Multicast PIM Auto RP

So, is this a property of mapping agent ? I will tell you where I am confused. As soon as we configure PIM on routers, they automatically have a **, 224.0.1.40) entry in their MRT. In order for them to generate PIM joins in sparse mode, some host needs to generate traffic. But in our example, we do not have any hosts which send an IGMP join to R3 and yet R2 sends the information about RP to it. Is this specific Mapping agent behaviour ?

Hi Suman,

Normally, we need a PIM join before a router will forward multicast traffic for a certain multicast group. With the mapping agent, it’s a bit different. The mapping agent will start advertising RP mapping information right away, no PIM join is needed to forward this information.

You can see this behavior when you try a “debug ip pim” on a mapping agent. It will advertise its messages on any PIM enabled interface right away.

Rene

Hi Renee,
Does autoRP provides RP redundancy? It seems that if the mapping agent dies, the RP is gone too.

Best regards,
Hans de Roode.

Hi rene, I am looking at aprod config and have a doubt

on one of Access Layer switch I see :-

  1. under some VLANs “ip pim passive” and “ip cgmp” —> what do you make out of this config ?

  2. under some interfaces : “ip pim sparse-dense mode” and under global “ip pim autorp listener”

why someone would configure both “ip pim sparse-dense mode” and “ip pim autorp listener” ?

Thanks in adv

Hi rene, I am looking at a prod config and have a doubt

on one of Access Layer switch I see :-

  1. under some VLANs “ip pim passive” and “ip cgmp” —> what do you make out of this config ?

  2. under some interfaces : “ip pim sparse-dense mode” and under global “ip pim autorp listener”

why someone would configure both “ip pim sparse-dense mode” and “ip pim autorp listener” ?

Thanks in adv

@Hans redundancy is no problem but you should configure multiple RPs AND multiple mapping agents. Otherwise a single mapping agent is your single point of failure.

@Abhishek CGMP is pretty old so it’s possible that this command is not even needed in your network. It’s used for within your L2 domain:

The ip pim passive command means that the switch/router won’t send any PIM messages on its interface, it will also not accept any PIM messages from other devices. It forces your switch/router to become the DR. If you need PIM and you are the only PIM-enabled device, you can use this. Otherwise you should disable it.

Adding ip pim autorp listener only makes sense if you use sparse mode, not dense or sparse-dense mode.

Awesome Thanks Rene !

How come R3/R4 are mapping agents chosen in ethernet segment? What makes them mapping agent? Without PIM being configured or statically configured ??

Hello Parajuli

Although Rene doesn’t go into details in the lesson, a router is configured as a mapping agent using the following command:

ip pim send-rp-discovery

You can find out more about this command at this Cisco Command Reference Documentation (search for ip pim send-rp-discovery.

It is possible to configure two or more mapping agents within a network. Each will function independently. Engineers often misunderstand how multiple mapping agents interact and often believe that there is more complexity to such a configuration than there really is. Receiving mapping data from two or more mapping agents is not a problem because both transmit identical mapping information, so there are never any conflicts.

Conversely, the command ip pim send-rp-announce configures a router as an RP.

I hope this has been helpful!

Laz

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If R3 has to be statically configured as MP beforehand, may be R4 can be done as well!
Regardless, in this topo, why is R4 not announcing itself as RP then?
Or this is for brevity; only for clarifying the technology?

Hello Deep

Routers will only play the role of a RP or an MA if they are statically configured as such. Specifically, to configure a router as a candidate RP and thus to have it announce itself as such, the following command must be present:

ip pim send-rp-announce

To configure a router as an MA the following command must be present:

ip pim send-rp-discovery

The above are configured on R1 and R2 respectively. R4 is not configured to announce itself as an RP so it will not do so. R4 does however recieve the information from the mapping agent because of the following command that is found on R3

ip pim autorp listener

This command allows the multicast traffic that reaches R3 to be flooded with dense mode, thus it will flood those packets to R4.

I hope this has been helpful!

Laz

Between Auto-RP and BSR, which one provides better fault tolerance? Both can have backup RP IPs.

Hello Chris

It really depends on what you want. A good explanation is given in this Cisco Community post:

The link to the white paper in the post is out of date. The following is the whitepaper related to this issue:

I hope this has been helpful!

Laz

It seems Cisco officially recommend to use sparse-dense mode rather than auto-rp listener, and dummy rp’s for the groups which don’t exist.

Which frankly makes little sense to me, as listener requires way less config!

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Hi All,

when I run a show ip pim rp to find out where the RP is on a network i get the following response:

(*, 239.0.78.141), 1w3s/00:02:54, RP 0.0.0.0

This entry is repeated for all multicast groups that transit the L3 switch. My question is, what does the 0.0.0.0 output mean? Is it saying this L3 device is the RP?

thanks

Mike

Hello Michael

A response of (*, 239.0.78.141), 1w3s/00:02:54, RP 0.0.0.0 tells us that an RP has not been defined. Either enable auto RP or manually define the RP in order to have that information added.

I hope this has been helpful!

Laz

Thanks Laz that makes sense, many thanks.

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A post was merged into an existing topic: VRF Lite Configuration on Cisco IOS

Dear Rene,

In the configuration example, R3 is getting the rp mappings before we configure the ip pim autorp listener command. That means R3 know the RP address. So R3 should be able to serve if R4 wants to join a feed which is coming from R1 (as RP). But it is not until we configure ip pim autorp listener command on R3. Why is that?

Hello Roshan

If R4 is a host that wants to participate in a multicast group, then yes you are right, it can receive all the necessary information for such a case. However, we don’t want R4 to function as a multicast participant, but as a multicast router that will allow hosts on networks connected to it to participate in multicast. In this case, R4 will not “know” anything because it is behind R3 and is not receiving any RP mapping packets. This is because we are using PIM sparse mode which means that it would only be forwarded when requested by a downstream router.

The solution is to use the ip pim autorp listener which ensures that traffic sent to 224.0.1.39 gets flooded with dense mode. Once enabled, packets will be flooded to R4.

I hope this has been helpful!

Laz