LANSpecific Issues with PIMDM and PIMSM
This section covers three small topics related to operations that only matter when PIM is used on LANs:
■ Prune Override
■ Assert messages
■ Designated routers
Both PIM-DM and PIM-SM use these features in the same way. Prune Override
In both PIM-DM and PIM-SM, the Prune process on multiaccess networks operates differently from how it operates on point-to-point links. The reason for this difference is that when one router sends a Prune message on a multiaccess network, other routers might not want the link pruned by the upstream router. Figure 20-11 shows an example of this problem, along with the solution through a PIM Join message that is called a Prune Override. In this figure, R1 is forwarding the group traffic for 239.9.9.9 on its fa0/0 interface, with R2 and R3 receiving the group traffic on their e0 interfaces. R2 does not have any connected group members, and its outgoing interface list would show null. The following list outlines the steps in logic shown in Figure 20-11, in which R3 needs to send a Prune Override:
1. R2 sends a Prune for group 239.9.9.9 because R2 has a null outgoing interface list for the group.
2. R1, realizing that it received the Prune on a multiaccess network, knows that other routers might still want to get the messages. So, instead of immediately pruning the interface, R1 sets a 3-second timer that must expire before R1 will prune the interface.
3. R3 also receives the Prune message sent by R2, because Prune messages are multicast to All-PIM-Routers group address 224.0.0.13. R3 still needs to get traffic for 239.9.9.9, so R3 sends a Join message on its e0 interface.
4. (Not shown in Figure 20-11) R1 receives the Join message from R3 before removing its LAN interface from the outgoing interface list. As a result, R1 does not prune its Fa0/0 interface.
Figure 20-11
POINT
Prune Override
Prune Override
This process is called Prune Override because R3 overrides the Prune sent by R2. The Prune Override is actually a Join message, sent by R3 in this case. The message itself is no different from a normal Join. As long as R1 receives a Join message from R3 before its 3-second timer expires, R3 continues to receive traffic without interruption.
Assert Message
The final PIM-DM message covered in this chapter is the PIM Assert message. The Assert message is used to prevent wasted effort when more than one router attaches to the same LAN. Rather than sending multiple copies of each multicast packet onto the LAN, the PIM Assert message allows the routers to negotiate. The winner gets the right to be responsible for forwarding multicasts onto the LAN.
Figure 20-12 shows an example of the need for the Assert message. R2 and R3 both attach to the same LAN, with H1 being an active member of the group 227.7.7.7. Both R2 and R3 are receiving the group traffic for 227.7.7.7 from the source 10.1.1.10.
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._ |
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a |
.10 |
Multicast Traffic Destination Address: 227.7.7.7 Source Address: 10.1.1.10
Multicast Traffic Destination Address: 227.7.7.7 Source Address: 10.1.1.10
I Assert *
Member of Group
The goal of the Assert message is to assign the responsibility of forwarding group traffic on the LAN to the router that is closest to the source. When R2 and R3 receive group traffic from the source on their s0 interfaces, they forward it on their e0 interfaces. Both of them have their s0 interfaces in the incoming interface list and e0 interfaces in the outgoing interface list. Now, R2 and R3 receive a multicast packet for the group on their e0 interfaces, which will cause them to send an Assert message to resolve who should be the forwarder.
The Assert process picks a winner based on the routing protocol and metric used to find the route to reach the unicast address of the source. In this example, that means that R2 or R3 will win based on the routes they each use to reach 10.1.1.10. R2 and R3 send and receive Assert messages that include their respective administrative distances of the routing protocols used to learn the route that matches 10.1.1.10, as well as the metric for those routes. The routers on the LAN compare their own routing protocol administrative distance and metrics to those learned in the Assert messages. The winner of the Assert process is determined as follows:
The router advertising the lowest administrative distance of the routing protocol used to learn the route wins.
If a tie, the router with the lowest advertised routing protocol metric for that route wins. If a tie, the router with the highest IP address on that LAN wins.
KEY 1. POINT
Designated Router
KEY PIM Hello messages are also used to elect a designated router (DR) on a multiaccess network. A POINT PIM-DM or PIM-SM router with the highest IP address becomes a DR.
The PIM DR concept applies mainly when IGMPv1 is used. IGMPv1 does not have a mechanism KEY to elect a Querier—that is to say that IGMPv1 has no way to decide which of the many routers on POINT a LAN should send IGMP Queries. When IGMPv1 is used, the PIM DR is used as the IGMP
Querier. IGMPv2 can directly elect a Querier (the router with the lowest IP address), so the PIM DR is not used as the IGMP Querier when IGMPv2 is used.
Note that on a LAN, one router might win the Assert process for a particular (S,G) SPT, while another might become the IGMP Querier (PIM DR for IGMPv1, IGMP Querier for IGMPv2). The winner of the Assert process is responsible for forwarding multicasts onto the LAN, whereas the IGMP Querier is responsible for managing the IGMP process by being responsible for sending IGMP Query messages on the LAN. Note also that the IGMPv2 Querier election chooses the lowest IP address, and the Assert process uses the highest IP address as a tiebreaker, making it slightly more likely that different routers are chosen for each function.
Summary of PIM-DM Messages
This section concludes the coverage of PIM-DM. Table 20-2 lists the key PIM-DM messages covered in this chapter, along with a brief definition of their use.
Table 20-2 Summary of PIM-DM Messages
KEY POINT
Table 20-2 Summary of PIM-DM Messages
|
PIM Message |
Definition |
|
Hello |
Used to form neighbor adjacencies with other PIM routers, and to maintain adjacencies by monitoring for received Hellos from each neighbor. Also used to elect a PIM DR on multiaccess networks. |
|
Prune |
Used to ask a neighboring router to remove the link over which the Prune flows from that neighboring router's outgoing interface list for a particular (S,G) SPT. |
|
State Refresh |
Used by a downstream router, sent to an upstream router on an RPF interface, to cause the upstream router to reset its Prune timer. This allows the downstream router to maintain the pruned state of a link, for a particular (S,G) SPT. |
|
Assert |
Used on multiaccess networks to determine which router wins the right to forward multicasts onto the LAN, for a particular (S,G) SPT. |
|
Prune Override (Join) |
On a LAN, a router may multicast a Prune message to its upstream routers. Other routers on the same LAN, wanting to prevent the upstream router from pruning the LAN, immediately send another Join message for the (S,G) SPT. (The Prune Override is not actually a Prune Override message—it is a Join. This is the only purpose of a Join message in PIM-DM, per RFC 3973.) |
|
Graft/Graft-Ack |
When a pruned link needs to be added back to an (S,G) SPT, a router sends a Graft message to its RPF neighbor. The RPF neighbor acknowledges with a Graft-Ack. |
The next two short sections introduce two other dense-mode protocols, DVMRP and MOSPF.
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