RFC 4611
Multicast Source Discovery Protocol (MSDP) Deployment Scenarios
RFC 4611: Multicast Source Discovery Protocol (MSDP) Deployment Scenarios ✨
okay bestie, let's get the basics first 🎀 this is BCP 121, which is a Best Current Practice, so it's basically the community's "here's how to do this well" guide 💅 It came out in August 2006 and it's written by M. McBride, J. Meylor, and D. Meyer (the Network Working Group, if you want the official tag). It doesn't list anything it obsoletes or updates, so it's not replacing or changing any other RFC 🫶
Status of This Memo 📜
this one's the official "what is this doc" part 🌸 It's an Internet Best Current Practices document for the Internet Community, and the authors want people to talk about it and suggest improvements. Anyone can share it, no limits 🦋
Copyright Notice ©️
Copyright belongs to The Internet Society (2006) 💜
Abstract 🔮
so the whole point is giving you best practices for deploying MSDP (Multicast Source Discovery Protocol) in two flavours: intra-domain (inside one domain) and inter-domain (between domains), always working alongside PIM-SM (Protocol Independent Multicast Sparse Mode) 🧚
Table of Contents 🗺️
1. Introduction ....................................................2
1.1. BCP, Experimental Protocols, and Normative References ......3
2. Inter-domain MSDP Peering Scenarios .............................4
2.1. Peering between PIM Border Routers .........................4
2.2. Peering between Non-Border Routers .........................5
2.3. MSDP Peering without BGP ...................................7
2.4. MSDP Peering at a Multicast Exchange .......................7
3. Intra-domain MSDP Peering Scenarios .............................7
3.1. Peering between MSDP- and MBGP-Configured Routers ..........8
3.2. MSDP Peer Is Not BGP Peer (or No BGP Peer) .................8
3.3. Hierarchical Mesh Groups ...................................9
3.4. MSDP and Route Reflectors .................................10
3.5. MSDP and Anycast RPs ......................................11
4. Security Considerations ........................................11
4.1. Filtering SA Messages .....................................11
4.2. SA Message State Limits ...................................12
5. Acknowledgements ...............................................12
6. References .....................................................12
6.1. Normative References ......................................12
6.2. Informative References ....................................131. Introduction 🌈
okay so first things first, MSDP [RFC3618] is mainly used in two situations 💖
- Between PIM Domains 🌍
MSDP can be used between PIM-SM [RFC4601] domains to pass along info about active sources that live in other domains 👀 The peering is usually one-to-one, and it follows the deterministic peer-RPF (Reverse Path Forwarding) rules from the MSDP spec. That means it doesn't use mesh-groups 🙅♀️ You can aggregate peerings onto a single MSDP peer, and that peer can typically have anywhere from one to hundreds of peerings. Honestly that's similar in scale to BGP peerings, so it's a whole vibe 🔥
- Within a PIM Domain 🏠
MSDP is often used between Anycast Rendezvous Points (Anycast-RPs) [RFC3446] inside a PIM domain, so they can sync up on the active sources each Anycast-RP peer is serving (because of IGP reachability) ✨ Peering here is usually based on MSDP mesh groups, with anywhere from two to tens of peers in a group. More than ten isn't typical though 🙈 One or more of these mesh-group peers might also have extra one-to-one peerings with MSDP peers outside that PIM domain, so they can find external sources. And honestly? MSDP for anycast-RP without external MSDP peering is a totally valid option and super common 💅
Current best practice for MSDP deployment uses PIM-SM and the Border Gateway Protocol with multi-protocol extensions (MBGP) [RFC4271, RFC2858] 🎀 This doc explains how these protocols work together to give you an intra-domain and inter-domain Any Source Multicast (ASM) service 🦋
The PIM-SM spec assumes that SM only works in one PIM domain 🧐 MSDP is what lets you use multiple PIM domains, by sharing the info about active multicast sources with the other domains. And because the multicast infrastructure gets split up into multiple PIM domains, MSDP also makes it possible to set policy on which groups and sources are visible 🔐
Transit IP providers usually deploy MSDP so they're part of the global multicast infrastructure, by connecting to their upstream and peer multicast networks over MSDP 🌐
Edge multicast networks usually have two choices. They can use their Internet provider's RP, or they can run their own RP and connect it to their ISP using MSDP 🤔 Running their own RP and MSDP means they can use internal multicast groups that the provider's RP can't see 👑 That helps internal multicast keep working if the connection to the provider has problems, or if the provider's RP/MSDP is having a rough time 😭 In the simplest cases, where there are no internal multicast groups needed, there's often no reason to deploy MSDP at all 💀
1.1. BCP, Experimental Protocols, and Normative References 🧪
okay so this doc is the best current practice for a widely deployed Experimental protocol, MSDP 🍓 There's no plan to move MSDP's status up (to something like Proposed Standard, for example). The reasons are:
- MSDP was originally meant to be a temporary protocol, to get replaced by whatever the IDMR working group came up with as an inter-domain protocol 😅 But the IDMR WG (or later, the BGMP WG) never made a protocol that could actually be deployed to replace it.
- One of the main reasons MSDP got classified as Experimental is that the MSDP Working Group came up with changes that they thought made it better, but implementors didn't see a reason to deploy them 🙄 Without those changes (like UDP or GRE encapsulation), MSDP can cause negative consequences for the initial packets in datagram streams 😬
- Scalability: even though we don't know what the hard limits are, readvertising everything you know every 60 seconds clearly limits how much state you can advertise 📉
- MSDP ended up with nearly ubiquitous deployment as the de facto standard inter-domain multicast protocol in the IPv4 Internet 👑 It's everywhere, bestie.
- No consensus could be reached on reworking MSDP to deal with all the concerns from the different groups within the IETF 🫠 So they decided to document what's (ubiquitously) deployed and move that doc to Experimental. Advancing MSDP to Proposed Standard was considered, but for the reasons above it was immediately discarded 🚫
- Protocols like source-specific multicast and bi-directional PIM, plus embedded RP techniques for IPv6, have further reduced consensus that a replacement for MSDP is needed for the IPv4 Internet 🌸
The RFC Editor's policy on references is that they get split into two categories, "normative" and "informative" 📚 Normative references are the docs you must read to understand or implement the tech in an RFC (or whose tech has to be there for the tech in the new RFC to work) [RFCED]. To understand this doc, you also have to understand both the PIM and MSDP docs, so references to those are normative 🧠
The IETF has a policy that BCPs must not have normative references to Experimental protocols 🚧 But this doc is a special case, because the underlying Experimental doc (MSDP) isn't planned to be advanced to Proposed Standard 🥹
The MBONED Working Group asked for approval under the Variance Procedure, as documented in RFC 2026 [RFC2026] 🙏 The IESG followed the Variance Procedure and, after an extra 4 week IETF Last Call, looked at the comments and status and approved this doc ✅
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119] 🎯
2. Inter-domain MSDP Peering Scenarios 🌐
these sections cover the most common inter-domain MSDP peering setups and how you can deploy them 💖
2.1. Peering between PIM Border Routers 🧱
so in this case, the MSDP peers inside the domain have their own RP, which sits inside a bounded PIM domain 🏰 The domain usually has its own Autonomous System (AS) number and one or more MBGP speakers too. It may also have multiple MSDP speakers. Each border router has an MSDP and MBGP peering with its peer routers. These external MSDP peering deployments usually set up the MBGP peering and the MSDP peering using the same directly connected next hop peer IP address, or some other IP address from the same router 🧩 Typical deployments like this are providers with direct peering with other providers, providers peering at an exchange, or providers who use their edge router to MSDP/MBGP peer with customers 🤝
For a direct peering inter-domain environment to work, the first AS in the MBGP best path to the originating RP should be the same as the AS of the MSDP peer ✨ For example, check out this topology:
AS1----AS2----AS4
| /
| /
| /
AS3in this case AS4 gets a Source Active (SA) message, originated by AS1, from AS2 📨 AS2 also has an MBGP peering with AS4. The MBGP first hop AS from AS4, in the best path to the originating RP, is AS2. The AS of the sending MSDP peer is also AS2. So the peer-Reverse Path Forwarding check (peer-RPF check) passes, and the SA message gets forwarded ✅
A peer-RPF failure would happen in this topology when the MBGP first hop AS, in the best path to the originating RP, is AS2, and the origin AS of the sending MSDP peer is AS3 🚫 This reliance on BGP AS PATH info is what stops SA packets from looping endlessly 🔁
Router code that has adopted the latest rules in the MSDP document will loosen the rules between AS's a bit 🪄 In the following topology, we have an MSDP peering between AS1<->AS3 and AS3<->AS4:
RP
AS1----AS2----AS3----AS4if the first AS in the best path to the RP doesn't equal the MSDP peer, MSDP peer-RPF fails 😤 So AS1 can't MSDP peer with AS3, since AS2 is the first AS in the MBGP best path to the AS4 RP. With code that's compliant with the latest MSDP document, AS1 will pick the peer in the closest AS along the best AS path to the RP 🗺️ Then AS1 will accept SA's coming from AS3 🎉 If there are multiple MSDP peers to routers within the same AS, the peer with the highest IP address is chosen as the RPF peer 👑
2.2. Peering between Non-Border Routers 🌸
Okay so, for MSDP peering between border routers, intra-domain MSDP scalability is restricted, because you also have to keep MBGP and MSDP peerings running internally towards your border routers 🫠. Inside the intra-domain, the border router becomes the announcer of the next hop towards the originating RP. That means every intra-domain MSDP peering has to mirror the MBGP path back towards the border router. External MSDP (eMSDP) peerings rely on AS path for peer RPF checking, while internal MSDP (iMSDP) peerings rely on the announcer of the next hop. Main character energy, fr 💅
While the eMBGP peer is typically directly connected between border routers, it's common for the eMSDP peer to sit deeper inside the transit provider's AS. Providers who want more flexibility in MSDP peering placement usually pick a few dedicated routers in their core networks for the inter-domain MSDP peerings to their customers. These core MSDP routers will also typically be in the provider's intra-domain MSDP mesh group and be configured for Anycast RP. All multicast routers in the provider's AS should statically point to the Anycast RP address. Static RP assignment is the most commonly used method for group-to-RP mapping due to its deterministic nature ✨. Auto-RP [RFC4601] and/or the Bootstrap Router (BSR) [BSR] dynamic RP mapping mechanisms could also be used to disseminate RP information within the provider's network.
For an SA message to be accepted in this (multi-hop peering) environment, we rely upon the next (or closest, with latest MSDP spec) AS in the best path towards the originating RP for the RPF check. The MSDP peer address should be in the same AS as the AS of the border router's MBGP peer. The MSDP peer address should be advertised via MBGP 🦋.
For example, in the diagram below, if customer R1 router is MBGP peering with the R2 router and if R1 is MSDP peering with the R3 router, then R2 and R3 must be in the same AS (or must appear, to AS1, to be from the same AS in the event that private AS numbers are deployed). The MSDP peer with the highest IP address will be chosen as the MSDP RPF peer. R1 must also have the MSDP peer address of R3 in its MBGP table.
+--+ +--+ +--+
|R1|----|R2|----|R3|
+--+ +--+ +--+
AS1 AS2 AS2From R3's perspective, AS1 (R1) is the MBGP next AS in the best path towards the originating RP. As long as AS1 is the next AS (or closest) in the best path towards the originating RP, RPF will succeed on SAs arriving from R1 🎀.
In contrast, with the single hop scenario, with R2 (instead of R3) border MSDP peering with R1 border, R2's MBGP address becomes the announcer of the next hop for R3, towards the originating RP, and R3 must peer with that R2 address. Moreover, all AS2 intra-domain MSDP peers need to follow iMBGP (or other IGP) peerings towards R2, since iMSDP has a dependence upon peering with the address of the MBGP (or other IGP) announcer of the next hop 🙈.
2.3. MSDP Peering without BGP 🧚
In this case, an enterprise maintains its own RP and has an MSDP peering with its service provider but does not BGP peer with them. MSDP relies upon BGP path information to learn the MSDP topology for the SA peer-RPF check. MSDP can be deployed without BGP, though, and as a result, there are some special cases where the requirement to perform a peer-RPF check on the BGP path information is suspended. These cases are:
- There is only a single MSDP peer connection.
- A default peer (default MSDP route) is configured.
- The originating RP is directly connected.
- A mesh group is used.
- An implementation is used that allows for an MSDP peer-RPF check using an IGP.
An enterprise will typically configure a unicast default route from its border router to the provider's border router and then MSDP peer with the provider's MSDP router. If internal MSDP peerings are also used within the enterprise, then an MSDP default peer will need to be configured on the border router that points to the provider. In this way, all external multicast sources will be learned, and internal sources can be advertised 🌈. If only a single MSDP peering was used (no internal MSDP peerings) towards the provider, then this stub site will MSDP default peer towards the provider and skip the peer-RPF check.
2.4. MSDP Peering at a Multicast Exchange 💜
Multicast exchanges allow multicast providers to peer at a common IP subnet (or by using point-to-point virtual LANs) and share MSDP SA updates. Each provider will MSDP and MBGP peer with each other's directly connected exchange IP address. Each exchange router will send/receive SAs to/from their MSDP peers. They will then be able to forward SAs throughout their domain to their customers and any direct provider peerings 🍓.
3. Intra-domain MSDP Peering Scenarios ✨
The following sections describe the different intra-domain MSDP peering possibilities and their deployment options.
3.1. Peering between MSDP- and MBGP-Configured Routers 🔥
The next hop IP address of the iBGP peer is typically used for the MSDP peer-RPF check (IGP can also be used). This is different from the inter-domain BGP/MSDP case, where AS path information is used for the peer-RPF check. For this reason, it is necessary for the IP address of the MSDP peer connection to be the same as the internal MBGP peer connection, whether or not the MSDP/MBGP peers are directly connected. A successful deployment would be similar to the following:
+----+
| Rb | 3.3.3.3
/ +----+
AS1 AS2 / |
+---+ +--+ / |
|RP1|---------|Ra| |
+---+ +--+ |
1.1.1.1 2.2.2.2 |
\ |
\ |
\ +-----+
| RP2 |
+-----+where RP2 MSDP and MBGP peers with Ra (using 2.2.2.2) and with Rb (using 3.3.3.3). When the MSDP SA update arrives on RP2 from Ra, the MSDP RPF check for 1.1.1.1 passes, because RP2 receives the SA update from MSDP peer 2.2.2.2, which is also the correct MBGP next hop for 1.1.1.1 ✅.
When RP2 receives the same SA update from MSDP peer 3.3.3.3, the MBGP lookup for 1.1.1.1 shows a next hop of 2.2.2.2, so RPF correctly fails, preventing a loop 🛑.
This deployment could also fail on an update from Ra to RP2 if RP2 was MBGP peering to an address other than 2.2.2.2 on Ra. Intra-domain deployments must have MSDP and MBGP (or other IGP) peering addresses that match, unless a method to skip the peer-RPF check is deployed.
3.2. MSDP Peer Is Not BGP Peer (or No BGP Peer) 🌙
This is a common MSDP intra-domain deployment in environments where few routers are running MBGP or where the domain is not running MBGP. The problem here is that the MSDP peer address needs to be the same as the MBGP peer address. To get around this requirement, the intra-domain MSDP RPF rules have been relaxed in the following topologies:
- By configuring the MSDP peer as a mesh group peer.
- By having the MSDP peer be the only MSDP peer.
- By configuring a default MSDP peer.
- By peering with the originating RP.
- By relying upon an IGP for MSDP peer-RPF.
The common choice around the intra-domain BGP peering requirement, when more than one MSDP peer is configured, is to deploy MSDP mesh groups. When an MSDP mesh group is deployed, there is no RPF check on arriving SA messages when they are received from a mesh group peer. Subsequently, SA messages are always accepted from mesh group peers. MSDP mesh groups were developed to reduce the amount of SA traffic in the network, since SAs which arrive from a mesh group peer are not flooded to peers within that same mesh group. Mesh groups must be fully meshed 🕸️.
If recent (but not currently widely deployed) router code is running that is fully compliant with the latest MSDP document, another option, to work around not having BGP to MSDP RPF peer, is to RPF using an IGP like OSPF, IS-IS, RIP, etc. This new capability will allow for enterprise customers, who are not running BGP and who don't want to run mesh groups, to use their existing IGP to satisfy the MSDP peer-RPF rules 💖.
3.3. Hierarchical Mesh Groups 👑
Hierarchical mesh groups are occasionally deployed in intra-domain environments where there are a large number of MSDP peers. Allowing multiple mesh groups to forward to one another can reduce the number of MSDP peerings per router (due to the full mesh requirement) and hence reduce router load. A good hierarchical mesh group implementation (one that prevents looping) contains a core mesh group in the backbone, and these core routers serve as mesh group aggregation routers:
[R2]{A,2}
/ \
/ \
/ \
/ \
/ \
/ \
/ \
{A,1}[R1]-------------[R3]{A,3}In this example, R1, R2, and R3 are in MSDP mesh group A (the core mesh group), and each serves as MSDP aggregation routers for their leaf (or second tier) mesh groups 1, 2, and 3. Since SA messages received from a mesh group peer are not forwarded to peers within that same mesh group, SA messages will not loop. Do not create topologies that connect mesh groups in a loop. In the above example, for instance, second-tier mesh groups 1, 2, and 3 must not directly exchange SA messages with each other, or an endless SA loop will occur 🌀.
Redundancy between mesh groups will also cause a loop, and is subsequently not available with hierarchical mesh groups. For instance, assume that R3 had two routers connecting its leaf mesh group 3 with the core mesh group A. A loop would be created between mesh group 3 and mesh group A, because each mesh group must be fully meshed between peers 🙃.
3.4. MSDP and Route Reflectors 🦋
okay so BGP has this strict rule: every iBGP speaker that isn't a route-reflector client or a confederation member has to be fully meshed, or loops happen 🔁. In the route reflector world, MSDP needs the RR clients to peer with the route reflector, because the RR is the one announcing the next hop toward the originating RP 👑. The RR isn't the BGP next hop itself. It's the announcer of the BGP next hop, and that announcer is usually the address MSDP peer-RPF checks use 🧐. Ngl this is giving very particular.
Here's the setup:
Ra--------RR
/|\
/ | \
A B CRa forwards MSDP SAs to the route reflector RR. Routers A, B, and C also MSDP peer with RR. When RR forwards the SA to A, B, and C, those RR clients accept it, because RR is the announcer of the next hop to the originating RP address. It's literally the main character move 💅.
Now the plot twist 😭. An SA will fail peer-RPF if Ra MSDP peers directly with Routers A, B, or C, because the announcer of the next hop is RR but the SA came from Ra. So the proper move is to have the RR clients MSDP peer with the RR 🫶. MSDP mesh groups can be used to work around this requirement 🌸. External MSDP peerings also get around it, since the next AS is compared between MBGP and MSDP peerings, not the IP address of the announcer of the next hop ✨.
Some recent MSDP implementations follow the latest MSDP document, which relaxes the requirement to peer with the Advertiser of the next hop (the Route Reflector) 🎀. The new rule lets you peer with the next hop as well as the Advertiser of the next hop. Going back to the example, if Ra is the next hop (maybe because of BGP's next hop self attribute), and A, B, and C are peering with Ra, then the SAs received from Ra will now succeed 🙌.
3.5. MSDP and Anycast RPs 🌈
A network with multiple RPs can get RP load sharing and redundancy by combining the Anycast RP mechanism with MSDP mesh groups [RFC3446]. Service providers and enterprises do this all the time inside their domains when they run several RPs 💼. Each of these RPs has the same IP address configured on a Loopback interface, and that becomes the Anycast address 🧚. The RPs MSDP peer with each other over a separate loopback interface and all sit in the same fully meshed MSDP mesh group. That MSDP loopback is typically reused for the MBGP peering too 🔄.
Every router in the provider's domain learns the Anycast RP address through Auto-RP, BSR, or a static RP assignment 📡. Each designated router sends source registers and group joins to the Anycast RP address, and unicast routing sends those registers and joins to the nearest Anycast RP 📍. If one Anycast RP router fails, unicast routing sends the next registers and joins to the nearest remaining Anycast RP instead 🛟. That RP then forwards an MSDP update to all peers in the Anycast MSDP mesh group. From there, each RP forwards SAs to external customers and providers (or receives them from them) 🍓.
4. Security Considerations 🔐
An MSDP service should be secured by explicitly controlling the state that gets created by, and passed within, the MSDP service. Just like unicast routing state, MSDP state should be controlled locally, at the edge origination points 🏰. Selective filtering at the multicast service edge makes sure only intended sources cause SA message creation, and that control helps lower the odds of state-aggregation problems in the core 😤. There are a bunch of spots where local policy should be applied to the MSDP service, so be thorough bestie.
4.1. Filtering SA Messages 🚫
Originating SA messages should be filtered so that only intended local sources actually get SA messages originated. MSDP speakers should also filter which SA messages get received and forwarded 🧹.
PIM-SM domains usually have a fair amount of (S,G) state that stays local to the domain. Without proper filtering though, SA messages carrying those local (S,G) announcements can get advertised to the global MSDP infrastructure 😬. Examples are domain-local applications that use global IP multicast addresses, and sources that use RFC 1918 addresses [RFC1918]. To make MSDP scale better and avoid global visibility of domain-local (S,G) information, an external SA filter list is recommended. It helps stop unnecessary creation, forwarding, and caching of well-known domain-local sources 🙈.
4.2. SA Message State Limits 🚨
Proper filtering on SA message origination, receipt, and forwarding will significantly cut down the odds of unintended and unexpected spikes in MSDP state 💖. Still, an SA-cache state limit SHOULD be configured as a final safeguard against state spikes 🛡️. Once an MSDP peering reaches a stable state (meaning the peering is established and the initial SA state has been transferred), it may also be desirable to configure a rate limiter for creating new SA state entries 🦋.
5. Acknowledgements 🫶
The authors want to thank Pekka Savola, John Zwiebel, Swapna Yelamanchi, Greg Shepherd, and Jay Ford for their feedback on earlier versions of this document 💜.
6. References 📚
6.1. Normative References
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas, "Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)", RFC 4601, August 2006.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G., and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, February 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2858] Bates, T., Rekhter, Y., Chandra, R., and D. Katz, "Multiprotocol Extensions for BGP-4", RFC 2858, June 2000.
[RFC3446] Kim, D., Meyer, D., Kilmer, H., and D. Farinacci, "Anycast Rendevous Point (RP) mechanism using Protocol Independent Multicast (PIM) and Multicast Source Discovery Protocol (MSDP)", RFC 3446, January 2003.
[RFC3618] Fenner, B. and D. Meyer, "Multicast Source Discovery Protocol (MSDP)", RFC 3618, October 2003.
6.2. Informative References
[BSR] Fenner, W., et. al., "Bootstrap Router (BSR) Mechanism for PIM Sparse Mode", Work in Progress, February 2003.
[RFCED] http://www.rfc-editor.org/policy.html
Authors' Addresses 🖊️
Mike McBride Cisco Systems EMail: mcbride@cisco.com
John Meylor Cisco Systems EMail: jmeylor@cisco.com
David Meyer EMail: dmm@1-4-5.net
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