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# Setting Up IP Multicast Forwarding

Configuring multicast forwarding
Forwarding from an MBONE router on a WAN link

## Configuring multicast forwarding

The multicast backbone (MBONE) is a virtual network layered on top of the Internet to support IP multicast routing across point-to-point links. It is used to transmit audio and video on the Internet in real time, because multicasting is a much cheaper and faster way to communicate the same information to multiple hosts.

When using the MBONE, the MAX looks like a multicast client. It responds as a client to Internet Group Membership Protocol (IGMP) packets it receives from MBONE routers. The packets can be IGMP version-1 or version-2, including IGMP multicast trace (MTRACE) packets.

To multicast clients on a WAN or Ethernet interface, the MAX looks like a multicast router. Like a router, it sends those clients IGMP queries, receives responses, and forwards multicast traffic. In this implementation, multicast clients are not allowed to source multicast packets. If they do, the MAX discards the packets.

Following are the parameters (shown with sample settings) for configuring multicast forwarding:

    Ethernet
          Mod Config
                Multicast...
                      Forwarding=Yes
                      Membership Timeout=60
                      Mbone Profile=
                      Client=No
                      Rate Limit=5
                      Grp Leave Delay=
                      HeartBeat Addr=224.0.1.1
                      HeartBeat Udp Port=123
                      HeartBeat Slot Time=10
                      HeartBeat Slot Count=10
                      Alarm threshold=3
                      Source Addr=128.232.0.0
                      Source Mask=0.0.0.0

    Ethernet
          Connections
                any Connection profile
                      Ip options...
                            Multicast Client=No
                            Multicast Rate Limit=5

### Understanding the multicast parameters

This section provides some background information about multicast parameters. For detailed information about each parameter, see the MAX Reference Guide.

#### Forwarding

The Forwarding parameter turns on multicast forwarding in the MAX.

When you change the Forwarding parameter from No to Yes, the multicast subsystem reads the values in the Ethernet profile and initiates the forwarding function.

Note: If you modify a multicast value in the Ethernet profile, you must set this parameter to No and then back to Yes again to force a read of the new value.

#### Membership Timeout

When you configure the Ascend unit as a multicast forwarder, it forwards polling messages generated by the multicast router and keeps track of active memberships from its client interfaces. To configure the timeout value for deactivating memberships, you can set the Membership Timeout parameter to a value from 60 seconds to 65535 seconds. The factory default is six minutes.

#### Mbone Profile

The multicast router resides in the MBONE interface. If it resides across the WAN, the Mbone Profile parameter must specify the name of a resident Connection profile to that router. If the Mbone Profile name is null and Multicast Forwarding is on, the MAX assumes that its Ethernet is the MBONE interface.

#### Client and Rate Limit

Each local or WAN interface that supports multicast clients must set the Client parameter to Yes (or set Multicast Client in each Client's Connection profile to Yes). With this setting, the MAX begins handling IGMP requests and responses on the interface. It does not begin forwarding multicast traffic until you set the Rate Limit parameter.

Rate Limit specifies the rate at which the MAX accepts multicast packets from its clients. It does not affect the MBONE interface. The default setting is 100, which disables multicast forwarding on the interface. The forwarder handles IGMP packets, but does not accept packets from clients or forward multicast packets from the MBONE router.

To begin forwarding multicast traffic on the interface, you must set the Rate Limit parameter to a number less than 100. For example, if you set it to 5, the MAX accepts a packet from multicast clients on the interface once every five seconds. The MAX discards any subsequent packets received in that five-second window.

#### Grp Leave Delay

Because multiple multicast clients can have multiple active sessions for identical IGMP groups via a single WAN interface on the MAX, you can configure the MAX to query each WAN interface from which it receives a `leave group` message, to make sure there are no clients with active multicast sessions for the same group on that interface.

When the MAX receives a `leave group` message for a WAN interface for which you configure a value for Grp Leave Delay, it sends a query to the WAN interface, requesting that any active members of the group respond. If the MAX receives a response within the time period of time you specify in the Grp Leave Delay parameter, it does not forward the leave group message to the MBONE. Otherwise, it sends a `leave group` message to the MBONE, and it clears the IGMP group session from its tables.

#### HeartBeat

When running as a multicast forwarder, the MAX continually receives multicast traffic. The heartbeat-monitoring feature enables the administrator to monitor possible connectivity problems by continuously polling for this traffic and generating an SNMP alarm trap in the event of a traffic breakdown. Following is the SNMP alarm trap:

    Trap type: TRAP_ENTERPRISE
    Code: TRAP_MULTICAST_TREE_BROKEN (19)
    Arguments:
    1) Multicast group address being monitored (4 bytes),
    2) Source address of last heartbeat packet received (4 bytes),
    3) Slot time interval configured in seconds (4 bytes),
    4) Number of slots configured (4 bytes).
    5) Total number of heartbeat packets received before the MAX started sending SNMP Alarms (4bytes).

Note: Heartbeat monitoring is optional. It is not required for multicast forwarding.

To set up heartbeat monitoring, you configure several parameters that define the packets to be monitored, how often and for how long to poll for multicast packets, and the threshold for generating an alarm. Following are the parameters you use to specify these settings:

#### Setting | #### Parameters
--- | ---
Packets to be monitored | HeartBeat Address specifies a multicast address. If set, causes the MAX to listen for packets to and from the specified address. HeartBeat UDP Port specifies a UDP port number. If set, causes the MAX to listen only to packets received through the specified port. Source Addr and Source Mask specify an IP address and subnet mask. If you specify an address, the MAX ignores packets from that source for monitoring purposes.
How often and for how long to poll for multicast packets | HeartBeat Slot Time specifies an interval (in seconds). The MAX polls for multicast traffic, waits for the duration of the interval, then polls again. HeartBeat Slot Count specifies how many times to poll before comparing the number of heartbeat packets received to the Alarm Threshold.
Threshold for generating an alarm | Heartbeat Alarm Threshold specifies a number. If the number of monitored packets falls below this number, the MAX sends the SNMP alarm trap.

#### Multicast Client

The Multicast Client parameter enables the MAX to respond to multicast clients on the WAN link. Clients cannot be supported on the MBONE interface, so this means another WAN link or the local Ethernet supports a multicast router.

When you set Multicast Client to Yes, the MAX begins handling IGMP requests and responses on the interface. It does not begin forwarding multicast traffic until the rate limit is set. When you set Multicast Client to No, the MAX specifies that the MAX not respond to multicast clients on the interface. The default is No.

#### Multicast Rate Limit

The Multicast Rate Limit parameter specifies the rate at which the MAX accepts multicast packets from clients on this interface. It does not affect the MBONE interface.

Note: By default, the Rate Limit t parameter is set to 100, which disables multicast forwarding on the interface. The forwarder handles IGMP packets, but does not accept packets from clients or forward multicast packets from the MBONE router.

To begin forwarding multicast traffic on the interface, you must set the rate limit to a number less than 100. For example if you set it to 5, the MAX accepts a packet from multicast clients on the interface every 5 seconds. Any subsequent packets received in that 5-second window are discarded. You can specify a number lower than the default 100 to begin forwarding multicast traffic on the interface.

#### Implicit priority setting for dropping multicast packets

For high-bandwidth data, voice, and audio multicast applications, the MAX supports both multicast rate limiting (as described in Client and Rate Limit) and prioritized packet dropping. If the MAX is the receiving device under extremely high loads, it drops packets according to a priority ranking, which the following UDP port ranges determine:

- Traffic on ports 0-16384 (unclassified traffic) has the lowest priority (50).

- Traffic on ports 16385-32768 (Audio traffic) has the highest priority (70).

- Traffic on ports 32769-49152 (Whiteboard traffic) has medium priority (60).

- Traffic on ports 49153-65536 (Video traffic) has low priority (55).

### Multicast interfaces

The MAX creates the following multicast interfaces at system startup:

#### Interface | #### Specified destination address
--- | ---
mcast | 224.0.0.0/4 Multicast addresses, except for special addresses discussed in this section, are directed to this interface.
local | 224.0.0.1/32 Multicast address for all systems on the local subnet. The MAX does not forward packets sent to this address.
local | 224.0.0.2/32. Multicast address for all routers on the local subnet. The MAX does not forward packets sent to this address.
local | 224.0.0.5/32. Multicast address for all OSPF routers on the network. The MAX does not forward packets sent to this address. If you disable OSPF routing, this route changes from local to a black-hole interface.
local | 224.0.0.6/32. This local address is the multicast address for all OSPF designated routers on the network. The MAX does not forward packets sent to this address. If you disable OSPF routing, this route changes from local to a black-hole interface.

### Forwarding from an MBONE router on Ethernet

Figure 12-1 shows a local multicast router on one of the MAX unit's Ethernet interfaces, and dial-in multicast clients.

Figure 12-1. MAX forwarding multicast traffic to dial-in multicast clients

Note: Heartbeat monitoring is an optional feature. You can operate multicast forwarding without it if you prefer.

As an example of this type of multicast configuration, the following procedure specifies the MBONE interface as the Ethernet port, and uses the heartbeat group address of 224.1.1.1:

1. Open Ethernet > Mod Config > Multicast.

2. Enable multicast forwarding, and leave the default values for the Mbone Profile, Client, and Rate Limit parameters:

       Ethernet
             Mod Config
                   Multicast...
                         Forwarding=Yes
                         Membership Timeout=60
                         Mbone Profile=
                         Client=No
                         Rate Limit=5

3. Specify a heartbeat group address and UDP port for monitoring heartbeat packets. For example:

                           HeartBeat Addr=224.1.1.1
                         HeartBeat Udp Port=16387

4. Specify the time, count, and alarm threshold parameters. For example:

                         HeartBeat Slot Time=10
                         HeartBeat Slot Count=10
                         Alarm threshold=3
                         Source Addr=0.0.0.0
                         Source Mask=0.0.0.0

5. Close the Ethernet profile.

To enable multicasting on WAN interfaces:

1. Open the Connection profile for a multicast client site.

2. Open the IP Options subprofile and set Multicast Client to Yes. If appropriate, specify a rate limit other than the default of 5.

       Ethernet
             Connections
                 any Connection profile
                       Ip options...
                             Multicast Client=Yes
                             Multicast Rate Limit=5

3. Close the Connection profile.

## Forwarding from an MBONE router on a WAN link

Figure 12-2 shows a multicast router on the WAN with local and dial-in multicast clients.

Figure 12-2. MAX acting as a multicast forwarder on Ethernet and WAN interfaces

This section presents a sample configuration for the local MAX unit in Figure 12-2. The configuration specifies the MBONE interface as a WAN link accessed through a Connection profile #20.

Note: This example does not use heartbeat monitoring. If you want to configure the MAX for heartbeat monitoring, see the sample settings in Forwarding from an MBONE router on Ethernet.

### Configuring the MAX to respond to multicast clients

To configure the MAX to respond to multicast clients on the Ethernet:

1. Open Ethernet > Mod Config > Multicast.

2. Enable multicast forwarding, specify the number of the Connection profile for the MBONE interface, and set Client to Yes:

   Ethernet
   Mod Config
   Multicast...
   Forwarding=Yes
   Membership Timeout=60
   Mbone Profile=20
   Client=Yes

3. In the same profile, set Multicast Rate Limit to a number lower than the default of 100:

                 Rate Limit=5

4. Close the Ethernet profile.

### Configuring the MBONE interface

To configure the MBONE interface:

1. Open the Connection profile for an MBONE interface (in this example, profile #20).

2. Open the IP options subprofile and set Multicast Rate Limit to a number lower than the default of 100:

       Ethernet
             Connections
                   profile #20...
                         Ip options...
                               Multicast Client=No
                               Multicast Rate Limit=5

3. Close the Connection profile.

### Configuring multicasting on WAN interfaces

To enable multicasting on WAN interfaces:

1. Open the Connection profile for a multicast client site.

2. Open the IP options subprofile.

3. Set Multicast Client to Yes, and set Multicast Rate Limit to a number lower than the default of 100:

       Ethernet
             Connections
                 any Connection profile
                       Ip options...
                             Multicast Client=Yes
                             Multicast Rate Limit=5

4. Close the Connection profile.

Copyright © 1998, Ascend Communications, Inc. All rights reserved.