Multicast Support Commands

Multicast Support Commands

Overview

The previous chapters have covered the operation and configuration of Cisco-supported IP multicast protocols. In this chapter, we will look at a number of multicast scenarios and multicast support commands. The support commands are not specific to any multicast routing protocols but are used to fine-tune your network.

Multicast Boundaries

The unicast IP address allocation reserved three sets of IP addresses for private use. An address block was reserved in each of the IP classes A, B, and C, as shown.

10.0.0.0

—

10.255.255.255

172.16.0.0

—

172.31.255.255

192.168.0.0

—

192.168.255.255

If these networks are used in a private intranet, then care must be taken not to advertise these networks on the Internet. Because multiple intranets may be using the same private IP address space, advertising them globally would cause confusion (see Figure 9-1). To prevent such confusion, private addresses should not be advertised outside the local intranet. Company A and Company B in Figure 9-1 would have to use Network Address Translation on their border routers to allow internal users Internet access. What has effectively been done is to form a boundary around the private addressed networks to prevent these addresses from being accessed through the Internet.

Figure 9-1: If private IP addresses are advertised over the Internet, then routing confusion can occur. For this reason, private IP addresses should not be advertised globally.

The multicast address space has a block of addresses assigned that are analogous to the private IP unicast address blocks. The block of Class D addresses from 239.0.0.0 to 239.255.255.255 are referred to as administratively scoped; the block is further subdivided, as shown in Table 9-1. Assume that in your company each department (finance, engineering, and marketing) wants to deploy multicasting, but they do not want to receive multicast traffic from the other departments. For this scenario, a multicast boundary will need to be set up around each department to prevent multicast traffic from crossing departmental boundaries (see Figure 9-2).

Table 9-1: Administratively Scoped Multicast Address Block

239.0.0.0—239.255.255.255

Administratively Scoped

239.0.0.0—239.63.255.255

Reserved

239.64.0.0—239.127.255.255

Reserved

239.128.0.0—239.191.255.255

Reserved

239.192.0.0—239.251.255.255

Organization—Local Scope

239.252.0.0—239.252.255.255

Site-Local Scope (Reserved)

239.253.0.0—239.253.255.255

Site-Local Scope (Reserved)

239.254.255.255—239.254.255.255

Site-Local Scope (Reserved)

239.255.0.0—239.255.255.255

Site-Local Scope

To configure a multicast boundary, use the interface command

ip multicast boundary access-list-number

no ip multicast boundary access-list-number

Standard IP access-list (1—99).

When configured on an interface, the ip multicast border command prevents multicast packets identified by the access list from flowing into or out of the interface. Each of the interfaces that connect border routers in Figure 9-2 would have the configuration as shown on the following page.

Figure 9-2: Multicast boundaries need to be established on the department border routers.

interface serial n

ip multicast boundary 1

access-list 1 deny 239.0.0.0 0.255.255.255

access-list 1 permit 224.0.0.0 15.255.255.255

The permit statement in the access list is required because every access list has an implicit deny any at the end of the list. In Chapter 7, we used the interface command ip pim border to prevent Bootstrap messages from passing through the interface, but allowed all other multicast traffic to pass. The ip multicast border command can be used in the same manner with regards to Auto-RP.

interface serial n

ip multicast boundary 1

access-list 1 deny 224.0.1.39

list 1 deny 224.0.1.40

access-list 1 permit 224.0.0.0 15.255.255.255

The ip multicast border command blocks Auto-RP and Mapping Agent messages from crossing the interface but allows all other multicast traffic. Although the ip multicast boundary command is usually used in conjunction with the administratively scoped block of multicast addresses, it can be used to block any multicast address on an interface.