CCIE SPv5.1 Labs
  • Intro
    • Setup
  • Purpose
  • Video Demonstration
  • Containerlab Tips
  • Labs
    • ISIS
      • Start
      • Topology
      • Prefix Suppression
      • Hello padding
      • Overload Bit
      • LSP size
      • Default metric
      • Hello/Hold Timer
      • Mesh groups
      • Prefix Summarization
      • Default Route Preference
      • ISIS Timers
      • Log Neighbor Changes
      • Troubleshooting 1 - No routes
      • Troubleshooting 2 - Adjacency
      • IPv6 Single Topology
      • IPv6 Single Topology Challenge
      • IPv6 Multi Topology
      • IPv6 Single to Multi Topology
      • Wide Metrics Explained
      • Route Filtering
      • Backdoor Link
      • Non-Optimal Intra-Area routing
      • Multi Area
      • Authentication
      • Conditional ATT Bit
      • Troubleshooting iBGP
      • Troubleshooting TE Tunnel
    • LDP
      • Start
      • Topology
      • LDP and ECMP
      • LDP and Static Routes
      • LDP Timers
      • LDP Authentication
      • LDP Session Protection
      • LDP/IGP Sync (OSPF)
      • LDP/IGP Sync (ISIS)
      • LDP Local Allocation Filtering
      • LDP Conditional Label Advertisement
      • LDP Inbound Label Advertisement Filtering
      • LDP Label Advertisement Filtering Challenge
      • LDP Implicit Withdraw
      • LDP Transport Address Troubleshooting
      • LDP Static Labels
    • MPLS-TE
      • Start
      • Topology
      • Basic TE Tunnel w/ OSPF
      • Basic TE Tunnel w/ ISIS
      • TE Tunnel using Admin Weight
      • TE Tunnel using Link Affinity
      • TE Tunnel with Explicit-Null
      • TE Tunnel with Conditional Attributes
      • RSVP message pacing
      • Reoptimization timer
      • IGP TE Flooding Thresholds
      • CSPF Tiebreakers
      • TE Tunnel Preemption
      • TE Tunnel Soft Preemption
      • Tunneling LDP inside RSVP
      • PE to P TE Tunnel
      • Autoroute Announce Metric (XE)
      • Autoroute Announce Metric (XR)
      • Autoroute Announce Absolute Metric
      • Autoroute Announce Backup Path
      • Forwarding Adjacency
      • Forwarding Adjacency with OSPF
      • TE Tunnels with UCMP
      • Auto-Bandwidth
      • FRR Link Protection (XE, BFD)
      • FRR Link Protection (XE, RSVP Hellos)
      • FRR Node Protection (XR)
      • FRR Path Protection
      • FRR Multiple Backup Tunnels (Node Protection)
      • FRR Multiple Backup Tunnels (Link Protection)
      • FRR Multiple Backup Tunnels (Backwidth/Link Protection)
      • FRR Backup Auto-Tunnels
      • FRR Backup Auto-Tunnels with SRLG
      • Full Mesh Auto-Tunnels
      • Full Mesh Dynamic Auto-Tunnels
      • One-Hop Auto-Tunnels
      • CBTS/PBTS
      • Traditional DS-TE
      • IETF DS-TE with MAM
      • IETF DS-TE with RDM
      • RDM w/ FRR Troubleshooting
      • Per-VRF TE Tunnels
      • Tactical TE Issues
      • Multicast and MPLS-TE
    • SR
      • Start
      • Topology
      • Basic SR with ISIS
      • Basic SR with OSPF
      • SRGB Modifcation
      • SR with ExpNull
      • SR Anycast SID
      • SR Adjacency SID
      • SR LAN Adjacency SID (Walkthrough)
      • SR and RSVP-TE interaction
      • SR Basic Inter-area with ISIS
      • SR Basic Inter-area with OSPF
      • SR Basic Inter-IGP (redistribution)
      • SR Basic Inter-AS using BGP
      • SR BGP Data Center (eBGP)
      • SR BGP Data Center (iBGP)
      • LFA
      • LFA Tiebreakers (ISIS)
      • LFA Tiebreakers (OSPF)
      • Remote LFA
      • RLFA Tiebreakers?
      • TI-LFA
      • Remote LFA or TILFA?
      • TI-LFA Node Protection
      • TI-LFA SRLG Protection
      • TI-LFA Protection Priorities (ISIS)
      • TI-LFA Protection Priorities (OSPF)
      • Microloop Avoidance
      • SR/LDP Interworking
      • SR/LDP SRMS OSPF Inter-Area
      • SR/LDP Design Challenge #1
      • SR/LDP Design Challenge #2
      • Migrate LDP to SR (ISIS)
      • OAM with SR
      • SR-MPLS using IPv6
      • Basic SR-TE with AS
      • Basic SR-TE with AS and ODN
      • SR-TE with AS Primary/Secondary Paths
      • SR-TE Dynamic Policies
      • SR-TE Dynamic Policy with Margin
      • SR-TE Explicit Paths
      • SR-TE Disjoint Planes using Anycast SIDs
      • SR-TE Flex-Algo w/ Latency
      • SR-TE Flex-Algo w/ Affinity
      • SR-TE Disjoint Planes using Flex-Algo
      • SR-TE BSIDs
      • SR-TE RSVP-TE Stitching
      • SR-TE Autoroute Include
      • SR Inter-IGP using PCE
      • SR-TE PCC Features
      • SR-TE PCE Instantiated Policy
      • SR-TE PCE Redundancy
      • SR-TE PCE Redundancy w/ Sync
      • SR-TE Basic BGP EPE
      • SR-TE BGP EPE for Unified MPLS
      • SR-TE Disjoint Paths
      • SR Converged SDN Transport Challenge
      • SR OAM DPM
      • SR OAM Tools
      • Performance-Measurement (Interface Delay)
    • SRv6
      • Start
      • Topology
      • Basic SRv6
      • SRv6 uSID
      • SRv6 uSID w/ EVPN-VPWS and BGP IPv4/IPv6
      • SRv6 uSID w/ SR-TE
      • SRv6 uSID w/ SR-TE Explicit Paths
      • SRv6 uSID w/ L3 IGW
      • SRv6 uSID w/ Dual-Connected PE
      • SRv6 uSID w/ Flex Algo
      • SRv6 uSID - Scale (Pt. 1)
      • SRv6 uSID - Scale (Pt. 2)
      • SRv6 uSID - Scale (Pt. 3) (UPA Walkthrough)
      • SRv6 uSID - Scale (Pt. 4) (Flex Algo)
      • SRv6 uSID w/ TI-LFA
    • Multicast
      • Start
      • Topology
      • Basic PIM-SSM
      • PIM-SSM Static Mapping
      • Basic PIM-SM
      • PIM-SM with Anycast RP
      • PIM-SM with Auto-RP
      • PIM-SM with BSR
      • PIM-SM with BSR for IPv6
      • PIM-BiDir
      • PIM-BiDir for IPv6
      • PIM-BiDir with Phantom RP
      • PIM Security
      • PIM Boundaries with AutoRP
      • PIM Boundaries with BSR
      • PIM-SM IPv6 using Embedded RP
      • PIM SSM Range Note
      • PIM RPF Troubleshooting #1
      • PIM RPF Troubleshooting #2
      • PIM RP Troubleshooting
      • PIM Duplicate Traffic Troubleshooting
      • Using IOS-XR as a Sender/Receiver
      • PIM-SM without Receiver IGMP Joins
      • RP Discovery Methods
      • Basic Interdomain Multicast w/o MSDP
      • Basic Interdomain Multicast w/ MSDP
      • MSDP Filtering
      • MSDP Flood Reduction
      • MSDP Default Peer
      • MSDP RPF Check (IOS-XR)
      • MSDP RPF Check (IOS-XE)
      • Interdomain MBGP Policies
      • PIM Boundaries using MSDP
    • MVPN
      • Start
      • Topology
      • Profile 0
      • Profile 0 with data MDTs
      • Profile 1
      • Profile 1 w/ Redundant Roots
      • Profile 1 with data MDTs
      • Profile 6
      • Profile 7
      • Profile 3
      • Profile 3 with S-PMSI
      • Profile 11
      • Profile 11 with S-PMSI
      • Profile 11 w/ Receiver-only Sites
      • Profile 9 with S-PMSI
      • Profile 12
      • Profile 13
      • UMH (Upstream Multicast Hop) Challenge
      • Profile 13 w/ Configuration Knobs
      • Profile 13 w/ PE RP
      • Profile 12 w/ PE Anycast RP
      • Profile 14 (Partitioned MDT)
      • Profile 14 with Extranet option #1
      • Profile 14 with Extranet option #2
      • Profile 14 w/ IPv6
      • Profile 17
      • Profile 19
      • Profile 21
    • MVPN SR
      • Start
      • Topology
      • Profile 27
      • Profile 27 w/ Constraints
      • Profile 27 w/ FRR
      • Profile 28
      • Profile 28 w/ Constraints and FRR
      • Profile 28 w/ Data MDTs
      • Profile 29
    • VPWS
      • Start
      • Topology
      • Basic VPWS
      • VPWS with Tag Manipulation
      • Redundant VPWS
      • Redundant VPWS (IOS-XR)
      • VPWS with PW interfaces
      • Manual VPWS
      • VPWS with Sequencing
      • Pseudowire Logging
      • VPWS with FAT-PW
      • MS-PS (Pseudowire stitching)
      • VPWS with BGP AD
    • VPLS
      • Start
      • Topology
      • Basic VPLS with LDP
      • VPLS with LDP and BGP
      • VPLS with BGP only
      • Hub and Spoke VPLS
      • Tunnel L2 Protocols over VPLS
      • Basic H-VPLS
      • H-VPLS with BGP
      • H-VPLS with QinQ
      • H-VPLS with Redundancy
      • VPLS with Routing
      • VPLS MAC Protection
      • Basic E-TREE
      • VPLS with LDP/BGP-AD and XRv RR
      • VPLS with BGP and XRv RR
      • VPLS with Storm Control
    • EVPN
      • Start
      • Topology
      • EVPN VPWS
      • EVPN VPWS Multihomed
      • EVPN VPWS Multihomed Single-Active
      • Basic Single-homed EVPN E-LAN
      • EVPN E-LAN Service Label Allocation
      • EVPN E-LAN Ethernet Tag
      • EVPN E-LAN Multihomed
      • EVPN E-LAN on XRv
      • EVPN IRB
      • EVPN-VPWS Multihomed IOS-XR (All-Active)
      • EVPN-VPWS Multihomed IOS-XR (Port-Active)
      • EVPN-VPWS Multihomed IOS-XR (Single-Active)
      • EVPN-VPWS Multihomed IOS-XR (Non-Bundle)
      • PBB-EVPN (Informational)
    • BGP Multi-Homing (XE)
      • Start
      • Topology
      • Lab1 ECMP
      • Lab2 UCMP
      • Lab3 Backup Path
      • Lab4 Shadow Session
      • Lab5 Shadow RR
      • Lab6 RR with Add-Path
      • Lab7 MPLS + Add Path ECMP
      • Lab8 MPLS + Shadow RR
      • Lab9 MPLS + RDs + UCMP
    • BGP Multi-Homing (XR)
      • Start
      • Topology
      • Lab1 ECMP
      • Lab2 UCMP
      • Lab3 Backup Path
      • Lab4 “Shadow Session”
      • Lab5 “Shadow RR”
      • Lab6 RR with Add-Path
      • Lab7 MPLS + Add Path ECMP
      • Lab8 MPLS + “Shadow RR”
      • Lab9 MPLS + RDs + UCMP
      • Lab10 MPLS + Same RD + Add-Path + UCMP
      • Lab11 MPLS + Same RD + Add-Path + Repair Path
    • BGP
      • Start
      • Conditional Advertisement
      • Aggregation and Deaggregation
      • Local AS
      • BGP QoS Policy Propagation
      • Non-Optimal eBGP Routing
      • Multihomed Enterprise Challenge
      • Provider Communities
      • Destination-Based RTBH
      • Destination-Based RTBH (Community-Based)
      • Source-Based RTBH
      • Source-Based RTBH (Community-Based)
      • Multihomed Enterprise Challenge (XRv)
      • Provider Communities (XRv)
      • DMZ Link BW Lab1
      • DMZ Link BW Lab2
      • PIC Edge in the Global Table
      • PIC Edge Troubleshooting
      • PIC Edge for VPNv4
      • AIGP
      • AIGP Translation
      • Cost-Community (iBGP)
      • Cost-Community (confed eBGP)
      • Destination-Based RTBH (VRF Provider-triggered)
      • Destination-Based RTBH (VRF CE-triggered)
      • Source-Based RTBH (VRF Provider-triggered)
      • Flowspec (Global IPv4/6PE)
      • Flowspec (VRF)
      • Flowspec (Global IPv4/6PE w/ Redirect)
      • Flowspec (Global IPv4/6PE w/ Redirect) T-Shoot
      • Flowspec (VRF w/ Redirect)
      • Flowspec (Global IPv4/6PE w/ CE Advertisement)
    • Intra-AS L3VPN
      • Start
      • Partitioned RRs
      • Partitioned RRs with IOS-XR
      • RT Filter
      • Non-Optimal Multi-Homed Routing
      • Troubleshoot #1 (BGP)
      • Troubleshoot #2 (OSPF)
      • Troubleshoot #3 (OSPF)
      • Troubleshoot #4 (OSPF Inter-AS)
      • VRF to Global Internet Access (IOS-XE)
      • VRF to Global Internet Access (IOS-XR)
    • Inter-AS L3VPN
      • Start
      • Inter-AS Option A
      • Inter-AS Option B
      • Inter-AS Option C
      • Inter-AS Option AB (D)
      • CSC
      • CSC with Option AB (D)
      • Inter-AS Option C - iBGP LU
      • Inter-AS Option B w/ RT Rewrite
      • Inter-AS Option C w/ RT Rewrite
      • Inter-AS Option A Multi-Homed
      • Inter-AS Option B Multi-Homed
      • Inter-AS Option C Multi-Homed
    • Russo Inter-AS
      • Start
      • Topology
      • Option A L3NNI
      • Option A L2NNI
      • Option A mVPN
      • Option B L3NNI
      • Option B mVPN
      • Option C L3NNI
      • Option C L3NNI w/ L2VPN
      • Option C mVPN
    • BGP RPKI
      • Start
      • RPKI on IOS-XE (Enabling the feature)
      • RPKI on IOS-XE (Validation)
      • RPKI on IOS-XR (Enabling the feature)
      • Enable SSH in Routinator
      • RPKI on IOS-XR (Validation)
      • RPKI on IOS-XR (RPKI Routes)
      • RPKI on IOS-XR (VRF)
      • RPKI iBGP Mesh (No Signaling)
      • RPKI iBGP Mesh (iBGP Signaling)
    • NAT
      • Start
      • Egress PE NAT44
      • NAT44 within an INET VRF
      • Internet Reachability between VRFs
      • CGNAT
      • NAT64 Stateful
      • NAT64 Stateful w/ Static NAT
      • NAT64 Stateless
      • MAP-T BR
    • BFD
      • Start
      • Topology
      • OSPF Hellos
      • ISIS Hellos
      • BGP Keepalives
      • PIM Hellos
      • Basic BFD for all protocols
      • BFD Asymmetric Timers
      • BFD Templates
      • BFD Tshoot #1
      • BFD for Static Routes
      • BFD Multi-Hop
      • BFD for VPNv4 Static Routes
      • BFD for VPNv6 Static Routes
      • BFD for Pseudowires
    • QoS
      • Start
      • QoS on IOS-XE
      • Advanced QoS on IOS-XE Pt. 1
      • Advanced QoS on IOS-XE Pt. 2
      • MPLS QoS Design
      • Notes - QoS on IOS-XR
    • NSO
      • Start
      • Basic NSO Usage
      • Basic NSO Template Service
      • Advanced NSO Template Service
      • Advanced NSO Template Service #2
      • NSO Template vs. Template Service
      • NSO API using Python
      • NSO API using Python #2
      • NSO API using Python #3
      • Using a NETCONF NED
      • Python Service
      • Nano Services
    • MDT
      • Start
      • MDT Server Setup
      • Basic Dial-Out
      • Filtering Data using XPATH
      • Finding the correct YANG model
      • Finding the correct YANG model #2
      • Event-Driven MDT
      • Basic Dial-In using gNMI
      • Dial-Out with TLS
      • Dial-In with TLS
      • Dial-In with two-way TLS
    • App-Hosting
      • Start
      • Lab - iperf3 Docker Container
      • Notes - LXC Container
      • Notes - Native Applications
      • Notes - Process Scripts
    • ZTP
      • Notes - Classic ZTP
      • Notes - Secure ZTP
    • L2 Connectivity Notes
      • 802.1ad (Q-in-Q)
      • MST-AG
      • MC-LAG
      • G.8032
    • Ethernet OAM
      • Start
      • Topology
      • CFM
      • y1731
      • Notes - y1564
    • Security
      • Start
      • Notes - Security ACLs
      • Notes - Hybrid ACLs
      • Notes - MPP (IOS-XR)
      • Notes - MPP (IOS-XE)
      • Notes - CoPP (IOS-XE)
      • Notes - LPTS (IOS-XR)
      • Notes - WAN MACsec White Paper
      • Notes - WAN MACsec Config Guide
      • Notes - AAA
      • Notes - uRPF
      • Notes - VTY lines (IOS-XR)
      • Lab - uRPF
      • Lab - MPP
      • Lab - AAA (IOS-XE)
      • Lab - AAA (IOS-XR)
      • Lab - CoPP and LPTS
    • Assurance
      • Start
      • Notes - Syslog on IOS-XE
      • Notes - Syslog on IOS-XR
      • Notes - SNMP Traps
      • Syslog (IOS-XR)
      • RMON
      • Netflow (IOS-XE)
      • Netflow (IOS-XR)
Powered by GitBook
On this page
  • Answer
  • Explanation
  • Verification
  • Summary
  1. Labs
  2. Multicast

Basic Interdomain Multicast w/ MSDP

PreviousBasic Interdomain Multicast w/o MSDPNextMSDP Filtering

Last updated 1 month ago

Load inter-as.multicast.msdp.init.cfg

#IOS-XE
config replace flash:inter-as.multicast.msdp.init.cfg
 
#IOS-XR
configure
load bootflash:inter-as.multicast.msdp.init.cfg
commit replace
y

Configure a partial mesh of MSDP peerings between all RPs. Do not peer CSR9 and XRv2. All MSDP peerings must use the loopbacks.

Use the password CCIE123 for all peering sessions, and set the keepalive/hold timer values to 10/25 seconds. (By default this is 30/75 seconds).

Join the group 226.1.2.3 on CSR3 and ping this from CSR2.

Answer

#CSR10
ip msdp peer 7.0.0.11 connect-source lo0
ip msdp peer 8.0.0.12 connect-source lo0
ip msdp peer 11.0.0.9 connect-source lo0
!
ip msdp password peer 7.0.0.11 CCIE123
ip msdp password peer 8.0.0.12 CCIE123
ip msdp password peer 11.0.0.9 CCIE123
!
ip msdp keepalive 7.0.0.11 10 25
ip msdp keepalive 8.0.0.12 10 25
ip msdp keepalive 11.0.0.9 10 25

#CSR9
ip msdp peer 7.0.0.11 connect-source lo0
ip msdp peer 9.0.0.10 connect-source lo0
!
ip msdp password peer 7.0.0.11 CCIE123
ip msdp password peer 9.0.0.10 CCIE123
!
ip msdp keepalive 7.0.0.11 10 25
ip msdp keepalive 9.0.0.10 10 25

#XR1
router msdp
 peer 9.0.0.10
  password encrypted 0130252D7E5A545C
  connect-source Loopback0
  keepalive 10 25
 !
 peer 11.0.0.9
  password encrypted 0130252D7E5A545C
  connect-source Loopback0
  keepalive 10 25
 !
 peer 8.0.0.12
  password encrypted 0130252D7E5A545C
  connect-source Loopback0
  keepalive 10 25

#XR2
router msdp
 peer 9.0.0.10
  password encrypted 0130252D7E5A545C
  connect-source Loopback0
  keepalive 10 25
 !
 peer 7.0.0.11
  password encrypted 0130252D7E5A545C
  connect-source Loopback0
  keepalive 10 25

#CSR3
int gi2.538
 ip igmp join-group 236.1.2.3

Explanation

MSDP allows RPs in separate domains to share active sources with one another. In intra-domain multicast, the FHR registers the source with the RP. However, when there are multiple RPs, every other RP besides the one the FHR registers with will be unaware of the active source. All of these RPs will be unable to “bring together” the source and interested receivers.

In MSDP, peers are defined in a similar fashion to BGP. The connect-source is the TCP source address. The originator-id (not used in this lab) is the originating RP address used when sourcing SA messages. This is important in an Anycast RP setup. (Unless there is only one default MSDP peer, an RFC check is done against the originator ID. If the receiving RP sees that its own Anycast RP address is in the origiantor ID, the SA message fails the RPF check. For this reason, you should manually set the originator-id to the unique loopback on each Anycast RP).

An MSDP peer will reflect an SA message to all other peers. In a way this works like eBGP. In order to prevent an SA message from looping endlessly, an RPF check is used. An SA message is only accepted if the RPF check succeeds. This is a complicated process which is described in later labs. In this lab, the RPF check that XR2 does for R10’s SA is successful because R10 is itself the originator of the SA.

Once a remote RP learns of an active source via an SA from an MSDP peer, and the RP has matching (*, G) state, the RP can pull in the traffic by joining a (S, G) tree to the source, just as it would in response to a PIM Register. This allows multicast to work between two different domains. As long as the remote AS has an RPF-valid path back to the source (which requires PIM to be enabled on the inter-AS links), traffic should work.

We have a few options when using MSDP. We can set a TCP password (similar to BGP), adjust the keepalive timers, and perform SA filtering. This lab tests your understanding of the MSDP password and timers. The timers do not need to match for MSDP to work. But if the hold time on one side is less than the keepalive on the other side, the MSDP peering will flap continuously. Also note that the TCP password can be set after the session is already up, and it won’t affect the peering session. It will only take effect once the session is restarted. (BGP works the same way).

MSDP is sometimes called a “napkin” protocol. It seems to have been developed a little hap-hazardly. The TCP session is opened and established with apparently no data at all. There is no negotiation or “open” message like with BGP. A simple three-way handshake and keepalives keep the session up. When a new source goes active, an SA is sent within the TCP session. The SA simply contains the encapsulated data, the Source, Group, and originating RP address.

Verification

On each router, you can use show ip msdp sum to view a summary of MSDP peers.

Currently, only CSR8 and XR2 have state for (*, 236.1.2.3) because R3 has joined this ASM group. CSR8 is the LHR and XR2 is the RP for the domain. The distribution tree is currently constrained to that domain. XR2 will need to be notified of new senders to distribute the traffic to R3.

On CSR2, ping this group (236.1.2.3). R5 will register the source with the RP, R10. R10 will send a SA message to all MSDP peers with the multicast data encapsulated in the SA message. An example of an SA message is shown below. The SA message is sent to R9, but the original multicast packet is encapsulated inside. The SA message simply has the RP address (set using the originator-ID command), and one or more S, G blocks. The S prefix length must always be 32.

Somewhat like eBGP, MSDP peers will propagate SAs to all other MSDP peers. For example, R9 forwards this SA message onto its other peer, XR1 (shown below). The SA message is reflected exactly as it is received. The RP address does not change. Because there’s no notion of an AS Path in MSDP, some other mechanism must be used for loop prevention. What is used is a complex set of RPF check rules, which we will detail later.

On R10 we can see the SA that has been originated to each peer using show ip msdp peer advertised-SAs.

Notice above that each entry is under the “from mroute table” section, not the “from SA cache” section. If an SA message is reflected to another MSDP peer, it will appear in the “from SA cache” section instead. For example, we can see that CSR9 reflected this SA message to XR1 from its SA cache, not from its mroute table.

We can see the SAs accepted from a peer using show ip msdp peer accepted-SAs

We can also see the entirety of the SA cache using show ip msdp sa-cache. It is important that each RP caches SA messages even if the RP does not have matching (*, G) state. This reduces the join latency if a receiver in the local domain joins this group later.

As a side note, MSDP has a mechanism to query other peers for SA messages. If a router does not want to cache SA state, it can instead rely on another router which is caching SAs, and upon receiving a new (*, G) join, it can ask the MSDP peer if it has any cached SAs for that group. However, IOS-XE does not appear to let us disable the default command ip msdp cache-sa-state, so this mechanism is not relevant to us. It seems IOS-XR allows us to filter which SAs we cache, but not query another router for SAs.

Back to our topology. Remember that to start, XR2 is the only router with (*, G) state. Upon learning of the source, XR2 joins the (S, G). Because PIM is running on the inter-AS links, and XR2 has a route to CSR2, the (S, G) forms successfully. On XR2, we see an E flag on the (S, G) entry, which indicates that the source is external (learned via MSDP):

On R10 we see an A flag on the (S, G) entry, because it is advertised via MSDP:

We can see on CSR6 that the (S, G) has an outgoing interface of the link to XR2, because XR2 has joined the (S, G) tree upon learning of the source.

On CSR8 we should see that the hardware forwarding count for this entry is increasing if we keep sending packets from CSR2. (CSR3 is not running ip multicast-routing, so it does not respond to the pings).

Let’s briefly explore the MRIB states when the roles are reversed: XR2 is advertising a source and R10 has joined the (S, G) tree. We’ll join a group from R2 and ping this from R3.

#R2
int Gi2.525
 ip igmp join-group 236.100.1.1

#R3
ping 236.100.1.1

R10 shows an M flag because the entry was created via an MSDP entry. The RPF check is done using MBGP.

XR2 does not appear to show anything indicating that it is advertising the mapping via MSDP. All we see is an L flag for “domain-local scope”:

Summary

MSDP is the key ingredient to achieving interdomain ASM. It is important that each domain has full control over its own RP. ASM relies on PIM Registers which allow the RP to learn of the active sources, and then pair the source traffic with the RPT multicast distribution tree. For this to work for interdomain ASM, each RP must announce active sources to all other RPs in other domains. This is the role of MSDP.

Interdomain ASM also requires PIM on all interdomain links. This allows PIM Joins to work, which build the interdomain (S, G) tree. PIM also requires the RPF check on the source to succeed as usual. Using BGP ipv4/multicast and ipv6/multicast, we can create a separate multicast topology from the unicast topology. The BGP ipv4/multicast or ipv6/multicast routes are preferred for the RPF check, and a PIM Join will be sent out the corresponding interface to the BGP route’s nexthop.

It’s interesting to realize that PIM has no notion of internal or external neighborships. PIM simply forms adjacencies on links on which it is enabled. The definition of PIM domains for ASM can then be thought as “where the RP information stops.” By bounding the propagation of the RP, for example by using BSR with ip pim bsr-border, you define the ASM domain. MSDP is run between the RPs in each domain simply to alert remote RPs of active senders.