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Chapter 2

IP Multicast in a Campus Network

IP Multicast Small Campus Design

RP of Last Resort

If the active RPs are no longer available or there are no RPs configured for a specific group, the default behavior is to dense-mode flood the multicast traffic. This is called dense-mode fallback. Typically, after deploying the recommendations in this document, an RP will always be available. However, in the event that something happens to all of the RPs or routing instability diverts access from the RPs, it is necessary to ensure that dense-mode fallback does not occur.

To prevent dense mode flooding, on every PIM-enabled router configure an access control list (ACL) and use the ip pim rp-address address group-list command to specify an “RP of last resort.” This way, if all else fails, the RP defined in this command will be used. It is recommended that a local loopback interface be configured on each router and that the address of this loopback interface be specified as the IP address of the RP of last resort. By configuring an RP of last resort, the local router will be aware of an RP and will not fallback to dense mode.


Do not advertise this loopback interface in the unicast routing table.

Example 2-1

RP of Last Resort (configured on every PIM-enabled router)

interface Loopback2 description Garbage-CAN RP ip address

ip pim rp-address 1

access-list 1 deny access-list 1 deny access-list 1 permit any

This not only helps with dense-mode fallback (by ensuring that an RP is always present on the local router if the main RPs become unavailable), but this also helps to guard against rogue sources that may stream unwanted multicast traffic. This blocking of unwanted multicast sources is sometimes referred to as a “Garbage-can RP.” For more information about using Garbage-can RPs, see Chapter 8, “Security, Timers, and Traffic Engineering in IP Multicast Networks.”

IP Multicast Small Campus Design

This section provides a sample design for IP multicast in a small campus network. In this design, there are Layer 3 interfaces on the backbone switches for HSRP. As shown in Figure 2-5, there is only one building. The VLANs are identified as either data VLANs or voice VLANs. HSRP is configured on both backbone switches and on their links connecting to the access switches. The addressing scheme for HSRP used in this design is 10.bulding_number.VLAN_number.role.

For example, the address layout for VLAN 2/building 1 is:


          —HSRP address (Default Gateway)


          —IP address of standby router (4kS3-left-BB)


          —IP address of active router (4kS3-right-BB).

All hosts on VLAN 2 use as their default gateway.


This type of addressing plan is used throughout this chapter.

Cisco AVVID Network Infrastructure IP Multicast Design



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