Multicast Scoping
A primary consideration when working with large-scale multicast domains is controlling the scope of the domain. You have read the discussion of the subject in Chapter 5, "Introduction to IP Multicast Routing," and you know that there are two methods of scoping multicast domains:
TTL scoping Administrative scoping
With TTL scoping, the TTL value of multicast packets is set in such a way that the packets can travel only a certain distance before the TTL is decremented to 0 and the packet is discarded. You can add some granularity to this rough method by setting boundaries on interfaces with the ip multicast ttl-threshold command. For example, an interface might be configured with ip multicast ttl-threshold 5. Only packets with TTL values greater than 5 are forwarded out of this interface. Any packets with TTL values of 5 or below are dropped. Table 7-1 shows an example of TTL scoping values. The values in this table, which is a repeat of Table 5-6, are a set of TTL values suggested for use with the MBone.
In Chapter 6, "Configuring and Troubleshooting IP Multicast Routing," you encountered several commands, such as the commands for enabling Auto-RP candidate RPs and mapping agents, that enable you to set the TTL values of the protocol messages for TTL scoping. You will encounter more commands in this chapter with the same option. However, you saw in Chapter 5 that TTL scoping lacks flexibility—a TTL boundary at an interface applies to all multicast packets. This is fine for an absolute boundary, but at times you will want some packets to be blocked and others to be forwarded.
Table 7-1. MBone TTL Thresholds|
| 0
| Restricted to same host
| | 1
| Restricted to same subnet
| | 15
| Restricted to same site
| | 63
| Restricted to same region
| | 127
| Worldwide
| | 191
| Worldwide limited bandwidth
| | 255
| Unrestricted
|
For this purpose, administrative scoping provides much more flexibility. Administrative scoping is just a procedure in which the multicast group address range 224.0.0.0–239.255.255.255 is partitioned in such a way that certain ranges of addresses are assigned certain scopes. Various domain boundaries can then be created by filtering on these address ranges. Administrative scoping is the subject of RFC 2365[1], and Table 7-2 shows the partitions that RFC suggests. You have already seen how the link-local scope of 224.0.0.0/24 is used. Packets with multicast addresses in this range—such as IGMP (224.0.0.1 and 224.0.0.2), OSPF (224.0.0.5 and 224.0.0.6), EIGRP (224.0.0.10), and PIM (224.0.0.13)—are never forwarded by a router and thus are restricted to the scope of the data link on which they were originated.
Table 7-2. RFC 2365 Administrative Partitions|
| 224.0.0.0/24
| Link-local scope
| | 224.0.1.0–238.255.255.255
| Global scope
| | 239.0.0.0/10
| Unassigned
| | 239.64.0.0/10
| Unassigned
| | 239.128.0.0/10
| Unassigned
| | 239.192.0.0/14
| Organization-local scope
| | 239.255.0.0/16
| Unassigned
|
Adding the ip multicast boundary command to an interface creates an administrative boundary. The command just references an IP access list, which specifies the group address range to be permitted or denied at the interface, as demonstrated in Example 7-1.
Example 7-1 Adding the ip multicast boundary Command to an Interface Creates an Administrative Boundary
interface Ethernet0
ip address 10.1.2.3 255.255.255.0
ip multicast boundary 10
!
interface Ethernet1
ip address 10.83.15.5 255.255.255.0
ip multicast boundary 20
!
access-list 10 deny 239.192.0.0 0.3.255.255
access-list 10 permit 224.0.0.0 15.255.255.255
access-list 20 permit 239.135.0.0 0.0.255.255
access-list 20 deny 224.0.0.0 15.255.255.255
Interface E0 marks a boundary at which organization-local packets, as defined in Table 7-2, are blocked, while global-scoped packets are passed. The boundary at E1 permits packets whose destination addresses fall within the 239.135.0.0/16 range and denies all other multicast packets. This address range falls within an undefined range in Table 7-2 and therefore has been given some special meaning by the local network administrator.
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