Exam 350-601 topic 1 question 25 discussion

Actual exam question from Cisco's 350-601

Question #: 25
Topic #: 1

DRAG DROP -

Refer to the exhibit. In a bidirectional PIM network using Phantom RP as an RP redundancy mechanism, two Cisco NX-OS routers have these requirements:
✑ R1 must be the active RP.
✑ R2 must be the backup RP that is used only if R1 is not reachable.
Drag and drop the configuration steps to complete the configuration for Router 2. Not all configuration steps are used.
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Reference:
https://community.cisco.com/t5/networking-documents/rp-redundancy-with-pim-bidir-phantom-rp/ta-p/3117191

by Havoc5280 at May 29, 2022, 5:53 p.m.

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GuyThatTakesDumps

1 year, 9 months ago

Answer is correct. Few remarks : * It's important the RP address is NOT the loopback address but one of the other address in the subnet defined on the loopback * By default, OSPF advertises all loopbacks as /32 so the need of 'ip ospf network point-to-point' command to advertise the subnet mask * We hardcode the OSPF RID to avoid any duplicate RID problem in case address of Loopback1 is the highest loopback * There is a need to define a mapping agent for auto-rp to work (not shown here) * BSR doesn't support yet the possibility to advertise as candidate RP an IP address which is not defined on any interface. This implies that BSR cannot be used with phantom RP. We need to use either Auto-RP or static RP.

upvoted 4 times

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Havoc5280

2 years ago

The solution is correct. Take a look at the reference and notice the mask change for the L1 address on R2.

upvoted 2 times

dan_pf

1 year, 8 months ago

That's a thing I don't want to understand... just to accept!

upvoted 3 times

pythonad

10 months ago

This confused me as well, until I read the reference. "This way, the 'active' RP is the candidate RP advertising the route with the longest subnet mask (no matter the metric)." Essentially, we're configuring the "active RP" with a /30, so it gets advertised through the network. Once it fails, the /29 takes over.

upvoted 3 times

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