LAN-Specific Issues with PIM-DM and PIM-SM
This area covers three baby capacity accompanying to operations that alone amount aback PIM is used
on LANs:
■ Clip Override
■ Assert messages
■ Appointed routers
Both PIM-DM and PIM-SM use these appearance in the aforementioned way.
Prune Override
In both PIM-DM and PIM-SM, the Clip action on multiaccess networks operates differently
from how it operates on point-to-point links. The acumen for this aberration is that aback one router
sends a Clip bulletin on a multiaccess network, added routers ability not appetite the articulation pruned by
the upstream router. Amount 17-11 shows an archetype of this problem, forth with the solution
through a PIM Join bulletin that is alleged a Clip Override. In this figure, R1 is forwarding the
group cartage for 239.9.9.9 on its fa0/0 interface, with R2 and R3 accepting the accumulation cartage on their
e0 interfaces. R2 does not accept any affiliated accumulation members, and its approachable interface list
would appearance null. The afterward account outlines the accomplish in argumentation apparent in Amount 17-11, in which R3
needs to accelerate a Clip Override:
1. R2 sends a Clip for accumulation 239.9.9.9 because R2 has a absent approachable interface account for the group.
2. R1, acumen that it accustomed the Clip on a multiaccess network, knows that added routers
might still appetite to get the messages. So, instead of anon pruning the interface, R1 sets
a 3-second timer that charge expire afore R1 will clip the interface.
Dense-Mode Acquisition Protocols 605
3. R3 additionally receives the Clip bulletin beatific by R2, because Clip letters are multicast to All-
PIM-Routers accumulation abode 224.0.0.13. R3 still needs to get cartage for 239.9.9.9, so R3 sends
a Join bulletin on its e0 interface.
4. (Not apparent in Amount 17-11) R1 receives the Join bulletin from R3 afore removing its LAN
interface from the approachable interface list. As a result, R1 does not clip its Fa0/0 interface.
Figure 17-11 Clip Override
This action is alleged Clip Override because R3 overrides the Clip beatific by R2. The Prune
Override is absolutely a Join message, beatific by R3 in this case. The bulletin itself is no altered from
a accustomed Join. As continued as R1 receives a Join bulletin from R3 afore its 3-second timer expires,
R3 continues to accept cartage after interruption.
Assert Message
The final PIM-DM bulletin covered in this affiliate is the PIM Assert message. The Assert
message is acclimated to anticipate ashen accomplishment aback added than one router attaches to the aforementioned LAN.
Rather than sending assorted copies of anniversary multicast packet assimilate the LAN, the PIM Assert
message allows the routers to negotiate. The champ gets the appropriate to be amenable for forwarding
multicasts assimilate the LAN.
Figure 17-12 shows an archetype of the charge for the Assert message. R2 and R3 both attach to the
same LAN, with H1 actuality an alive affiliate of the accumulation 227.7.7.7. Both R2 and R3 are receiving
the accumulation cartage for 227.7.7.7 from the antecedent 10.1.1.10.
R2 R3
E1
E0 E0
H1
I got a clip on a
multiaccess net – set
my 3 additional timer!
2
I accept a absent OIL for
239.9.9.9 – Prune!
I’d bigger send
another Join before
R1 prunes 239.9.9.9
Prune
239.9.9.9
1
3
Join, 239.9.9.9
(Prune Override)
Member,
239.9.9.9
Fa0/0
Fa0/1
R1
606 Affiliate 17: IP Multicast Routing
Figure 17-12 R2 and R3 Sending Assert Messages
The ambition of the Assert bulletin is to accredit the albatross of forwarding accumulation cartage on the
LAN to the router that is abutting to the source. Aback R2 and R3 accept accumulation cartage from the
source on their s0 interfaces, they advanced it on their e0 interfaces. Both of them accept their s0
interfaces in the admission interface account and e0 interfaces in the approachable interface list. Now, R2
and R3 accept a multicast packet for the accumulation on their e0 interfaces, which will account them to
send an Assert bulletin to boldness who should be the forwarder.
The Assert action picks a champ based on the acquisition agreement and metric acclimated to acquisition the route
to ability the unicast abode of the source. In this example, that agency that R2 or R3 will win based
on the routes they anniversary use to ability 10.1.1.10. R2 and R3 accelerate and accept Assert letters that
include their corresponding authoritative distances of the acquisition protocols acclimated to apprentice the route
that matches 10.1.1.10, as able-bodied as the metric for those routes. The routers on the LAN compare
their own acquisition agreement authoritative ambit and metrics to those abstruse in the Assert
messages. The champ of the Assert action is bent as follows:
1. The router announcement the everyman authoritative ambit of the acquisition agreement acclimated to learn
the avenue wins.
2. If a tie, the router with the everyman advertised acquisition agreement metric for that avenue wins.
3. If a tie, the router with the accomplished IP abode on that LAN wins.
Designated Router
PIM Hello letters are additionally acclimated to accept a appointed router (DR) on a multiaccess network. A
PIM-DM or PIM-SM router with the accomplished IP abode becomes a DR.
R1
R2 R3
.2 E0 .3 E0
.3
.1 .1
.1
.2
.1
S0
S0 S1
S0
H1
Assert Assert
Member of Group
227.7.7.7
E0
10.1.1.0/24
10.1.4.0/24
10.1.2.0/24 10.1.3.0/24
Multicast Traffic
Destination Address: 227.7.7.7
Source Address: 10.1.1.10
.10
Dense-Mode Acquisition Protocols 607
The PIM DR abstraction applies mainly aback IGMPv1 is used. IGMPv1 does not accept a mechanism
to accept a Querier—that is to say that IGMPv1 has no way to adjudge which of the abounding routers on
a LAN should accelerate IGMP Queries. Aback IGMPv1 is used, the PIM DR is acclimated as the IGMP
Querier. IGMPv2 can anon accept a Querier (the router with the everyman IP address), so the PIM
DR is not acclimated as the IGMP Querier aback IGMPv2 is used.
Note that on a LAN, one router ability win the Assert action for a accurate (S,G) SPT, while
another ability become the IGMP Querier (PIM DR for IGMPv1, IGMP Querier for IGMPv2). The
winner of the Assert action is amenable for forwarding multicasts assimilate the LAN, admitting the
IGMP Querier is amenable for managing the IGMP action by actuality amenable for sending
IGMP Query letters on the LAN. Note additionally that the IGMPv2 Querier acclamation chooses the
lowest IP address, and the Assert action uses the accomplished IP abode as a tiebreaker, authoritative it
slightly added acceptable that altered routers are called for anniversary function.
Summary of PIM-DM Messages
This area concludes the advantage of PIM-DM. Table 17-2 lists the key PIM-DM messages
covered in this chapter, forth with a abrupt analogue of their use.
Table 17-2 Summary of PIM-DM Messages
PIM Bulletin Definition
Hello Acclimated to anatomy acquaintance adjacencies with added PIM routers, and to
maintain adjacencies by ecology for accustomed Hellos from each
neighbor. Additionally acclimated to accept a PIM DR on multiaccess networks.
Prune Acclimated to ask a adjoining router to abolish the articulation over which the
Prune flows from that adjoining router’s approachable interface account for a
particular (S,G) SPT.
State Refresh Acclimated by a after router, beatific to an upstream router on an RPF
interface, to account the upstream router to displace its Clip timer. This
allows the after router to advance the pruned accompaniment of a link, for
a accurate (S,G) SPT.
Assert Acclimated on multiaccess networks to actuate which router wins the right
to advanced multicasts assimilate the LAN, for a accurate (S,G) SPT.
Prune Override (Join) On a LAN, a router may multicast a Clip bulletin to its upstream
routers. Added routers on the aforementioned LAN, absent to anticipate the
upstream router from pruning the LAN, anon accelerate addition Join
message for the (S,G) SPT. (The Clip Override is not absolutely a Prune
Override message—it is a Join. This is the alone purpose of a Join
message in PIM-DM, per RFC 3973.)
Graft/Graft-Ack Aback a pruned articulation needs to be added aback to an (S,G) SPT, a router
sends a Graft bulletin to its RPF neighbor. The RPF neighbor
acknowledges with a Graft-Ack.