Anycast RP

Answered Question
Aug 28th, 2009

When configuring anycast RP...can this be done in Auto-RP and BSR apart from static.

I have this problem too.
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Correct Answer by Peter Paluch about 7 years 3 months ago

Hello Mohamed,

I am familiar with the technical documentation and in fact, it supports my standpoints. Perhaps there is a misunderstanding between the terms we both use, despite talking about the same thing. Let me discuss these things with you.

My standpoint is that AnycastRP cannot be successfully created without the help of MSDP. I am not saying that AutoRP or BSR are unusable or incompatible with AnycastRP - on the contrary, they are necessary and required also with AnycastRP, especially in large networks. What I am saying is that having AutoRP or BSR alone is not sufficient to provide AnycastRP service. You may have the AutoRP or BSR - absolutely no problem with that - but you must have MSDP in addition to those protocols in order to have a workable AnycastRP.

Quoting from the document "Anycast RP" at:

http://www.cisco.com/en/US/docs/ios/solutions_docs/ip_multicast/White_papers/anycast.html

Anycast RP allows two or more rendezvous points (RPs) to share the load for source registration and the ability to act as hot backup routers for each other. Multicast Source Discovery Protocol (MSDP) is the key protocol that makes Anycast RP possible.

So, the AnycastRP is a configuration of multiple RPs that all have the same IP address which has certain benefits. The means of making this IP address known to multicast routers in your domain is another business - you may use BSR, AutoRP or static configuration for that. The problem here is that if two or more routers advertise the same IP address in a routing protocol, the network will be effectively partitioned - each router will use the closer RP, according to the routing protocol metrics. Now, consider that there are two RPs with the same IP address, A and B. A multicast sender close to router A starts sending the multicast stream. The multicast stream will be sent to the RP A. Further on, consider that there is a recipient of this traffic close to the RP B. A shared tree will be created for this recipient with the root in RP B. However, the RP B has no idea that there is already is a sender for this multicast group because it has never received a multicast packet from it. Without an additional protocol, the recipient closer to the RP B will never start receiving the multicast stream sent by a sender close to RP A. This is where the MSDP comes in. As soon as RP A notices there is a sender for a particular multicast group, it informs its MSDP neighbors (the RP B in this case) about the existence of a sender to a particular multicast group. The RP B will now know about it, and when a recipient subscribes to this group, the RP B is able to send the Join towards the sender, thereby creating a branch of multicast distribution tree through which the multicast stream can flow natively. Notice that neither AutoRP nor BSR can help you here because they do not disseminate information about multicast senders. You absolutely must use the MSDP here, otherwise, you can't have a true AnycastRP.

Regarding the multiple mapping agents: Quoting from document "Multicast Quick-Start Configuration Guide" at:

http://www.cisco.com/en/US/tech/tk828/technologies_tech_note09186a0080094821.shtml

You can use one RP that also serves as the mapping agent, or you can configure multiple RPs and multiple mapping agents for redundancy purposes.

Also, quting from the internet draft where the AutoRP was first submitted to IETF (but never made it to RFC) available at:

ftp://ftp-eng.cisco.com/ipmulticast/pim-autorp-spec01.txt

For robustness purpose, multiple RP-mapping agents may be configured inside the same administrative domain. Each RP-mapping agent also listens to the the RP mapping discovery group CISCO-RP-DISCOVERY, and suppresses the sending of its own RP-mapping packets if it hears RP-mapping packets originated from another RP-mapping agent with a higher IP address.

I hope this clarifies the confusion in this field.

Best regards,

Peter

Correct Answer by Peter Paluch about 7 years 3 months ago

Hi,

For Anycast RP, you need to use the Multicast Source Discovery Protocol (MSDP) in addition to providing routers with RP knowledge, be it static, BSR or AutoRP. The mechanisms for basic RP discovery like BSR or AutoRP or even static RP configuration are not capable alone to provide the Anycast RP functionality. Of course, you can combine any of them with MSDP.

You might want to read this document for further information and for configuration examples:

http://www.cisco.com/en/US/docs/ios/solutions_docs/ip_multicast/White_papers/anycast.html

http://www.cisco.com/en/US/docs/ios/solutions_docs/ip_multicast/White_papers/rps.html

Best regards,

Peter

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Correct Answer
Peter Paluch Fri, 08/28/2009 - 11:42

Hi,

For Anycast RP, you need to use the Multicast Source Discovery Protocol (MSDP) in addition to providing routers with RP knowledge, be it static, BSR or AutoRP. The mechanisms for basic RP discovery like BSR or AutoRP or even static RP configuration are not capable alone to provide the Anycast RP functionality. Of course, you can combine any of them with MSDP.

You might want to read this document for further information and for configuration examples:

http://www.cisco.com/en/US/docs/ios/solutions_docs/ip_multicast/White_papers/anycast.html

http://www.cisco.com/en/US/docs/ios/solutions_docs/ip_multicast/White_papers/rps.html

Best regards,

Peter

Mohamed Sobair Fri, 08/28/2009 - 11:43

Hi,

Anycast RP, allows for redundancy and Loadsharing purpose, when one of the RP fails the IGP reconverge to th second RP.

The RP with the lowest cost path prefered.

Also if the the total cost to both RPs are the same, the traffic to both RPs are loadshared.

Its certainly possible to have 2 or More RPs announce themselves through Auto-rp and BSR messages. But keep in mind that you cant have more than 1 mapping Agent announce it self to all routers part of the group.

HTH

Mohamed

Peter Paluch Fri, 08/28/2009 - 12:50

Hello Mohamed,

Why do you believe you cannot have more than one mapping agent? The AutoRP architecture allows you to have as many RP candidates as well as mapping agents.

But nevertheless, the main point of the OP was in my opinion whether the BSR or AutoRP by themselves provide the AnycastRP functionality. The fact is that they do not. Additional work by MSDP is required.

Best regards,

Peter

Mohamed Sobair Fri, 08/28/2009 - 13:05

Hello Peter,

I am not saying that MSDP is not required, I am saying its certainly possible to be done in Auto-rp and BSR.

Second point, its not my believes that Mapping agent cant be more than 1 , its technically the way its, I recall from my studies (CCIE studies) that you cant have more than one mapping Agent as you have for the RPs to be announced.

If you are not sure, please check the technical documentation in Cisco , and you will find the correct answer.

Pls rate all helpful posts ,

HTH

Mohamed

Correct Answer
Peter Paluch Fri, 08/28/2009 - 13:32

Hello Mohamed,

I am familiar with the technical documentation and in fact, it supports my standpoints. Perhaps there is a misunderstanding between the terms we both use, despite talking about the same thing. Let me discuss these things with you.

My standpoint is that AnycastRP cannot be successfully created without the help of MSDP. I am not saying that AutoRP or BSR are unusable or incompatible with AnycastRP - on the contrary, they are necessary and required also with AnycastRP, especially in large networks. What I am saying is that having AutoRP or BSR alone is not sufficient to provide AnycastRP service. You may have the AutoRP or BSR - absolutely no problem with that - but you must have MSDP in addition to those protocols in order to have a workable AnycastRP.

Quoting from the document "Anycast RP" at:

http://www.cisco.com/en/US/docs/ios/solutions_docs/ip_multicast/White_papers/anycast.html

Anycast RP allows two or more rendezvous points (RPs) to share the load for source registration and the ability to act as hot backup routers for each other. Multicast Source Discovery Protocol (MSDP) is the key protocol that makes Anycast RP possible.

So, the AnycastRP is a configuration of multiple RPs that all have the same IP address which has certain benefits. The means of making this IP address known to multicast routers in your domain is another business - you may use BSR, AutoRP or static configuration for that. The problem here is that if two or more routers advertise the same IP address in a routing protocol, the network will be effectively partitioned - each router will use the closer RP, according to the routing protocol metrics. Now, consider that there are two RPs with the same IP address, A and B. A multicast sender close to router A starts sending the multicast stream. The multicast stream will be sent to the RP A. Further on, consider that there is a recipient of this traffic close to the RP B. A shared tree will be created for this recipient with the root in RP B. However, the RP B has no idea that there is already is a sender for this multicast group because it has never received a multicast packet from it. Without an additional protocol, the recipient closer to the RP B will never start receiving the multicast stream sent by a sender close to RP A. This is where the MSDP comes in. As soon as RP A notices there is a sender for a particular multicast group, it informs its MSDP neighbors (the RP B in this case) about the existence of a sender to a particular multicast group. The RP B will now know about it, and when a recipient subscribes to this group, the RP B is able to send the Join towards the sender, thereby creating a branch of multicast distribution tree through which the multicast stream can flow natively. Notice that neither AutoRP nor BSR can help you here because they do not disseminate information about multicast senders. You absolutely must use the MSDP here, otherwise, you can't have a true AnycastRP.

Regarding the multiple mapping agents: Quoting from document "Multicast Quick-Start Configuration Guide" at:

http://www.cisco.com/en/US/tech/tk828/technologies_tech_note09186a0080094821.shtml

You can use one RP that also serves as the mapping agent, or you can configure multiple RPs and multiple mapping agents for redundancy purposes.

Also, quting from the internet draft where the AutoRP was first submitted to IETF (but never made it to RFC) available at:

ftp://ftp-eng.cisco.com/ipmulticast/pim-autorp-spec01.txt

For robustness purpose, multiple RP-mapping agents may be configured inside the same administrative domain. Each RP-mapping agent also listens to the the RP mapping discovery group CISCO-RP-DISCOVERY, and suppresses the sending of its own RP-mapping packets if it hears RP-mapping packets originated from another RP-mapping agent with a higher IP address.

I hope this clarifies the confusion in this field.

Best regards,

Peter

Mohamed Sobair Fri, 08/28/2009 - 13:55

Hello Peter,

please look at the bellow paragraph from Multicast Quick start Guide:

(Auto-RP with Multiple RPs

If two RPs announce their availability to be RPs for the same group(s), the mapping agent(s) resolve these conflicts with "the highest IP address wins" rule.)

Ok, I have one question if you allow me:

IF Two Mapping Agents announces their availability for the same Group, the router part of that group would choose which Mapping Agent?

Awaiting your reply,

Mohamed

Peter Paluch Fri, 08/28/2009 - 14:05

Hello Mohamed,

Regarding your question: there is no "choosing" of a mapping agent by a multicast router. A multicast router simply receives all mapping messages from any mapping agent. Refer to the quotation of the internet draft I have quoted earlier: If two or more mapping agents are configured, they also listen to each other. If a mapping agent receives a message from a mapping agent with a higher IP address, it stops sending its own mappings. After a while, only the mapping agent with the highest IP address remains speaking, all other just watch if it still lives.

However, notice that even if multiple mapping agents were sending their mappings simultaneously, they would be announcing the same mappings because the algorithm is simple and straightfoward. Therefore, even if a multicast router received mapping messages from multiple mapping agents, their contents would be identical in a well-configured network. So for a router, it is largely irrelevant how many mapping agents are out there - the important thing is that it receives the mappings at all.

Best regards,

Peter

Mohamed Sobair Sun, 08/30/2009 - 15:05

Hello Peter,

Ok, I agree that you could have multiple mapping agents but STILL you cant have more than one mapping agent active at a time as indicated in the documentation.

So my point is still valid, as routers dont recieve thier mapping information from more than one router at a time, and if the active Mapping agent fails , the secondary takes over.

HTH

Mohamed

Peter Paluch Sun, 08/30/2009 - 18:26

Hello Mohamed,

I agree with that. Originally, I had the feeling that you have excluded the possibility of having multiple mapping agents configured.

I am glad we reached a common point. It's been a pleasure for me to discuss these things with you.

Best regards,

Peter

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