IP MULTICAST ROUTING
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Joined: Sep 2010
02-11-2010, 03:14 PM
ip_multicasting.ppt (Size: 392.5 KB / Downloads: 204)
IP MULTICAST ROUTING
What is Multicasting?
IP Multicast Addressing
Multicast Forwarding Algorithms
Simple, Source-Based Tree, Shared-Tree
Multicast Routing Protocols
Dense-mode (DVMRP, MOSPF, PIM - DM)
Sparse-mode (PIM - SM, CBT)
Time To Live (TTL)
Scope-limiting parameter for IP Multicast datagrams
Controls the number of hops that a IP Multicast packet is allowed to propagate
TTL = 1: local network multicast
TTL > 1: Multicast router(s) attached to the local network forward IP Multicast datagrams
Joined: Apr 2012
27-08-2012, 11:06 AM
Multicast Routing and Forwarding
Multicast Routing.pdf (Size: 171.28 KB / Downloads: 39)
Introduction to Multicast Routing and Forwarding
In multicast implementations, multicast routing and forwarding are implemented by three types of tables:
Each multicast routing protocol has its own multicast routing table, such as PIM routing table.
The information of different multicast routing protocols forms a general multicast routing table.
The multicast forwarding table is directly used to control the forwarding of multicast packets.
A multicast forwarding table consists of a set of (S, G) entries, each indicating the routing information for delivering multicast data from a multicast source to a multicast group. If a router supports multiple multicast protocols, its multicast routing table will include routes generated by multiple protocols. The router chooses the optimal route from the multicast routing table based on the configured multicast routing and forwarding policy and installs the route entry into its multicast forwarding table.
When creating multicast routing table entries, a multicast routing protocol uses the reverse path forwarding (RPF) mechanism to ensure multicast data delivery along the correct path.
The RPF mechanism enables routers to correctly forward multicast packets based on the multicast route configuration. In addition, the RPF mechanism also helps avoid data loops caused by various reasons.
Implementation of the RPF mechanism
Upon receiving a multicast packet that a multicast source S sends to a multicast group G, the router first searches its multicast forwarding table:
1) If the corresponding (S, G) entry exists, and the interface on which the packet actually arrived is the incoming interface in the multicast forwarding table, the router forwards the packet to all the outgoing interfaces.
2) If the corresponding (S, G) entry exists, but the interface on which the packet actually arrived is not the incoming interface in the multicast forwarding table, the multicast packet is subject to an RPF check.
The basis for an RPF check is a unicast route or a multicast static route. A unicast routing table contains the shortest path to each destination subnet, while a multicast static routing table lists the RPF routing information defined by the user through static configuration. A multicast routing protocol does not independently maintain any type of unicast route; instead, it relies on the existing unicast routing information or multicast static routes in creating multicast routing entries.
When performing an RPF check, a router searches its unicast routing table and multicast static routing table at the same time. The specific process is as follows:
1) The router first chooses an optimal route from the unicast routing table and multicast static routing table:
The router automatically chooses an optimal unicast route by searching its unicast routing table, using the IP address of the “packet source” as the destination address. The outgoing interface in the corresponding routing entry is the RPF interface and the next hop is the RPF neighbor. The router considers the path along which the packet from the RPF neighbor arrived on the RPF interface to be the shortest path that leads back to the source.
The router automatically chooses an optimal multicast static route by searching its multicast static routing table, using the IP address of the “packet source” as the destination address. The corresponding routing entry explicitly defines the RPF interface and the RPF neighbor
Multicast Static Routes
If the topology structure of a multicast network is the same as that of a unicast network, receivers can receive multicast data via unicast routes. However, the topology structure of a multicast network may differ from that of a unicast network, and some routers may support only unicast but not multicast. In this case, you can configure multicast static routes to provide multicast transmission paths that are different from those for unicast traffic. Note the following two points:
A multicast static route only affects RPF checks, and not guides multicast forwarding, so it is also called an RPF static route.
A multicast static route is effective on the multicast router on which it is configured, and will not be broadcast throughout the network or injected to other routers.
A multicast static route is an important basis for RPF checks. With a multicast static route configured on a router, the router searches the unicast routing table and the multicast static routing table simultaneously in a RPF check, chooses the optimal unicast RPF route and the optimal multicast static route respectively from the routing tables, and uses one of them as the RPF route after comparison.
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