Multicast Flashcards
Describe 3 types of filters that can be used with AutoRP?
ip pim send-rp announce group-list: cRP announces for which group they want to be RP’s
rp-announce-filter: mapping agents specify which RP is permitted for which group, requires two ACL’s one for RP and one for group
ip pim accept-rp: normal routers can specify which RP permitted for which group.
In a sparse-dense mode configuration, what are two ways of confirming if a group is being switched in dense mode instead of sparse?
- In the show ip rp mapping command, the group will have a “minus” symbol next to it in brackets
- In the show ip mroute output there will be no mroute associated with the group
In a topology where AutoRP is used how would you configure a scenario where there are two routers (1 and 2), 1 is the RP for GroupA and 2 is the RP for GroupB but if R1 fails then R2 can also be the RP for GroupB?
The way to achieve this is to configure e.g R1 for its primary groups with a longer prefix e.g 224.0.0.0/5 and then to configure its backup 232.0.0.0/4 with a shorter prefix and vice versa for R2. This way the backup is never preferred as the other router has a longer prefix match of the group.
What are two methods of changing multicast routing paths/next-hops without modifying the underlying unicast IGP?
- Static mroute
2. Multicast BGP (address family ipv4 multicast)
What are two methods of configuring multicast boundaries for AutoRP?
- Use the TTL scope command either with the RP announce or MA discover
- Configure a multicast boundary ACL to specify which group is blocked, it can also be used to
What is the difference between AutoRP and BSR RP election?
BSR can be assigned a priority whereas AutoRP cannot and the highest IP wins.
The MA elects the RP in AutoRP whereas the routers elect the RP in BSR
What feature can be used so that only a specific source or sources are accepted by the RP for a group? What specific things to remember with this feature?
The ip pim accept-register feature can be used to define which sources will be accepted by an RP for a group. The two caveats to remember are:
- The acl has to be an extended acl
- If the RP is the DR for a segment then the filter will not work
How do you configure BGP for multicast?
Configure the prefixes in question under address family ipv4 multicast
If you have three BSR RP candidates R1, R2 and R3 and you dont want R3 to be chosen as an RP, what can be configured to acheive this
All cRP’s are configured with a priority of 0 when using the command ip pim rp-candidate. If R3 configures itself with any number above 0 then the other routers who have priority 0 will be preferred
What configuration can you use so that only a specific source can send traffic for a multicast group and where would you configure it
ip pim accept-register list and this is configured in the RP.
What are the different type of PIM messages and what do they do?
Hello - Multicast sent every 10 seconds by default
Register - Encapsulated packets tunnelled to the RP by the DR on behalf of a source and sent as unicast
Register-stop - Sent by RP if no receivers for the group or source hasnt started sending traffic
Join/Prune - Join for SM and Prune for SM/DM. Sent upstream via multicast and shared tree is established sent periodically or to form a spt
Assert - When there are multiple routers on a segment, assert message used to elect one forwarder for the segment. AD -> routing metric -> highest IP used for election.
Graft - Used in PIM-DM to rejoin a group after it has been pruned rather than wait 3 minutes for the next flood/prune cycle
cPR Advertisement: Used to advertise cPR in AutoRP
Bootstrap: Flooded by BSR routers to each other to work out who has the highest priority and will become BSR
DF Election: Used to elect a DF for every BiDir group
