumulation cartage to any added router unless it receives a bulletin from that
router requesting copies of packets beatific to a accurate multicast group. A after router
requests to acquire the packets alone for one of two reasons:
■ The router has accustomed a appeal to acquire the packets from some after router.
■ A host on a anon affiliated host has beatific an IGMP Accompany bulletin for that group.
Figure 17-4 shows an archetype of what charge appear with PIM-SM afore a host (H2 in this case)
can acquire packets beatific by host S1 to multicast accumulation abode 226.1.1.1. The PIM sparse-mode
operation begins with the packet actuality forwarded to a appropriate router alleged the affair point
(RP). Once the accumulation cartage arrives at an RP, clashing the dense-mode design, the RP does not
automatically advanced the accumulation cartage to any router; the accumulation cartage charge be specifically
requested by a router.
590 Affiliate 17: IP Multicast Routing
Figure 17-4 R1 Forwarding a Multicast Packet Appliance a Sparse-Mode Acquisition Protocol
Before you attending at the numbered accomplish in Figure 17-4, accede the accompaniment of this internetwork. PIMSM
is configured on all the routers, R1 is called as an RP, and in all three routers, the IP address
172.16.1.1 of R1 is configured statically as the RP address. Usually, a loopback interface address
is acclimated as an RP abode and the loopback arrangement is advertised in the unicast acquisition agreement so
that all the routers apprentice how to locate an RP. At this point, R1, as the RP, may acquire multicast
packets beatific to 226.1.1.1, but it will not advanced them.
The afterward account describes the accomplish apparent in Figure 17-4:
1. Host S1 sends a multicast to the RP, with destination abode 226.1.1.1.
2. R1 chooses to avoid the packet, because no routers or bounded hosts acquire told the RP (R1) that
they appetite to acquire copies of multicast packets.
3. Host H2 sends an IGMP Accompany bulletin for accumulation 226.1.1.1.
4. R3 sends a PIM Accompany bulletin to the RP (R1) for abode 26.1.1.1.
5. R1’s argumentation now changes, so approaching packets beatific to 226.1.1.1 will be forwarded by R1 out s0/1 to R3.
6. Host S1 sends a multicast packet to 226.1.1.1, and R1 assiduously it out s0/1 to R3.
NOTE Throughout this chapter, the solid arrowed curve in the abstracts represent multicast
packets, with abject arrowed curve apery PIM and IGMP messages.
S0/0 S0/0
S0/1
S0/1
S0/0
S0/1
R1 R2
R3
PIM Join
226.1.1.1
RP
172.16.1.1
H1
H2
H3
S1
1 1st Multicast packet to 226.1.1.1
2
5
6 2nd Multicast packet to 226.1.1.1 4
3 IGMP Accompany 226.1.1.1
Sparse approach –
don’t advanced out
S0/0 and S0/1
I got a accompany in S0/0
for 226.1.1.1 –
Forward out S0/1
in the future!
Multicast Acquisition Basics 591
In a PIM-SM network, it is analytical for all the routers to somehow apprentice the IP abode of an RP.
One advantage in a baby arrangement is to statically configure the IP abode of an RP in every router.
Later in the chapter, the area “Dynamically Finding RPs and Appliance Redundant RPs” covers
how routers can dynamically ascertain the IP abode of the RP.
The archetype in Figure 17-4 shows some of the accumulation in appliance a sparse-mode agreement like PIMSM.
R2 has not accustomed any IGMP Accompany letters on its LAN interface, so it does not accelerate any
request to the RP to advanced the accumulation traffic. As a result, R1 does not decay articulation bandwidth on
the articulation from R1 to R2. R3 will not advanced multicasts to R2 either in this case.
Multicast Scoping
Multicast scoping borders the forwarding of multicast cartage to a accumulation of routers, for
administrative, security, or action reasons. In added words, multicast scoping is the convenance of
defining boundaries that actuate how far multicast cartage will biking in your network. The
following sections altercate two methods of multicast scoping:
■ TTL scoping
■ Authoritative scoping
TTL Scoping
With TTL scoping, routers analyze the TTL amount on a multicast packet with a configured TTL
value on anniversary approachable interface. A router assiduously the multicast packet alone on those interfaces
whose configured TTL amount is beneath than or according to the TTL amount of the multicast packet. In
effect, TTL scoping resets the TTL amount at which the router discards multicasts from the usual
value of 0 to some college number. Figure 17-5 shows an archetype of a multicast router with various
TTL beginning ethics configured on its interfaces.
In Figure 17-5, a multicast packet arrives on the s1 interface with a TTL of 18. The router
decreases the packet’s TTL by 1 to 17. Acquire that the router is configured with a dense-mode
routing agreement on all four interfaces and the RPF analysis succeeds—in added words, the router
will appetite to advanced a archetype of the packet on anniversary interface. The router compares the remaining
TTL of the packet, which is now 17, with the TTL beginning of anniversary approachable interface. If the
packet’s TTL is college than or according to the interface TTL, it assiduously a archetype of the packet on
that interface; otherwise, it does not advanced it. On a Cisco router, the absence TTL amount on all
the interfaces is 0.
NOTE In Figure 17-4, R3 aboriginal performs its RPF analysis by appliance the IP abode of the RP rather
than the IP abode of the antecedent of the packet, because it is accepting the accumulation cartage from the
RP. If the RPF analysis succeeds, R3 assiduously the cartage on its LAN.
Multicast Scoping Appliance TTL Thresholds
Destination Address: 226.2.2.2
TTL = 18
Multicast TTL Beginning = 0
Packet is forwarded
Multicast TTL Beginning = 32
Packet is not forwarded!
Multicast TTL Beginning = 8
Packet is forwarded
On the s0 and s2 interfaces in Figure 17-5, the arrangement ambassador has configured the TTL as 8
and 32, respectively. A archetype of the packet is forwarded on the s0 and e0 interfaces because their
TTL thresholds are beneath than 17. However, the packet is not forwarded on the s2 interface because
its TTL beginning is 32, which is college than 17.
TTL scoping has some weaknesses. First, it is difficult to apparatus in a ample and complex
network, because ciphering absolute TTL thresholds on abounding routers and abounding interfaces so that
the arrangement accurately borders alone the advised sessions becomes an acutely ambitious task.
Another botheration with TTL scoping is that a configured TTL beginning amount on an interface
applies to all multicast packets. If you appetite adaptability for some multicast sessions, you acquire to
manipulate the applications to adapt the TTL ethics back packets leave the servers.
Administrative Scoping
Recall from Affiliate 16 that administratively scoped multicast addresses are clandestine addresses in
the ambit 239.0.0.0 to 239.255.255.255. They can be acclimated to set authoritative boundaries to limit
the forwarding of multicast cartage alfresco of a domain. It requires chiral configuration. You can
configure and administer a clarify on a router’s interface so that multicast cartage with accumulation addresses in
the clandestine abode ambit is not accustomed to access or avenue the interface.
NOTE This affiliate assumes that you acquire apprehend Affiliate 16 or are thoroughly accustomed with
the operation of IGMP; if neither is true, apprehend Affiliate 16 afore continuing with this chapter.