CCNP 642-811 BCMSN Exam Certification Guide - Cisco press

Switching Multicast Traffic 367

Next, you must identify each of the candidate RP routers. Configure each RP with the following global configuration command:

Switch(config)# ip pim rp-candidate type mod/num ttl group-list access-list-number

Finally, by default, the bootstrap messages permeate the entire PIM domain. You can limit the scope of the advertisements by defining PIMv2 border routers, which will not forward the bootstrap messages. Use the following global configuration command:

Switch(config)# ip pim border

Switching Multicast Traffic

Routers or multilayer switches can build multicast trees and set up forwarding in an efficient, intelligent manner. At Layer 2, however, a switch can examine only the Ethernet frame header to find the source and destination MAC addresses. These switches cannot enjoy the luxury of ondemand multicast forwarding at all; the best information they have is the destination multicast address, and that signifies only that the frame needs to be flooded out all ports on the VLAN.

Two methods have been developed to help switches make intelligent forwarding decisions for multicast traffic: IGMP snooping and CGMP. One method requires more sophisticated switching hardware, whereas the other method leans on a nearby router for assistance.

IGMP Snooping

In normal operation, a host desiring multicast group membership must contact a local router so that it gets added into the multicast tree. IGMP snooping allows a switch to eavesdrop on these IGMP membership reports, so that it can find out who is requesting which group.

Recall that to join a group, a host must send its IGMP membership report to the multicast address of the group itself. A Layer 2-only switch must listen to every multicast frame to find the IGMP information. Clearly, this becomes a burden to the switch CPU.

A multilayer or Layer 3 switch has a clear advantage—it can inherently pick out Layer 3 information within frames. This type of switch must listen only to every IGMP packet. When a membership report is overheard, the switch adds the multicast group’s MAC address to its Content Addressable Memory (CAM) table (if it doesn’t already exist), along with the source switch port where the IGMP packet was received. This links the group address with the host who requested membership.

As other hosts request membership to the group, the respective switch ports are added to the CAM table list for the group address. Now, when a frame destined for the multicast group arrives, it can be replicated out exactly the right ports to reach the recipients.

Verifying Multicast Routing and Switching 369

NOTE As a rule, IGMP snooping and CGMP are mutually exclusive—they cannot both be used simultaneously on a switch. For switches that have IGMP snooping capability, IGMP snooping is enabled by default. For switches that cannot do IGMP snooping, CGMP is enabled by default.

If you are configuring IP multicast support in your network, be sure to identify any legacy Layer 2 switches that are capable only of flooding multicast traffic. Enable CGMP on these switches, and then enable CGMP on an upstream multicast router or multilayer switch. This way, your entire network will be able to intelligently constrain the flooding of multicast traffic.

Verifying Multicast Routing and Switching

To verify the operation of the features discussed in this chapter, you can use the commands listed in the sections that follow. In particular, look for the active router, standby or backup routers, and loadbalancing methods in use.

Multicast Routing with PIM

Remember that PIM is based on the unicast routing table; no separate multicast routing table is kept. Table 15-2 lists those commands that you need to verify that the multicast routing with PIM operations is working as intended.

Table 15-2 Commands for Verifying Multicast Routing with PIM

Multicast Switching

Table 15-3 lists those commands that you need to verify that IGMP snooping is configured and working as intended.

370 Chapter 15: Multicast

Table 15-3 Commands for Verifying IGMP Operation

You can also perform some multicast testing with multilayer switches. Choose a multicast group address that several switches can join. Configure a specific interface to join the group with the following interface configuration command:

Switch(config-if)# ip igmp join-group multicast-address

Then, you can use show commands to see information about the group and the IGMP membership. After several switches have joined the group, you can also issue a ping to the multicast group IP address. Every switch that has joined the group should answer with a reply.

What Would Happen Without a Multicast Router?

Suppose that a network is so small that a multicast router is not available. Instead, only one or more Layer 2 or Layer 3 switches are used. Can a multicast group be supported for users that want to view a streaming video from a server?

Multicast can always be supported, with or without a router. When a host sends an IGMP membership report to join a group, it does so blindly. After all, a host does not know about multicast routers at all—it just sends out a request to join and hopes that it will start receiving traffic destined for the multicast group address. Even if a multicast router is present, it doesn’t send a reply to a host that has joined a group. A router only periodically sends out membership queries asking if hosts are still wanting to remain a member of the group.

In this small network, Layer 2 switches will simply flood the server’s multicast traffic out all ports on the VLAN. No CGMP router is available to lend its intelligence. Layer 3 switches can use IGMP snooping, however, to constrain the multicast flooding. Although a router isn’t present, the switch can still listen to the membership reports being sent to the nonexistent router.

Foundation Summary 371

Foundation Summary

The Foundation Summary is a collection of information that provides a convenient review of many key concepts in this chapter. If you are already comfortable with the topics in this chapter, this summary can help you recall a few details. If you just read this chapter, this review should help solidify some key facts. If you are doing your final preparation before the exam, this information is a convenient way to review the day before the exam.

■IP multicast addresses range from 224.0.0.0 to 239.255.255.255 (highest 4 bits are always 1110).

■Multicast MAC addresses always start with 0100.5e (next bit is always 0). The lower 28 bits of the IP address are mapped over into the lower 23 bits of the MAC address.

■RPF decides whether a multicast packet can be forwarded—if it arrived on an interface where the source can be found, it is forwarded; if not, it is dropped.

■IGMP is the protocol used to join and leave (as well as to maintain) multicast groups.

■PIM is a multicast routing protocol used to build multicast tree topologies.

Table 15-4 A Comparison of PIM Modes

372 Chapter 15: Multicast

Table 15-5 IP PIM Multicast Configuration Commands

■IGMP snooping allows a switch to intercept IGMP messages to learn of group members and their port locations.

■CGMP allows a router to relay IGMP join and leave requests to Layer 2 switches. The switches then learn of group members and their port locations.

Q&A 373

Q&A

The questions and scenarios in this book are more difficult than what you should experience on the actual exam. The questions do not attempt to cover more breadth or depth than the exam; however, they are designed to make sure that you know the answers. Rather than allowing you to derive the answers from clues hidden inside the questions themselves, the questions challenge your understanding and recall of the subject. Hopefully, these questions will help limit the number of exam questions on which you narrow your choices to two options and then guess.

The answers to these questions can be found in Appendix A.

1.By default, what does a router or Layer 3 switch do with multicast packets?

2.By default, what does a Layer 2 switch do with a multicast packet?

3.What high-order bit combination signals that an IP address is used for multicast?

4.If the IP-to-MAC multicast address mapping is somewhat ambiguous, how can a frame be forwarded to the correct destination group?

5.What IP multicast address range is set aside for use only on the local network segment?

6.For the RPF check, the source IP address is looked up in the unicast routing table. To forward the packet, what should the result of the test be?

7.What important difference exists between IGMPv1 and IGMPv2?

8.To join a multicast group, what type of message is sent? Where is this message sent?

9.What is the purpose of the IGMP Querier?

10.For PIM Dense Mode, how is the multicast tree built?

11.Where is the root of the PIM Sparse Mode tree located?

12.With PIM Sparse-Dense Mode, is the PIM mode determined per interface or per group?

13.What routing table is used for PIM?

14.What command is used to configure an interface for the hybrid PIM mode?

15.When a switch performs IGMP snooping, what is it snooping for?

16.What else does a Layer 2 switch need when it is configured for CGMP?

17.When should IGMP snooping and CGMP be used together on a switch?

18.At a trade show, several PCs and servers are connected to a single Layer 2 switch. The switch has CGMP enabled. When a server begins to send video data to a multicast address, what happens to that traffic?

PART IV: Campus Network

Services

Chapter 16 Quality of Service Overview

Chapter 17 Diffserv QoS Configuration

Chapter 18 IP Telephony

Chapter 19 Securing Switch Access

Chapter 20 Securing with VLANs

This part of the book covers the following BCMSN exam topics:

■Describe the quality issues with voice traffic on a switched data network, including jitter and delay.

■Describe the QoS solutions that address voice-quality issues.

■Describe the features and operation of network analysis modules on Catalyst switches to improve network traffic management.

■Implement IP technology on a switched network with auxiliary VLANs.

■Configure QoS features on multilayer switched networks to provide optimal quality and bandwidth utilization for applications and data.

■Describe the general design models when implementing IP telephony in a switched network environment.

■Plan QoS implementation within a multilayer switched network.