The first four bits of multicast IP addresses are always 1110, and the last 23 bits map to the MAC. That leaves five bits that are dropped. Remember that this is an issue, because every multicast MAC maps to many multicast IPs! Example 7-5 shows how a MAC address of 0100.5E40.0305 could map to several different multicast IP addresses. Notice that the first two octets can vary in the IP addresses.
Multicast IP Addresses
Multicasts use the IP address range 224.0.0.0 to 239.255.255.255. The first four bits of the first octet are always binary 1110, and the remaining 28 bits identify the multicast group. Some addresses are reserved:
- 224.0.0.1 is the all-hosts group.
- 224.0.0.2 is the all-routers group.
- The rest of the 224.0.0.0/24 range is reserved for link-local protocols.
- 224.0.1.0 to 238.255.255.255 are for use over the Internet, and they are called globally-scoped addresses.
- Source-specific multicast uses 232.0.0.0/8 addresses.
- 233.0.0.0/8 is used to assign static multicast addresses for use by an organization. The second and third octets of the address are the organization’s Autonomous System number. This is called GLOP, which is a combination of global and scope.
- 239.0.0.0/8 is for local use within an organization, and it is called a limited scope or an administratively scoped address.
Protocol Independent Multicast (PIM)
PIM is a protocol used between routers to keep track of where to forward traffic for each multicast group. It can use information gathered from any routing protocol. PIM can run in dense mode or sparse mode.
PIM Dense Mode
PIM dense mode uses source-based trees. When running in dense mode, PIM assumes that every router needs to receive multicasts. Any router that doesn’t want to receive it must send a prune message upstream to the server. PIM dense mode is most appropriate when:
- Multicast servers and receivers are near each other.
- There are just a few servers and many receivers.
- You have a high volume of multicast traffic.
- The multicast stream is fairly constant.
PIM sparse mode uses shared distribution trees. It does not assume that any routers want to receive the multicast, but instead waits to hear an explicit message from them, joining the group. Then it adds branches to the tree to reach the hosts behind those routers. PIM sparse mode uses RPs to connect hosts and servers. After the connection is made, PIM switches over to a source tree. Sparse mode is used when:
- Pockets of users are widely dispersed around the network.
- Multicast traffic is intermittent.
An interface can be configured in sparse-dense mode. Then, if the router knows of an RP for its group, it uses sparse mode. Otherwise, it uses dense mode. In addition, it makes the interface capable of receiving multicasts from both sparse and dense-mode groups.
Configuring Multicast Routing and PIM
Use the following command to enable multicast routing:
(config)# ip multicast-routing
PIM mode must be configured at each interface with the following command. Configuring PIM on an interface also enables Internet Group Management Protocol (IGMP) on that interface:
(config-if)# ip pim {sparse-mode | dense-mode | sparse-dense-mode}
When using sparse mode, an RP must be specified. A router knows that it is an RP when it sees its own address in the command:
(config)# ip pim rp-address ip-address
Auto-RP
Auto-RP automates the discovery of RPs in a sparse or sparse-dense PIM network. RPs advertise themselves to a router designated as an RP mapping agent. The mapping agent then decides on one RP per group and sends that information to the other routers.
To configure a router as an RP, type the following:
(config)# ip pim send-rp-announce type number scope ttl group-list access-list-number
To configure a router as a mapping agent, type the following:
(config)# ip pim send-rp-discovery scope ttl
PIM Version 2
Cisco routers with recent versions of the IOS use PIM Version 2 by default. Some differences between PIM Version 1 and PIM Version 2 include:
- PIM Version 1 is Cisco proprietary, whereas PIM Version 2 is standards-based.
- Both versions can dynamically map RPs to multicast groups. PIM Version 1 uses an auto-RP mapping agent, and PIM Version 2 uses a bootstrap router (BSR).
- PIM Version 1 uses a Time-to-Live value to bound its announcements, and PIM Version 2 uses a configured domain border.
- In PIM Version 2, sparse and dense mode are group properties, not interface properties.
To configure PIM Version 2, configure at least one router as a BSR, and selected routers as RPs. To configure a BSR, use the following:
(config)# ip pim bsr-candidate interface hash-mask-length [priority]
To configure a router as a candidate RP, use the following:
(config)# ip pim rp-candidate type number ttl group-list access-list-number
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