PIM-SM router types

Within a PIM-SM domain, PIM-SM routers can be configured to fill one or more of the following roles:

  • Designated router (DR): A router performing this function forwards multicast traffic from a unicast source to the appropriate distribution (rendezvous) point.

  • Bootstrap router (BSR): A router elected to this function keeps all routers in a PIM-SM domain informed of the currently assigned rendezvous point (RP) for each multicast group currently known in the domain.

  • Rendezvous point (RP): A router elected as an RP for a multicast group receives requested multicast traffic from a DR and forwards it toward the multicast receivers requesting the traffic. An RP can be manually configured or dynamically elected through the BSR process.

  • Static RP: This option forwards traffic in the same way as an RP, but requires manual configuration on all routers in the domain to be effective.

  • Candidate RP (C-RP): The C-RP periodically sends advertisement messages to the BSR, which collects RP-set information for RP election. The BSR starts a holdtime timer for a C-RP after it receives an advertisement message. If the BSR does not receive any advertisement message when the timer expires, it considers the C-RP failed or unreachable.

All of these can be enabled on each of several routers in a PIM-SM domain.

DR

In a LAN segment populated by one or more routers running PIM-SM, one such router is elected the DR for that LAN segment. When the DR receives a Join request from a multicast receiver on that LAN segment, it forwards the join toward the router operating as the RP for the requested multicast group.

Where multiple PIM-SM routers exist in a LAN segment, the following criteria is used to elect a DR:

  1. The router configured with the highest DR priority in the LAN segment is elected.

  2. If multiple routers in the LAN segment are configured with the same DR priority, the router having the highest IP address is elected.

In a given domain, each LAN segment capable of receiving multicast traffic from a unicast source should have at least one DR. (Enabling PIM-SM on a LAN segment automatically enables the router as a DR for that LAN segment.) Because there is an election process for DR on each LAN segment, all routers on a LAN segment must be enabled for DR. Where it is important to ensure that a particular router is elected as the DR for a given LAN segment, you can increase the DR priority on that LAN segment configuration for that router.

If it is necessary to prevent a router from operating as a DR on a given LAN segment, disable DR operation by configuring the DR priority as zero (0).

BSR

Before a DR can forward encapsulated packets for a specific multicast group to an RP, it must know which router in the domain is the elected RP for that multicast group. The BSR function enables this operation by doing the following:

  1. Learns the group-to-RP mappings on the C-RPs in the domain by reading the periodic advertisements each one sends to the BSR.

  2. Distributes the aggregate Candidate-RP (C-RP) information as an RP-set to the PIM-SM routers in the domain. This is followed by an election to assign a specific multicast group or range of groups to the C-RPs in the domain. The software supports assignment of up to four multicast addresses and/or ranges of multicast addresses to a Candidate Rendezvous Point.

The BSR periodically sends bootstrap messages to the other PIM-SM routers in the domain to maintain and update the RP-set data throughout the domain, and to maintain its status as the elected BSR.

RP

Instead of flooding multicast traffic as is done with PIM-DM, PIM-SM uses a set of multiple routers to operate as RPs. Each RP controls multicast traffic forwarding for one or more multicast groups as follows:

  • Receives traffic from multicast sources (S) through a DR.

  • Receives multicast joins from routers requesting multicast traffic.

  • Forwards the requested multicast traffic to the requesting routers.

The routers requesting multicast traffic are either edge routers or intermediate routers. Edge routers are directly connected to specific multicast receivers using IGMP to request traffic. Intermediate routers are on the path between edge routers and the RP. This is known as an RP Tree (RPT) where only the multicast address appears in the routing table. For example:

( *, G ), where:

* = a variable (wildcard) representing the IP address of any multicast source

G = a particular multicast group address.

C-RP

Within a PIM-SM domain, different RPs support different multicast addresses or ranges of multicast addresses. That is, a given PIM-SM multicast group or range of groups is supported by only one active RP, although other C-RPs can also be configured with overlapping or identical support.

A C-RP's group-prefix configuration identifies the multicast groups the RP is enabled to support.

If multiple C-RPs have group-prefixes configured so that any of these RPs can support a given multicast group, then the following criteria are used to select the RP to support the group:

  1. The C-RP configured with the longest group-prefix mask applicable to the multicast group is selected to support the group. Step 2 of this procedure applies if multiple RP candidates meet this criterion.

  2. The C-RP configured with the highest priority is selected. Step 3 of this procedure applies if multiple RP candidates meet this criterion

  3. A hash function (using the configured bsr-candidate hash-mask-length value) generates a series of mask length values that are individually assigned to the set of eligible C-RPs. If the hash function matches a single RP candidate to a longer mask length than the other candidates, that candidate is selected to support the group. Apply step 4 of this procedure if the hash function matches the longest mask length to multiple RP candidates.

  4. The C-RP having the highest IP address is selected to support the group.

Also, within a PIM-SM domain, a router can be configured as a C-RP available for a given multicast group or range of groups and as the static RP for a given multicast group or range of groups. The recommended practice is to use C-RPs for all multicast groups unless there is a need to ensure that a specific group or range of groups is always supported by the same routing switch.

Static RP

Like C-RPs, static RPs control multicast forwarding of specific multicast groups or ranges of contiguous groups. However, static RPs are not dynamically learned, and increase the configuration and monitoring effort to maintain them. As a result, static RPs are not recommended for use except where one of the following conditions applies:

  • It is desirable to designate a specific router interface as a backup RP for specific groups.

  • Specific multicast groups are expected, and a static RP would help to avoid overloading a given RP with a high volume of multicast traffic.

  • A C-RP for the same groups is less reliable than another RP that would not normally be elected to support the groups.

  • Tighter traffic control or a higher priority is desired for specific multicast groups.