All Cisco-Network Study Notes: Dynamically Finding the RP Application BSR

Dynamically Finding the RP Application BSR

Cisco provided the proprietary Auto-RP affection to break a brace of specific problems. PIM

Version 2, which came later, provided a altered band-aid to the aforementioned problem, namely the

BootStrap Router (BSR) feature. From a actual accepted perspective, BSR works analogously to

Auto-RP. Anniversary RP sends a bulletin to addition router, which collects the group-to-RP mapping

information. That router again distributes the mapping advice to the PIM routers. However,

any assay of BSR above that akin of detail shows that these two accoutrement do alter in

many ways.

It is accessible to aboriginal accept the abstraction of the bootstrap router, or BSR router, afore thinking

about the RPs. One router acts as BSR, which is agnate to the mapping abettor in Auto-RP. The

BSR receives mapping advice from the RPs, and again it advertises the advice to other

routers. However, there are some specific differences amid the accomplishments of the BSR, and their

implications, and the accomplishments of the Auto-RP mapping agent:

■ The BSR router does not aces the best RP for anniversary multicast group; instead, the BSR router

sends all group-to-RP mapping advice to the added PIM routers central bootstrap messages.

■ PIM routers anniversary apart aces the currently best RP for anniversary multicast accumulation by

running the aforementioned assortment algorithm on the advice in the bootstrap message.

■ The BSR floods the mapping advice in a bootstrap bulletin beatific to the all-PIM-routers

multicast abode (224.0.0.13).

■ The calamity of the bootstrap bulletin does not crave the routers to accept a accepted RP or to

support close mode. (This will be declared in added detail in the abutting few pages.)

Figure 17-21 shows an example, declared next, of how the BSR floods the bootstrap message.

PIMv2 creates specific rules for BSR bootstrap messages, advertence that PIM routers should flood

these messages. PIM-SM routers flood bootstrap letters out all non-RPF interfaces, which in

effect guarantees that at atomic one archetype of the bulletin makes it to every router. Note that this logic

is not abased on a alive dense- or spare-mode implementation. As a result, BSR overcomes

the chicken-and-egg botheration of Auto-RP.

For example, in Figure 17-21, brainstorm that R4’s s1 interface is its RPF interface to ability R2, and

R5’s RPF interface to ability R2 is its s0 interface. So, they anniversary advanced the bootstrap letters at

Step 3 of Figure 17-21. However, because R4 receives the bootstrap bulletin from R5 on one of

R4’s non-RPF interfaces, R4 discards the packet, thereby preventing loops. R5 additionally does not

forward the bootstrap bulletin any added for the aforementioned basal reasons.

Sparse-Mode Routing Protocols 627

Figure 17-21 BSR Calamity Bootstrap Messages

The added important allotment of BSR operation is for anniversary applicant RP (c-RP) to acquaint the BSR

router that it is an RP and to analyze the multicast groups it supports. This allotment of the action with

BSR is simple if you accumulate in apperception the afterward point:

All PIM routers already apperceive the unicast IP abode of the BSR based on the earlier

receipt of bootstrap messages.

So, the c-RPs artlessly accelerate unicast messages, alleged c-RP Advertisements, to the BSR. These c-RP

advertisements accommodate the IP abode acclimated by the c-RP, and the groups it supports.

The BSR affection supports bombastic RPs and bombastic BSRs. As mentioned earlier, the bootstrap

message beatific by the BSR router includes all applicant RPs, with anniversary router application the aforementioned hash

algorithm to aces the currently best RP for anniversary multicast group. The mapping advice can list

multiple RPs that abutment the aforementioned accumulation addresses.

Additionally, assorted BSR routers can be configured. In that case, anniversary applicant BSR (c-BSR)

router sends bootstrap letters that accommodate the antecedence of the BSR router and its IP address. The

highest-priority BSR wins, or if a tie occurs, the accomplished BSR IP abode wins. Then, the winning

BSR, alleged the adopted BSR, continues to accelerate bootstrap messages, while the added BSRs

monitor those messages. If the adopted BSR’s bootstrap letters cease, the bombastic BSRs can

attempt to booty over.

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