WB1 8.18 PIM Bootstrap Router

8.18 PIM Bootstrap Router

• Remove the Auto-RP configuration on SW2, SW4 and R1.
• Remove the multicast boundary configuration on R5.
• Configure R5 to advertise itself as the RP for all multicast groups using the
standards-based protocol.

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Bootstrap router or BSR is a standards-based solution available with PIMv2,
which performs the same function as Auto-RP, i.e. disseminates RP information.
Both protocols use the concept of a candidate RP. However, BSR does not use
any dense-mode groups to distribute RP-to-Group mapping information. Instead,
the information is flooded using PIM messages, on a hop-by-hop basis. That is,
when a router receives a candidate RP announcement inside a PIM message, it
applies an RPF check, validating that the announcement was received on the
interface that is on the shortest path to the RP. If the RPF check succeeds, the
message is flooded out of all PIM-enabled interfaces.

In order to configure a candidate RP use the command

ip pim rp-candidate <PIM-Enabled-Interface> [group-list <Standard-ACL>]
[interval <Seconds>] [priority <0-255>].

If you omit all arguments,
the router will start advertising itself as the RP for all groups. You may specify a
list of groups using the group-list argument. All entries in this list are
“positive”, if you compare that to Auto-RP, and you cannot use “negative” groups.
The priority value is used when the routers select the best RP for a given group,
and lower values are preferred. The default priority is zero (which is against the
standard, which specifies a value of 192) and which is the highest possible value.
You would very rarely want to change the priority value for a candidate RP,
possibly only in cases when you want to gracefully take the RP out of service.

Now, the part where BSR differs from Auto-RP is the bootstrap router itself. This
router performs the role similar to the Auto-RP MA, by listening to candidate RP
announcements. However, unlike the Auto-RP MA, BSR does not elect the best
RP for every group range. Instead of this, for every group range it builds a set of
candidate RPs, including all routers that advertised their willingness to service
this group range. This is called the group range to RP set mapping.

The resulting array of group range to RP set mappings is distributed by the BSR
using PIM messages and the same flooding procedure described above. The
command to configure a router as a BSR is

ip pim bsr-candidate <Interface-Name> [hash-mask-length] [priority].

Ignore the hashmask-length parameter for the moment, and notice the priority field.
By default,the priority of zero is advertised in all BSR messages. The higher the priority
value, the more preferred the BSR. The IP address of the interface used to
source the BSR messages is used as a tie-breaker in case two priorities match –
the higher IP is preferred. If there are multiple BSRs, they all listen to other
potential BSR messages. If a BSR hears a message with a higher priority or IP
address, it immediately stops its own BSR advertisements. This process ensures
a unique BSR in the domain while maintaining some redundancy.

The bootstrap messages are received by every multicast router and used to
populate their RP cache. Note that it’s up to the routers to select the best
matching RP from the sets advertised by the BSR router. In order to facilitate RP
load-balancing, routers may use a special hash function to select the best RP
from a set that services the same group range.

R1:
no ip pim send-rp-discovery Loopback 0

R5:
ip pim rp-candidate Loopback0
ip pim bsr-candidate Loopback0
!
interface FastEthernet 0/0
no ip multicast boundary PERMITTED_GROUPS filter-autorp

SW2:
no ip pim send-rp-announce Loopback 0

SW4:
no ip pim send-rp-announce Loopback 0

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debug ip pim bsr
show ip pim bsr-router
show ip pim rp mapping