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CX11x, CX31x, CX710 (Earlier Than V6.03), and CX91x Series Switch Modules V100R001C10 Configuration Guide 12

The documents describe the configuration of various services supported by the CX11x&CX31x&CX91x series switch modules The description covers configuration examples and function configurations.
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Principles

Principles

This section describes the implementation of multicast route management (IPv6).

Multicast Routing and Forwarding

Devices that play different roles on a multicast network maintain different multicast tables, including the MLD group table, multicast protocol routing table, multicast routing table, and multicast forwarding table. The following describes the functions of these tables in multicast routing and forwarding.

MLD Group

A multicast router creates an MLD group entry when receiving an MLD Report message (MLD Join) that a host sends to join a group. The router maintains group memberships in MLD group entries and instructs a multicast routing protocol, usually the Protocol Independent Multicast (PIM) protocol, to create matching (*, G) entries. The router maintains an MLD group entry for each interface as long as the interfaces have MLD enabled and have received MLD Join messages. The following is an example of a group entry on an interface:

<HUAWEI> display mld group
Interface group report information
 Vlanif100(FE80::4E1F:CCFF:FE44:FFF0):
  Total 1 MLD Group reported
   Group Address:FF1E::1
    Last Reporter:FE80::1
    Uptime:00:13:20
    Expires:00:04:00 

Table 8-89 explains the fields in an MLD group entry.

Table 8-89 Description of fields in an MLD group entry
Field Description
Group Address Address of a group that an interface has joined.
Last Reporter IP address of the last user that sent an MLD Join message to the group.
Uptime Time that elapsed since the group was created.
Expires Time before the group will be aged out.
Multicast Protocol Routing Table

Multicast routing protocols maintain their own routing tables to guide multicast routing and forwarding. PIM is the most widely used multicast routing protocol. The following is an example of a PIM routing table:

<HUAWEI> display pim ipv6 routing-table
VPN-Instance: public net
 Total 0 (*, G) entry; 1 (S, G) entry

 (2001::2, FFE3::1)
     Protocol: pim-sm, Flag: SPT LOC ACT
     UpTime: 00:04:24
     Upstream interface: Vlanif20
         Upstream neighbor: FE80::A01:100:1
         RPF prime neighbor: FE80::A01:100:1
     Downstream interface(s) information:
     Total number of downstreams: 1
         1: Vlanif10
             Protocol: pim-sm, UpTime: 00:04:24, Expires: 00:02:47

Table 8-90 explains the fields in a PIM routing entry.

Table 8-90 Description of fields in a PIM routing entry
Field Description
(2001::2, FFE3::1) (S, G) entry.
Protocol: pim-sm Protocol type. The first Protocol field in an entry indicates the protocol that generates the entry, and the second Protocol field indicates the protocol that generates the downstream interfaces.
Flag: SPT LOC ACT Flag of a PIM routing entry.
UpTime: 00:04:24 Life time. The first UpTime field in an entry indicates how long the entry has existed, and the second UpTime field indicates how long a downstream interface has existed.
Upstream interface: Vlanif20 Upstream interface.
Upstream neighbor: FE80::A01:100:1 Upstream neighbor. NULL indicates that no upstream neighbor is available.
RPF prime neighbor: FE80::A01:100:1 RPF neighbor. NULL indicates that no RPF neighbor is available.
Downstream interface(s) information: Information about downstream interfaces.
Total number of downstreams: 1 Number of downstream interfaces.
Expires: 00:02:47 Aging time of a downstream interface.

For details about PIM routing entries, see Configuring PIM (IPv6) in the PIM feature description.

Multicast Routing Table

A multicast routing table is generated and maintained by the multicast route management module of a router. If a router supports multiple multicast protocols, its multicast routing table contains the optimal routes selected from routing tables of these protocols. In unicast routing, routing tables of various routing protocols such as Open Shortest Path First version 3 (OSPFv3) and Routing Information Protocol next generation (RIPng) constitute an IP routing table. Similarly, routing tables of different multicast protocols constitute a multicast table. Routers deliver multicast routing entries to their multicast forwarding tables to guide multicast data forwarding. The following is an example of a multicast routing table:

<HUAWEI> display multicast ipv6 routing-table
IPv6 multicast routing table
 Total 1 entry

 00001. (2001::2, FFE3::1)
       Uptime: 00:00:14
       Upstream Interface: Vlanif10
       List of 1 downstream interface
           1:  Vlanif20

Table 8-91 explains the fields in a multicast routing entry.

Table 8-91 Description of fields in a multicast routing entry
Field Description
00001. (2001::2, FFE3::1) Entry number 00001, in the (S, G) format.
Uptime: 00:00:14 Time that elapsed since the multicast routing entry was updated.
Upstream Interface: Vlanif10 Upstream interface.
List of 1 downstream interface List of downstream interfaces.
Multicast Forwarding Table

A multicast forwarding table, usually called a multicast forwarding information base (MFIB), is created and maintained by the route management module of a router according to multicast routing information. Routers forward multicast data according to their MFIBs. An MFIB has the same functions as a unicast FIB. The following is an example of an MFIB.

<HUAWEI> display multicast ipv6 fib
IPv6 Multicast Forwarding Table
Total 1 entry, 1 matched
 
1.(2002:1::3, FF1E::1)
     Index   : 10
     Flags   : 0
     Timeout : 00:03:29
     Incoming interface : Vlanif10
     Outgoing interfaces: 1 
       1: Vlanif20
     Matched packets :954900 packets(9549000 bytes)
     Wrong interface :0 packets
     Forwarded       :954900 packets(9549000 bytes) 

Table 8-92 explains the fields in a multicast forwarding entry.

Table 8-92 Description of fields in a multicast forwarding entry
Field Description

1.(2002:1::3, FF1E::1)

Entry number 1, in the (S, G) format.

Index: 10

Unique identifier of the multicast forwarding entry in the MFIB, which is used to rapidly search the multicast forwarding table.
Flags: 0 Flag of the multicast forwarding entry.

Timeout: 00:03:29

How soon the multicast forwarding entry will time out.
Incoming interface: Vlanif10 Inbound interface in the multicast forwarding entry.

Outgoing interfaces: 1

List of outbound interfaces.

Matched packets :954900 packets(9549000 bytes)

Number of packets that match the multicast forwarding entry.

Wrong interface :0 packets

Number of packets that arrive on the incorrect inbound interfaces.

Forwarded :954900 packets(9549000 bytes)

Number of forwarded packets.

The preceding information shows that multicast data is actually forwarded according to the MFIB. Each multicast forwarding entry records statistics about packets that are forwarded according to the entry.

RPF Check

RPF Check Basics

In unicast routing and forwarding, unicast packets are transmitted along a point-to-point path. Routers only need to know the destination address of a packet to find the outbound interface. In multicast route management, routers cannot know the location of a receiver because the destination address of a multicast packet identifies a group of receivers. However, routers can know the source of a multicast packet according to the source address, and they ensure correct forwarding paths for multicast packets by checking source addresses of the packets.

When a router receives a multicast packet, it searches the unicast routing table for the route to the source address of the packet. After finding the route, the router checks whether the outbound interface of the route is the same as the inbound interface of the multicast packet. If they are the same, the router considers that the multicast packet is received from a correct interface. This process is called an RPF check, which ensures correct forwarding paths for multicast packets.

The correct interface is called an RPF interface.

Process of an RPF Check

When receiving the first multicast packet, a router selects an optimal route from the unicast routing table based on the source address of the packet. The outbound interface of the unicast route is an RPF interface, and the next hop of the route is the RPF neighbor. The router compares the inbound interface of the packet with the RPF interface of the selected RPF route. If the inbound interface is the same as the RPF interface, the router considers that the packet has arrived on the correct path from the source and forwards the packet to downstream interfaces. If the inbound interface is different from the RPF interface, the packet fails the RPF check. The router considers that the packet is received from an incorrect interface and discards the packet.

As shown in Figure 8-97, a multicast stream sent from the source 2001::1 arrives at interface Int1 of the router. The router checks the routing table and finds that the multicast stream from this source should arrive at interface Int0. Therefore, the RPF check fails and the multicast stream is dropped by the router.
Figure 8-97 RPF check fails

As shown in Figure 8-98, a multicast stream sent from the source 2001::1 arrives at interface Int0 of the router. The router checks the routing table and finds that the RPF interface is also Int0. The RPF check succeeds, and the multicast stream is correctly forwarded.
Figure 8-98 RPF check succeeds

RPF Check in Multicast Data Forwarding

Multicast routing protocols determine the upstream and downstream neighbors and create multicast routing entries according to existing unicast routes. The RPF check mechanism enables multicast data streams to be transmitted along the multicast distribution tree and prevents loops on forwarding paths.

If a router searches the unicast routing table to perform an RPF check on every multicast data packet received, many system resources are consumed. To save system resources, a router first searches for the matching (S, G) entry after receiving a data packet sent from a source S to a group G.

  • If no matching (S, G) entry is found, the router performs an RPF check to find the RPF interface for the packet. The router then creates a multicast route with the RPF interface as the upstream interface and delivers the route to the multicast forwarding table. If the RPF check succeeds, the inbound interface of the packet is the RPF interface, and the router forwards the packet to all the downstream interfaces in the forwarding entry. If the RPF check fails, the packet is forwarded along an incorrect path, and the router drops the packet.
  • If a matching (S, G) entry is found and the inbound interface of the packet is the same as the upstream interface in the entry, the router forwards the packet to all the downstream interfaces specified in the entry.
  • If a matching (S, G) entry is found but the inbound interface of the packet is different from the upstream interface in the entry, the router performs an RPF check on the packet. The router processes the packet according to the RPF check as follows:
    • If the RPF interface is the same as the upstream interface in the entry, the (S, G) entry is correct and the packet is forwarded along an incorrect path. The router drops the packet.
    • If the RPF interface is different from the upstream interface in the entry, the (S, G) entry is outdated, and the router changes the upstream interface in the entry to the RPF interface. The router then compares the RPF interface with the inbound interface of the packet. If the inbound interface is the RPF interface, the router forwards the packet to all the downstream interfaces specified in the (S, G) entry. If the inbound interface is not the RPF interface, the router drops the packet.

Multicast Load Splitting

Load splitting and load balancing are different. Load splitting provides a way to distribute data streams destined for the same destination to multiple equal-cost paths, which may not result in a balanced traffic load on the paths. Load balancing is a special form of load splitting and distributes even data traffic loads on multiple equal-cost paths.

Implementation

By default, if multiple equal-cost optimal routes are available, a router selects the route with the largest next-hop address from the IGP routing table according to the RPF check policy.

Multicast load splitting enables a router to distribute multicast traffic to multiple equal-cost routes, instead of selecting only one route according to the RPF check policy.

As shown in Figure 8-99, the multicast source (Source) sends multicast streams to group G. RouterA and RouterD run an Interior Gateway Protocol (IGP), OSPF for example, to implement IP interworking. Two equal-cost paths are available: RouterA-> RouterB-> RouterD and RouterA-> RouterC-> RouterD. According to the default RPF check policy, the multicast streams are forwarded through interface Int1 of RouterA because interface Int1 has a larger IP address than interface Int0. After multicast load splitting is configured on RouterA, RouterA does not select forwarding paths by comparing the next-hop IP addresses. Multicast streams are forwarded through both the two equal-cost paths.
Figure 8-99 Multicast forwarding without and with multicast load splitting
Multicast Load Splitting Modes

Various methods are available to load split (*, G) and (S, G) data streams in different scenarios, as described in the following table.

  • Load splitting based on group addresses

    As shown in Figure 8-100, the source sends data streams to different multicast groups (G1 to G10). Router7, Router6, and Router5 each have two equal-cost paths towards the source. These routers use route selection algorithms to select an optimal path for data sent to each group. In this load splitting mode, streams transmitted on different paths are sent to different groups.

    Figure 8-100 Load splitting based on group addresses
  • Load splitting based on source addresses

    As shown in Figure 8-101, different sources (S1 to S10) send data streams to the same group. Router7, Router6, and Router5 each have two equal-cost paths towards the sources. These routers use route selection algorithms to select an optimal path for data from each source. In this load splitting mode, streams transmitted on different paths are sent from different sources.

    Figure 8-101 Load splitting based on source addresses
  • Load splitting based on source and group addresses

    As shown in Figure 8-102, different sources (S1 to S10) send data streams to different groups (G1 to G10). Router7, Router6, and Router5 each have two equal-cost paths towards the sources. These routers use route selection algorithms to select an optimal path for each (S, G) stream. In this load splitting mode, streams transmitted on different paths have different source and group addresses.

    Figure 8-102 Load splitting based on source and group addresses
  • Other load splitting methods

    Figure 8-103 Other load splitting methods
    • Stable-preferred load splitting

      As shown in Figure 8-103, when route flapping occurs on a multicast network, frequent changes of multicast traffic distribution on equal-cost paths will worsen route flapping. Stable-preferred load splitting can be configured to solve the problem. When route flapping occurs, a router with stable-preferred load splitting adjusts traffic distribution on equal-cost paths until route flapping ends. When the network topology becomes stable, the router evenly distributes (S, G) streams from the same source to the equal-cost paths.

    • Load splitting based on link bandwidth

      As shown in Figure 8-103, transmission paths on a network have different load splitting capabilities. If routing entries are evenly distributed on the equal-cost paths, the bandwidth of each path cannot be fully leveraged. After load splitting based on link bandwidth, routing entries can be distributed on paths based on link bandwidth.

Multicast NSR

Non-stop routing (NSR) is a reliability technology applied to a system with double control planes. (The system can be a standalone device with double switch module units or a stack system with master and standby member devices. The following descriptions use a standalone device with double switch module units as an example). This technology shields switchovers between system control planes from the routing protocol control plane. When a switchover occurs between the switch module units on a device, its neighbors are unaware of the switchover.

For details about NSR, see "NSR" in the CX11x&CX31x&CX710&CX91x Series Switch Modules Configuration Guide - Reliability.

Principles

IP multicast services, such as IPTV, video conferencing, and tele-education, require high network reliability. Without the multicast NSR technology, multicast forwarding is interrupted if a device on a multicast network experiences an active/standby switchover.

Multicast NSR technology can ensure non-stop multicast forwarding during an active/standby switchover. When the standby switch module unit starts, multicast NSR starts to back up multicast routing information and multicast protocol control information (including neighbor, multicast distribution tree, RP set, and user access information) to the standby switch module unit. When a switchover occurs between the active and standby switch module units, multicast data can still be forwarded according to the backup multicast routing information. In addition, the local device maintains the PIM neighbor relationships using the backup protocol control information so that neighbors are unaware of the switchover. Backup of multicast routing information and multicast protocol control information ensures non-stop multicast routing, delivering high reliability for multicast services.

Implementation

On IPv6 multicast networks, multicast NSR can be implemented for the Multicast Listener Discovery (MLD) and Protocol Independent Multicast (PIM IPv6). Multicast NSR involves the following mechanisms:

  1. Batch backup

    When the standby switch module unit starts, the active switch module unit backs up all multicast routing information and multicast protocol control information to the standby switch module unit, ensuring consistent information on the two switch module units.

  2. Real-time backup

    During operation of multicast protocols, the active switch module unit backs up changed multicast routing information and multicast protocol control information to the standby switch module unit in real time, ensuring data synchronization between the two switch module units.

  3. Switchover

    If the active switch module unit fails, the standby switch module unit takes over services. The standby switch module unit saves the same multicast routing information and multicast protocol control information as the active switch module unit, so multicast forwarding is not interrupted during the switchover.

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Updated: 2019-08-09

Document ID: EDOC1000041694

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