CCNP Tutorial For The BSCI Exam:
Introduction To Multicasting, Reserved Multicast Addresses, And The RPF Check
Chris Bryant, CCIE #12933
Vital Reading And Watching For All Cisco Certification Candidates:
The July 2010 CCNP Changes (And How To Beat Them!)
Ever since you picked up your first CCNA book, you've heard about multicasting, gotten a fair idea of what it is, and you've memorized a couple of reserved multicasting IP addresses.
Now as you prepare to pass the BSCI exam and become a CCNP, you've got to take that knowledge to the next level and gain a true understanding of multicasting. Those of you with an eye on the CCIE will truly have to become multicasting experts! Having said that, we're going to briefly review the basics of multicasting first, and then look at the different ways in which multicasting can be configured on Cisco routers and switches. Be sure to check out our CCNP Guaranteed Pass offer, and enjoy the tutorial, which starts below!
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What Is Multicasting? Once upon a time, we had two basic kinds of network transmissions - unicasts and broadcasts.
A unicast is data that is sent from one host directly to another, while a broadcast is data sent from a host that is destined for "all" host addresses. By "all", we can mean all hosts on a subnet, or all hosts on a network.
What if we don't want to send data to every possible host, but we need to send that data to a group of hosts? That's where multicasts come in.
A multicast is simply the middle ground between a unicast and a broadcast. Data send to a multicast address will be accepted by hosts belonging to that group - such as all EIGRP speakers accepting packets destined for 224.0.0.10 - but hosts that are not members of that group will not accept such packets.
Some other multicasting factors to be aware of:
I know that you knew that last one, but you might not have known that the overall range of Class D addresses contains several other reserved address ranges.
The 224.0.0.0 - 224.0.0.255 range is reserved for network protocols. Packets in this range will not be forwarded by routers, so these packets cannot leave the local segment. This block of addresses is the local network control block.
Just as Class A, Class B, and Class C networks have private address ranges, so does Class D. The Class D private address range is 239.0.0.0 - 239.255.255.255.
Like the other private ranges, these addresses can't be routed, so they can be reused from one network to another. This block of addresses is the administratively scoped block. These addresses are also called limited scope addresses.
The 224.0.1.0 - 238.255.255.255 range is the globally scoped address range, and these addresses are acceptable to assign to internet-based hosts - with a lot of exceptions.
Here are some other individual reserved multicast addresses:
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224.0.0.5 - All OSPF Routers
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224.0.0.6 - All OSPF DRs
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224.0.0.9 - All RIPv2 routers
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224.0.0.10 - All EIGRP routers
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224.0.0.12 - HSRP v2
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224.0.1.1 - Network Time Protocol (NTP)
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224.0.0.1 - "All hosts"
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224.0.0.2 - "All multicast routers"
Other reserved multicast address blocks:
- 232.0.0.0 /8, reserved for Source-Specific Multicast (SSM)
- 233.0.0.8 /8, GLOP address range (no, I'm not kidding about that name - read RFC 2770)
There are some individual addresses in the Class D range that you should not use. Called unusable multicast addresses or unstable multicast addresses , there's quite a few of these and you should be aware of them when planning a real-world multicast deployment.
The actual addresses are beyond the scope of the BCSI exam, but you can find them easily using your favorite search engine.
When you're configuring multicasting, there's one big difference between unicasting and multicasting that you have to keep in mind. I'll be the first to say that this one takes a little getting used to, but it becomes second nature after a awhile ... and the difference is the RPF Check.
The RPF Check A fundamental difference between unicasting and multicasting is that a unicast is routed by sending it toward the destination , while a multicast is routed by sending it away from its source .
The difference between "toward the destination" and "away from its source" might sound like nitpicking, but it's not. A unicast has a destination of one and only one host IP address.
With multicast routing, the destination is a multicast IP group address. It's the multicast router's job to decide which paths will lead back to the source (upstream) and which paths are downstream from the source. Reverse Path Forwarding refers to the router's behavior of sending multicast packets away from the source rather than toward a specific destination.
The RPF Check is run against any incoming multicast packet. The multicast router examines the interface that the packet arrived on. If the packet comes in on an "upstream interface" - that is, an interface found on the reverse path that leads back to the source - the packet passes the check and will be forwarded.
If the packet comes in on any other interface, the packet is dropped.
Like I said, this concept takes some getting used to, but it's a fundamental concept in multicasting - so we gotta get used to it!
As a veteran of Cisco certification exams, it won't surprise you to learn that there are multiple ways to configure multicasting - and we have to know all about each way! We'll tackle the first multicasting configuration option in the next installment of this BSCI tutorial, coming March 16!
Bookmark the BSCI Exam Resource Page and visit every day in March and April for new CCNP study material!
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