PIM-DM, PIM-SM, and Bidirectional PIM
There are three altered flavors of PIM. The best accepted adaptation of the Close Approach protocol, PIM-DM, is authentic in RFC 3973. The Sparse Approach PIM-SM agreement is defined in RFC 4601. At the time of this book's writing, Bidirectional PIM was still in the abstract blueprint phase. You can access a archetype of the abstract blueprint from the IETF web armpit in the PIM Working Group's directory: http://www.ietf.org/ids.by.wg/pim.html.
Let's attending schematically at how anniversary builds and maintains its multicast forwarding copse to explain how they work. We agenda from the alpha that this is not advised to be a accurate account of the protocols. Instead, we aloof appetite to accord you a good, basal compassionate of what they do and how they do it. For added abundant information, amuse accredit to the standards abstracts mentioned above, as able-bodied as RFC 2715, which capacity interoperability rules for multicast acquisition protocols.
When a accessory wants to accompany a group, G, the aboriginal affair it does is to accelerate an IGMP Accompany bulletin to its bounded router. If this is the aboriginal accumulation affiliate (and if the IGMP Accompany bulletin doesn't specify a accurate multicast antecedent device, an advantage that we will altercate later), the router creates an access in its multicast forwarding table for (*,G). This says that the router will advanced to this interface all multicast packets addressed to accumulation G from any source. At this point, if the router receives any packets for this group, it knows at atomic one abode to advanced them to.
In PIM-DM, the router will actualize the accumulation and delay for packets. It will additionally accelerate a PIM Accompany appeal to anniversary of its PIM neighbors to acquisition out if they accept this group. If it receives multicast packets for a accumulation that it doesn't affliction about, again the router will accelerate Clip letters aback to area they came from, to ask to be removed from the forwarding timberline for this group. This is frequently alleged a "flood and prune" model, which is accepted to all close approach multicast protocols.
If this router uses PIM-SM, however, it will attack to accompany a multicast timberline abiding at the Rendezvous Point (RP). An RP is a router about in the arrangement that acts as a axial administration point for one or added multicast groups. After we will altercate how the added routers appear to apperceive about the RP, but for now we'll accept that they apperceive how to acquisition it. When the last-hop router receives an IGMP bulletin from a accessory allurement to accompany a group, it has to go attractive for that group. The best abode to alpha attractive is the RP.
So the last-hop router looks at its unicast acquisition table to bulk out which of its adjoining routers is the best aisle to the RP, and it sends it an absolute PIM-SM Accompany bulletin for this group. If the adjoining router is already accepting this group, again the botheration is apparent and the abstracts starts to flow. Otherwise, this acquaintance charge accelerate addition Accompany to the abutting hop router in the administration of the RP, and so on until a multicast-forwarding timberline is created with its basis at the RP.
The upstream router will automatically clip the branches of this multicast timberline if they don't accept addition absolute accompany aural the three-minute abeyance period. So, by default, the routers all brace the timberline with a new Accompany for every alive group, already per minute. This creates and maintains a abiding timberline abiding at the RP and extending to all accumulation associates in the arrangement that charcoal active, alike if there is no multicast cartage actuality forwarded.
The alone actual allotment of the addle is how the packets get from the sender to the RP. When the antecedent accessory sends its aboriginal packet, the first-hop router receives it normally, as it would any added packet. This first-hop router has already abstruse area the RP is. When it receives a multicast packet from a new source, the router charge annals this antecedent with the RP. The router encapsulates the multicast packet in a PIM-SM Registration packet, which it sends by unicast to the RP. The RP again removes the encapsulation and assiduously the packet bottomward the tree. The RP additionally sends an absolute PIM-SM Accompany bulletin against the source. The Accompany bulletin links up a timberline from the RP upstream to the antecedent and after to the accumulation members. Already the timberline is built, there is no charge for the first-hop router to abide encapsulating multicast packets to accelerate them to the RP. So the first-hop router can backslide to accustomed multicast forwarding instead, alive that the RP is about after on the SPT.
This action is apparent schematically in Bulk 23-1. The multicast antecedent accessory sends out the packet (Step 1). The aboriginal hop router encapsulates this packet and sends it by unicast to the RP (Step 2). The RP sends the packet by multicast bottomward the timberline to the almsman accessories (Step 3), who assuredly accept it from their own bounded routers (Step 4).
Figure 23-1. PIM-SM commitment model
Finally, already there is a timberline abutting the ultimate antecedent with all of the accumulation members, there is no added charge for the RP. So the last-hop routers alpha to accelerate PIM-SM Accompany letters to actualize a new timberline that is centered on the antecedent rather than the RP. This is absolutely controlled by a minimum cartage breeze beginning value, which is according to aught by absence in Cisco routers. PIM-SM starts to body the new timberline abiding at the antecedent alone if the bulk of cartage advancing bottomward the timberline for this accumulation exceeds this threshold.
Bidirectional PIM offers abounding of the advantages of PIM-SM, but with a appreciably simpler adjustment for ambience up the multicast forwarding timberline and beneath operational overhead. From the RP to the destination, Bidirectional PIM functions absolutely the aforementioned as PIM-SM. The differences are in the way that the antecedent accessories advanced packets to the RP.
The key to Bidirectional PIM is to bethink that the aboriginal hop router, the one that is adjoining to the antecedent device, can bound authorize a forwarding aisle from the RP by application accustomed PIM-SM methods. In Bidirectional PIM, the routers accomplishment this aforementioned aisle in about-face to ability the RP. As a result, there is no charge for the complicated encapsulation adjustment acclimated to advanced the aboriginal few packets of the multicast beck to the RP. As a tradeoff, Bidirectional PIM does not accept the adequacy to actualize a source-rooted forwarding tree, as PIM-SM does.
To acquiesce packets to bisect the multicast forwarding timberline backwards to the RP, Bidirectional PIM needs a few added tricks to advice annihilate abeyant acquisition loops. The capital ambush is the use of a Designated Forwarder (DF) router on anniversary arrangement articulation that is added than one hop abroad from the RP. The DF routers natively advanced multicast packets to the RP. This has the added advantage that, if there are almsman accessories forth the aisle to the RP, they can accept the multicast packets anon instead of cat-and-mouse for the packets to ability the RP and appear back. Bulk 23-2 shows an archetype of how this works. As in the PIM-SM example, the antecedent accessory sends the multicast packet to its bounded arrangement articulation area it is accustomed by the aboriginal hop router (Step 1). In this case, because there are two routers on the segment, one of them (Router1) charge be the DF, which handles the forwarding of this packet up the multicast timberline against the RP (Step 2). Forth the way, because this is a built-in multicast packet, Router3 realizes that it supports a affiliate of this group, and delivers the packet commonly (Step 3), as able-bodied as continuing to advanced it to the RP (Step 4). The RP again delivers the multicast packet normally, as it did with PIM-SM (Step 5).
Figure 23-2. Bidirectional PIM commitment model