IP Multicast Configuration and Verification

I finished up the rest of chapter 9 on Multicast by learning a little mor PIM theory along with simple configuration of multicast. When configuring PIM-DM (Dense Mode), it initially floods unicast traffic being sent by the source throughout the entire network. As each router receives multicast traffic via its RPF interface (the interface in the direction of the source), it forwards the multicast traffic to all of its PIM-DM neighbors.

PIM-DM prune messages are sent to stop unwanted traffic. Prune messages are sent on a RPF interface when the router has no downstream receivers for multicast traffic for that source. Prune messages are sent to non-RPF interfaces to shut off the flow of multicast traffic because it is arriving via an interface that is not the shortest path to the source.

PIM-SM (Sparse Mode) uses shared distribution trees with RP's (Rendezvous Points) but may uses source distribution trees as well. PIM-SM is based on a pull model so that traffic is forwarded only to those parts of the network that need it. PIM-SM uses an RP to coordinate forwarding of multicast traffic from a source to the receivers. PIM-SM is appropriate for wide-scale deployment for both densely and sparsly populated groups in the enterprise network. It is preferred over PIM-DM for all production networks regardless of size and membership density.

There are many optimizations and enhancements to PIM, including the following:

• Bidirectional PIM mode, which is designed for many-to-many applications (that is, many host all multicasting to each other)

• Source Specific Multicast (SSM), which is a variant of PIM-SM that builds only source specific shortest path trees and does not need an active RP for source-specific groups (in the address range 232.0.0.0/8)
  

Multicast IGMP and PIM

I learned a little more about IGMP and PIM this morning. Hosts use IGMP (Internet Group Management Protocol) to register with the router to join or leave specific multicast groups. The router is then aware that it needs to forward the data stream destined to a specific multicast group to the registered hosts. There are currently three versions of IGMP, versions 1, 2, and 3.

• IGMPv1 - periodically sends membership queries (60-120 sec) to the all-hosts multicast address 224.0.0.1. IGMPv4 doesn't have a mechanism defined for hosts to leave the multicast group. There for IGMP routers learn that a group is no longer available when it times out from not receiving any queries from that particular group.

• IGMPv2 - has group-specific queries that allows a router to query membership only in a single group instead for all groups. Instead of waiting for a timeout from a particular group, the last hosts that are apart of a multicast group sends the router a specific message that it's leaving said group.

• IGMPv3 - is still being designed and proposed. Version 3 adds the ability to filter multicasts based on multicast source so that hosts can indicate that they want to recieve traffic only from particular sources within a multicast group.

In order for Layer 2 devices to recognize multicast packets it uses either CGMP (Cisco Group Management Protocol) or IGMP Snooping. As you might have guessed, CGMP is a Cisco proprietary protocol designed for Cisco switches specifically. It allows you to maunally configure specific switch ports for multicast traffic but this feature isn't scable because of that reason. IGMP Snooping allows a switch to eavesdrop on IGMP messages sent between routers and hosts, and updates its MAC address table accordingly.

PIM (Protocol Independent Multicast) is used by routers that are forwarding multicast packets. PIM uses the normal IP routing table in its multicast calculations. PIM uses what's called distribution trees to forward multicast packets. There's two types of trees

• Source Tree - A source tree is created for each source sending to each multicast group. The source tree has its root at the source and has branches through the network to the receivers.
• Shared Tree - Is a single tree that is shared between all sources for each multicast group. The shared tree has a single common root, called a rendezvous point (RP). Sources initially send their multicast packets to the RP, which in turn forwards data through a shared tree to the members of the group.

PIM uses two modes that determines the type of distribution tree to use including one hybrid mode:

• PIM Sparse Mode (PIM-SM) - Sparse mode uses a "pull" model to send multicast traffic. it usres a shared tree and therefore requires an RP to be defined.
  
• PIM Dense Mode (PIM-DM) - Dense mode uses a "push" model that floods multicast traffic to the entire network. Dense mode uses source trees.
  
• PIN Sparse Dense Mode - uses both Sparse and Dense modes throughout its network
  

Multicast Overview

I'm nearing the end of my CCNP Study Guide Book, with only this chapter on multicasting and the last chapter on IPv6 to go. Multicasting enables data to be sent over networks to a group of destinations in the most efficient way. The data is sent from the source as one stream; this single data stream travels through the network. Other network devices only replicate the data through the network if they have other members on their interfaces that are apart of this destination group.

Multicast groups are identified by Class D IP addresses, which are in the range from 224.0.0.0 to 239.255.255.255. Muticast uses the Internet Group Management Protocol (IGMP) and Cisco Group Management Protocol (CGMP) for determining which network devices require the multicast data stream. Protocol Independent Multicast (PIM) is used for determining the best way to route multicast traffic.

There are many differences between Multicast and Unicast packets. Unicast duplicates a packet for each reciever that it needs to send the data too (one copy for each reciver). Multicast sends one packet stream as mentioned previously, downstream routers replicate the packets only on links where receiving hosts exist. Multicast provides the following advantages over unicast:

• Enhanced efficiency
• Optimized Performance
• Support for distributed applications

The disadvantage of multicast is that it uses UDP (User Datagram Protocol) as it's transport protocol. This means that packets are only sent on "best-effort" delivery and that packets aren't sent reliably. In order to cut down on unreliable packets, the multicast applications them selves may need to provide some sort of reliability mechanisms to prevent huge data lost. This could mean more processing power needed on the hosts them selves.

Configure the Network Full CCNA Lab Preview

Hey all, I just wanted to give you a quick snippet of one of the labs that will be available when you purchase the full version of the Configure the Network Lab book. View the picture above for a peek at what Task 8 will have you configuring for your CCNA exam!

Don't forget to download the free CCNA ICND1 Lab book at www.configurethenetwork.com

End of Chapter BGP Labs

The end of chapter 8 consists of 3 BGP labs to test your knowledge of the material you've learned so far. I went through 2 labs today and will complete the last one tomorrow along with the end of chapter questions. The nice thing about setting up and configuring the network examples in the book is that by time you get to the end of chapter labs you will know how to configure everything with ease. I noted today than I configured a total of 75 network labs so far during my BSCI studies the last few months! I'm sure that I'm going to easily pass 100 labs by time I take the exam for the first time, I just want to be as prepped as possible going in.

BGP MED Configuration

When using BGP, sometimes the Autonomous System (AS) wants to influence another AS's routing decision on how incoming packets are routed to its AS. Multiexit-discriminator (MED) is used to configure this setting for BGP peers. Unfortunately MED is one of the last considered processes in the BGP routing process. That means that if local preference is used in the BGP process it can not knowingly override any MED settings you are sending to that remote AS.

Route maps can be used with MED to better load balance settings, using the command default-metric under the BGP configuration will set MED settings. However if two BGP peers in the same AS have different MED values, the lower cost will always be preferred. While the other peer will receive little to almost no routing traffic besides BGP updates. What would be better is creating a route map that specifies which networks should have lower metrics on each BGP peer as shown in the example above.

Multiple AS BGP Lab

I spent this evening creating one of the bigger labs I usually do. My task was to setup a BGP network with 5 AS's in which every AS should be able to reach other loop-free (requirement of BGP anyways). I was also supposed to make Router C choose Router A to send its packets destined for AS 65004. Well I was able to get the BGP network up without to many hitches which is a long way than just a week or two ago. I however could not get my route-map statement for Router A to set the local preference to AS 65004 higher than Router B to work. I'm not sure why but as i finish up this chapter i will review why this command didn't take. I think it might have been the way I applied it to the BGP router configuration (had the route-map set to in instead of out). All in all I would say it was a pretty good success setting up a lab of this size without having to really reference the book for command guidance.