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CLI-based Configuration Guide - IP Multicast

AR120, AR150, AR160, AR200, AR1200, AR2200, AR3200, and AR3600 V200R007

This document describes the principles and configurations of IP multicast, and provides configuration examples.
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Switch-MDT Switchover

Switch-MDT Switchover

Background

According to the process of establishing a Share-multicast distribution tree (Share-MDT) described in the previous section, you can find that the VPN instance bound to PE3 has no receivers but PE3 still receives the VPN multicast data packet of the group (192.1.1.1, 225.1.1.1). This is a defect of the multicast domain (MD) scheme: All the PEs belonging to the same MD can receive multicast data packets regardless of whether they have receivers. This wastes the bandwidth and imposes extra burden on PEs.

In MVPN, an optimized solution, Switch-MDT, is provided so that multicast data can be transmitted on demand. It allows on-demand multicast transmission. Traffic will be switched from the Share-MDT to the Switch-MDT if multicast traffic on PEs reaches the maximum. Only the PEs that have receivers connected to them will receive multicast data from the Switch-MDT. This reduces the stress on PEs and bandwidth consumption.

Implementation

Figure 7-12 shows the switch-MDT implementation process based on the assumption that a share-MDT has been successfully established.

Figure 7-12  Switch-MDT implementation

  1. On PE1, the range of the switch-group-pool for Switch-MDT is set to 238.1.1.0-238.1.1.255 and the data forwarding rate threshold that triggers Switch-MDT switchover is set.

  2. When the rate of the data forwarded by the source connected with CE1 exceeds the configured threshold, PE1 selects a group address 238.1.1.0 from the switch-group-pool and sends signaling packets to other PEs through the Share-MDT periodically to inform them to switch to the Switch-MDT.

  3. If PE2 has a receiver, after receiving the signaling packet, PE2 joins the group 238.1.1.0 and then a Switch-MDT is set up accordingly. The process of establishing a Switch-MDT is similar to that of a Share-MDT. If PE3 has no receiver, after receiving the signaling packet, it does not join the Switch-MDT. As a result, only PE2 can receive the VPN multicast data packet of the group (192.1.1.1, 225.1.1.1). Note that the PIM control packets are still transmitted over the Share-MDT.

    The Switch-MDT switchover is triggered if the following conditions are met:

    • The source and group addresses of VPN multicast data packets match the source and group address ranges defined in the ACL filtering rules. Otherwise, the packets are still forwarded along the Share-MDT.

    • The forwarding rate of VPN multicast data packets exceeds the switchover threshold, and this situation keeps for a certain period.

  4. In some cases, the forwarding rate of VPN multicast data packets fluctuates around the switchover threshold. To prevent the multicast data packets from being frequently switched between the Share-MDT and Switch-MDT, the system does not perform the switchover immediately after the system finds that the forwarding rate is greater than the switchover threshold. Instead, the system starts the switch-delay timer. During the setup of the Switch-MDT, the Share-MDT is still used for multicast data packet forwarding. That is, the switch-delay timer can ensure non-stop forwarding during switch from the Share-MDT to the Switch-MDT. Before the switch-delay timer times out, the system continues to detect the data forwarding rate. If the rate remains higher than the switchover threshold, data packets are switched to the Switch-MDT. Otherwise, the packets are still forwarded through the Share-MDT.

Switchback from the Switch-MDT to Share-MDT

When the conditions change after the VPN multicast data is switched to the Switch-MDT, the switchover conditions may not be met. In this case, PE1 switches the VPN multicast data from the Switch-MDT to the Share-MDT. The reverse switchover conditions are as follows:

  • The forwarding rate of VPN multicast data packets should be lower than the specified threshold and remain unchanged in the switch-Holddown period.

  • In some cases, the forwarding rate of VPN multicast data packets fluctuates around the switchover threshold. To prevent the multicast data flow from being frequently switched between the Switch-MDT and Share-MDT, the system does not perform the switchover when the system finds that the forwarding rate is lower than the switchover threshold. Instead, the system starts the Holddown timer. The timeout period of the timer is configured through the related command. Before the Holddown timer expires, the system continues to detect the data forwarding rate. If the rate is always lower than the switchover threshold, the data packets are switched back to the Share-MDT. Otherwise, the packets are still forwarded through the Switch-MDT.

  • When the switch-group-pool is changed, the switch-group address used to encapsulate the VPN multicast data should be outside the switch-group-pool.

  • If the advanced ACL rules used to control the switchover of VPN multicast data packets to the Switch-MDT change, the VPN multicast data packets cannot pass the filtering of new ACL rules.

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Updated: 2019-06-12

Document ID: EDOC1000097181

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