The root of the distribution router in a shared tree is a router located in the core part of the network. In PIM-SM, it is important to select one or more routers to function as rendezvous points (RPs). Cisco defines RPs as a single common root placed at a chosen point of a shared distribution tree that can be configured statically on each router or learned dynamically.
In a PIM-SM network, the sources are obliged to send their traffic to the rendezvous point, which is then forwarded to the receivers through a shared distribution tree. The first-hop router of the receiver learns the source by default and will send a join message to the source, where it creates a source-based distribution tree between the source and the receiver. The source tree exempts the RP unless the RP is situated within the shortest path between the source and the receiver.
To join a rendezvous-point tree (RPT) as it is known in PIM sparse mode protocols, the router needs to follow the following steps:
- The RP IP address should be defined.
- A shared tree for the group must be built.
- Send a join message from the router interface using the correct multicast protocol, which is most likely PIM sparse mode, informing the upstream multicast routers that it is interested in joining that particular group’s shared tree.
Rendezvous point can be, as stated above, configured statically for a multicast group range by configuring the RP address on every router included in the multicast domain. It is simple to configure a static RP, and it can be done with a couple of configurations on each router. If there are various rendezvous points that are defined in the network or there is not much of a change on the rendezvous points, using a static RP can be the most basic method to define an RP. Also, you can use a static RP if your network is relatively small.
The downside of configuring a static RP is that the administrative overhead increases when implemented on a larger scale, making the network more complicated. Each router in the network must have the same RP address, which means if you decide to change the RP address, you must reconfigure all the RP addresses on the network, which can be really tedious if you have a reasonably large network.
Another concern is that if multiple rendezvous points are active on various groups, information about handling which RP is handling a specific multicast group should be known by all the routers. Multiple configuration commands are needed for this kind of information to be complete, and if the static RP configurations fail, there is no backup that will handle this issue.
There are a couple of ways to dynamically configure a multicast router as an RP. One is through Auto-RP, and the other is through the PIM Bootstrap Router (BSR).
Cisco has developed a proprietary mechanism known as Auto-RP that addresses the shortcoming of static RP by automating the distribution of group-to-RP mappings in a PIM sparse mode network.
According to Cisco, below are the advantages that can be experienced using Auto-RP:
- Multiple RPs in a network serving different group ranges can be used easily.
- Load splitting between various RPs.
- Simplified RP replacement in the multicast groups based on the location of the participant.
- Prevents manual static RP configuration inconsistencies.
- Multiple RPs can be used to serve different group ranges or to serve as backups for each other.
- The Auto-RP mechanism operates using two basic components, candidate RPs (C-RPs) and RP mapping agents (MAs).
C-RP expresses its willingness to advertise to become an RP via RP announcement messages. It shows its availability, IP address, and the multicast group range it wants to support. The RP messages are sent every 60 seconds to the multicast IP group 22.214.171.124, known as Cisco-RP-Announce.
An RP message consists of the default group range of 126.96.36.199/4, the C-RP IP address, and the hold time which is thrice the RP announce interval. The C-RP with the highest IP address is preferred if multiple RPs are being considered.
RP Mapping Agents
RP mapping agents store the information that is embedded in the RP announcements in a cache inside a group-to-RP mapping. The RP mapping agents join a multicast group 188.8.131.52, where they receive the RP announcements.
RP MAs send advertisements to a popular multicast group known as Cisco-RP-Discovery (184.108.40.206). These pieces of information are sent every 60 seconds and contain the elected RPs. Every multicast router in the PIM domain advertises the same group-to-RP mapping details.
PIM Bootstrap Router (BSR)
According to RFC 5059, a PIM BSR is an open-source mechanism that gives a fault-tolerant, automated RP discovery and distribution mechanism. BSR is being used by PIM to discover and announce rendezvous point set information for all the group prefix to each multicast router in the PIM domain.
Bootstrap Router is a part of PIM version 2 specification that contains the following components below:
- RP Priority
- Hash Mask Length
- SM/Bidir flag
- Multicast group range
- RP IP addresses
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