November 20, 2020

My multicast network is a mess! Please help!

by Fabrizio Maccioni

Multicast IP Routing protocols have been increasingly popular over the last several years. 

They are used to efficiently distribute data to a group of destination hosts simultaneously in a one-to-many or many-to-many model

Typical examples of applications where the use of multicast is very common are audio/video streaming services (e.g content delivery networks, IPTV) and financial trading platforms (e.g Stock Exchanges). 

This blog is not intended to be a multicast tutorial but, but it aims to showcase how Forward can map and search multicast-enabled networks. Before going there, just a quick recap, at a very high level, of the multicast components: 

  • Multicast Source: a host sending a multicast stream of data  
  • Multicast Client: a host interested in receiving a multicast stream of data 
  • Multicast Router: a network device that is able to forward multicast traffic 
  • Rendezvous Point (RP): a router that acts as a shared root for a multicast shared tree 
  • Multicast Routing Protocol: a protocol used by Multicast Routers to forward multicast traffic from a source to clients 
  • Group Management Protocol: a protocol used by clients to signal that they are interested in receiving the streaming of data 
  • Multicast IP address: an IP address in the range 224.0.0.0 through 239.255.255.255 for IPv4 while in IPv6 it’s an IP address with a prefix ff00::/8 
  • Multicast Group: a group of hosts that includes all the senders and all the receivers. It is identified by a Multicast IP address. 

Putting all together, a Multicast Client signals that it’s interested in receiving a multicast stream identified by a Multicast IP address using a Group Management Protocol (e.g. IGMPv3). 

Multicast Source sends a multicast stream of data to a Multicast Group and the Multicast Routers forward the multicast streams to the Multicast Clients using a Multicast Routing Protocol (e.g. PIM-SM). 

If your background is more on applications, the concept is similar to a Pub/Sub (Publisher/Subscriber) messaging service where the Publishers in multicast are the Multicast Sources and the Subscribers are the Multicast Clients. 

Easy right? 

Well, the concept might be easy, and the basic configuration can be fairly straightforward but when something doesn’t work it’s a nightmare to troubleshoot a multicast network. 

This is where Forward Network can help you make, one more time, your life way easier! 

Forward Enterprise allows you to get full visibility and analysis of your multicast network by searching for multicast insights and performing multicast path analysis in the Forward Search application. 

Search for multicast group 

To search for multicast groups, all you have to do is navigate to the Search application and type the multicast group IP in the search bar. 

Forward Search automatically recognizes the IP address as a Multicast group. 

Figure 1 – Multicast group auto-discovery 

Let’s dive into the info provided by the Forward platform for the given multicast group: 

Figure 2 – Multicast Group IP query results 

The Search Results pane provides a Multicast group category where you can find information about the Rendezvous Point name and IP address, the VRF where the group is configured in and direct links to the Rendezvous Point configuration and state for the given multicast group. 

The Multicast Group Card in the middle pane provides an at-a-glance view of all the group members as well as direct links to the configuration and state for the given multicast group for each member. 

The Topology map on the right is context-aware and highlights the devices relevant to the content you are navigating, like the Rendezvous Point, the devices in the VRF where the RP is configured, and specific members. 

You can also search multicast groups by subnet. 

In this case, all the matching multicast groups are shown in the Search Results along with any RP information. At this point you can select one multicast group to get more information as in the previous example. 

Figure 3 – Multicast groups subnet query 


Multicast Path Analysis 

There are several IP Multicast protocols but the most commonly used are Protocol-Independent Multicast – Sparse Mode (or PIM-SM) at L3 and IGMPv3 at L2. 

With PIM-SM, multicast traffic flows on primarily two types of routes: (S,G) or s-coma-g and (*,G) or star-coma-g.
The (S,G) routes flow from a specific Multicast Source to a Multicast Group while traffic on a (*,G) route flows from any Multicast Source to the RP and from the RP to all the Multicast Clients of group G. 

Forward Enterprise provides PIM-SM Control Plane Modeling to compute complete (*, G) and (S, G) trees and show them in the Forward Enterprise topology along with the tree’s details. 

For instance, you can do a (*, G) path analysis to find out all the paths between the RP and the potential Receivers by issuing a query from the multicast group IP with Destination IP the multicast group IP: 

Figure 4 – PIM-SM (*,G) query example 

You can see all the different paths from the RP of the multicast group destined to the group members using the Paths selector and see hop by hop summary information as well as specific device state at Layer 2 and Layer 3. 

Figure 5 – hop L3 information 

You can also select a specific last hop egress interface in the Filters pane to verify if everything is ok for potential receivers connected to that interface.

Figure 6 – Filter by egres interface 

To perform a (S,G) path analysis, simply provide the multicast sender IP address and the multicast group IP address. 

Figure 7 – PIM-SM (S,G) query example 

In this case, the path analysis provides not only all the paths between the Source and the Receivers at a data plane level as in a common Path Analysis for unicast traffic, but it provides a Control Plane validation for each path as well. 

If both indicators in the Control plane validation section are green, it means that Forward Enterprise didn’t find any issue in the Control Plane, neither from the Sender to the RP nor from the Receivers to the RP, as you would expect. If either of the Control Plane validations are shown in red, it means Forward Enterprise has found some issues and it provides some insight that can help you find  whether connectivity exists from the sender to the RP and from the RP to the receiver. 

It also shows you what interface would be selected in case a receiver will show up based on the Reverse-Path Forwarding (RPF) for the given Sender and a brief explanation: 

As usual, more capabilities will be added in the future to make it even easier to work with your Multicast network, so stay tuned and, in the meantime, watch the demo video above and happy multicasting with Forward Networks!!! 

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