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Example: Configuring Multicast Snooping

Requirements

This example uses the following hardware components:

• One MX Series router
• One Layer 3 device functioning as a multicast router

• Configure the interfaces. See the Junos OS Network Interfaces Library for Routing Devices.
• Configure an interior gateway protocol. See the Junos OS Routing Protocols Library for Routing Devices.
• Configure a multicast protocol. This feature works with the following multicast protocols:
  • DVMRP
  • PIM-DM
  • PIM-SM
  • PIM-SSM

Overview and Topology

IGMP snooping prevents Layer 2 devices from indiscriminately flooding multicast traffic out all interfaces. The settings that you configure for multicast snooping help manage the behavior of IGMP snooping.

You can configure multicast snooping options on the default master instance and on individual bridge or VPLS instances. The default master instance configuration is global and applies to all individual bridge or VPLS instances in the logical router. The configuration for the individual instances overrides the global configuration.

• flood-groups—Enables you to list multicast group addresses for which traffic must be flooded. This setting if useful for making sure that IGMP snooping does not prevent necessary multicast flooding. The block of multicast addresses from 224.0.0.1 through 224.0.0.255 is reserved for local wire use. Groups in this range are assigned for various uses, including routing protocols and local discovery mechanisms. For example, OSPF uses 224.0.0.5 for all OSPF routers.
• forwarding-cache—Specifies how forwarding entries are aged out and how the number of entries is controlled.

  You can configure threshold values on the forwarding cache to suppress (suspend) snooping when the cache entries reach a certain maximum and reuse the cache when the number falls to another threshold value. By default, no threshold values are enabled on the router.

  The suppress threshold suppresses new multicast forwarding cache entries. An optional reuse threshold specifies the point at which the router begins to create new multicast forwarding cache entries. The range for both thresholds is from 1 through 200,000. If configured, the reuse value must be less than the suppression value. The suppression value is mandatory. If you do not specify the optional reuse value, then the number of multicast forwarding cache entries is limited to the suppression value. A new entry is created as soon as the number of multicast forwarding cache entries falls below the suppression value.

• graceful-restart—Configures the time after which routes learned before a restart are replaced with routes relearned. If graceful restart for multicast snooping is disabled, snooping information is lost after a Routing Engine restart.

  By default, the graceful restart duration is 180 seconds (3 minutes). You can set this value between 0 and 300 seconds. If you set the duration to 0, graceful restart is effectively disabled. Set this value slightly larger than the IGMP query response interval.

• ignore-stp-topology-change—Configures the MX Series router to ignore messages about the spanning-tree topology state change.

  By default the IGMP snooping process on an MX Series router detects interface state changes made by any of the spanning tree protocols (STPs).

  In a VPLS multihoming environment where two PE routers are connected to two interconnected CE routers and STP root protection is enabled on the PE routers, one of the PE router interfaces is in forwarding state and the other is in blocking state.

  If the link interconnecting the two CE routers fails, the PE router interface in blocking state transitions to the forwarding state.

  The PE router interface does not wait to receive membership reports in response to the next general or group-specific query. Instead, the IGMP snooping process sends a general query message toward the CE router. The hosts connected to the CE router reply with reports for all groups they are interested in.

  When the link interconnecting the two CE routers is restored, the original spanning-tree state on both PE routers is restored. The forwarding PE receives a spanning-tree topology change message and sends a general query message toward the CE router to immediately reconstruct the group membership state.

  Note: The ignore-stp-topology-change statement is supported for the virtual-switch routing instance type only.

Figure 1 shows a VPLS multihoming topology in which a customer network has two CE devices with a link between them. Each CE is connected to one PE.

Figure 1: VPLS Multihoming Topology

Configuration

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.

Step-by-Step Procedure

1. Configure multicast snooping settings in the master routing instance.

   [edit bridge-domains domain1]user@host# set multicast-snooping-options forwarding-cache threshold suppress 100 reuse 50user@host# set multicast-snooping-options graceful-restart 120

2. Configure the routing instance.

   [edit routing-instances ce1]user@host# set instance-type virtual-switch

3. Configure the bridge domain in the routing instance.

   [edit routing-instances ce1 bridge-domains domain1]user@host# set domain-type bridgeuser@host# set interface ge-0/0/6.0user@host# set interface ge-0/3/9.0user@host# set vlan-id 100

4. Configure flood groups.

   [edit routing-instances ce1 bridge-domains domain1]user@host# set multicast-snooping-options flood-groups 224.0.0.5

5. Configure the router to ignore messages about spanning-tree topology state changes.

   [edit routing-instances ce1 bridge-domains domain1]user@host# set multicast-snooping-options ignore-stp-topology-change

6. If you are done configuring the device, commit the configuration.

   user@host# commit

Results

Confirm your configuration by entering the show bridge-domains and show routing-instances commands.

Verification

• show igmp snooping interface
• show igmp snooping membership
• show igmp snooping statistics
• show multicast snooping route
• show multicast snooping statistics
• show route table