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Dell Configuration Guide for the S4048–ON System 9.14.2.5

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Electing an RP using the BSR Mechanism

Every PIM router within a domain must map a particular multicast group address to the same RP. The group-to-RP mapping may be statically or dynamically configured. RFC 5059 specifies a dynamic, self-configuring method called the Bootstrap Router (BSR) mechanism, by which an RP is elected from a pool of RP candidates (C-RPs).

Some routers within the domain are configured to be C-RPs. Other routers are configured to be Bootstrap Router candidates (C-BSRs); one router is elected the BSR for the domain and the BSR is responsible for forwarding BSM containing RP-set information to other routers.

The RP election process is as follows:

  1. C-BSRs flood their candidacy throughout the domain in a BSM. Each message contains a BSR priority value, and the C-BSR with the highest priority value becomes the BSR.
  2. Each C-RP unicasts periodic Candidate-RP-Advertisements to the BSR. Each message contains an RP priority value and the group ranges for which it is a C-RP.
  3. The BSR collects the most efficient group-to-RP mappings and periodically updates it to all PIM routes in the network.
  4. The BSR floods the RP-Set throughout the domain periodically in case new C-RPs are announced, or an RP failure occurs.

Constraints

  1. When a multicast group range is removed from the ACL group list, the E-BSR sends the advertisements to the group with hold-time as 0 only when the C-RP timer expires. Till the timer expires, the C-RP will act as a RP for that multicast group.

  2. In E-BSR, if the C-RP advertisements are not in synchronization with the standby, first few BCM C-RP advertisement might not have the complete list of RP mappings. Due to this, there is a possibility of RP mapping timeout and momentary traffic loss in the network.

  3. If you configure a secondary VLT peer as an E-BSR and in case of ICL flap or failover, the VLT lag will be down resulting a BSM timeout in the PIM domain and a new BSR will be elected. Hence, it is recommended to configure the primary VLT peer as E-BSR.

NOTE BSR configuration in the multicast topology should ensure that secondary VLT node is not selected as E-BSR. If selected as E-BSR during ICL flap or VLT failover, traffic disruption will be reported.

To enable BSR election for IPv4 or IPv6, perform the following steps:

  1. Enter the following IPv4 or IPv6 command to make a PIM router a BSR candidate:
    CONFIGURATION
    ip pim bsr-candidate
    ipv6 pim bsr-candidate
  2. Enter the following IPv4 or IPv6 command to make a PIM router a RP candidate:
    CONFIGURATION
    ip pim rp-candidate
    ipv6 pim rp-candidate
  3. Display IPv4 or IPv6 Bootstrap Router information.
    EXEC Privilege
    show ip pim bsr-router
    Example: 
    DellEMC# show ip pim bsr-router
PIMv2 Bootstrap information
This system is the Bootstrap Router (v2)
   BSR address: 7.7.7.7 (?)
   BSR Priority: 0, Hash mask length: 30
   Next bootstrap message in 00:00:08
This system is a candidate BSR
    Candidate BSR address: 7.7.7.7, priority: 0, hash mask length: 30
DellEMC#
    show ipv6 pim bsr-router
    Example: 
    DellEMC#show ipv6 pim bsr-router 
PIMv2 Bootstrap information
  BSR address: 200::1 (?)
  BSR Priority: 0, Hash mask length: 126
  Expires:     00:01:43

This system is a candidate BSR
  Candidate BSR address: 100::1, priority: 0, hash mask length: 126

Next Cand_RP_advertisement in 00:00:25
  RP: 100::1(Lo 0)
DellEMC#

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