15.5.4 Notes on Network Configuration

(a) Configuration that requires attention

Take care of the following if using PIM-SM or PIM-SSM in the following configurations.

• In a configuration such as the one in the figure below, in which multiple routers are directly connected to the host on the same network, make sure that PIM-SM always runs at the interface.

  If IGMP runs without running PIM-SM at an interface on which multiple routers exist on the same network, multicast data might be forwarded twice.

  Figure 15-28: Configuration requiring attention (connection between multiple routers and hosts)

• Host A,B exist on the same network, and each host periodically joins or leaves the same grouping, as shown in the following diagram. In this environment, if multicast forwarding is started by joining Host B immediately after stopping multicast forwarding due to host A's detachment, it may appear that packet loss occurs immediately after multicast forwarding starts on Host B due to the propagation delay of the control packet.

  In such cases, Last Member Query Time of the Switch can be avoided by linking it with IGMP snooping of the Switch or an external device so that it is larger than the interval for joining or leaving the Switch.

  Figure 15-29: Configuration that requires attention (connection between a router and multiple hosts)

• In configurations such as the one in the figure below, in which the PIM protocol cannot detect the upstream router, multicast communication cannot be performed. This is because the environment has a static route for which the virtual interface set as the gateway has VRRP set by Switch C for Switch A and for Switch B.

  To perform multicast communication in this configuration, a static route needs to be set for Switch C, with the real address of Switch A or Switch B used as the gateway address to the rendezvous point address, BSR address, and multicast data source address.

  Figure 15-30: Configuration requiring attention (when a VRRP is set)

• In an environment in which multiple multicast forwarding is periodically switched from multiple multicast forwarding to multiple hosts, the delay between switching multicast forwarding and starting the selected multicast forwarding may increase. In such cases, setting the configuration command ip pim fast-mcache-setting improves the setting.

(b) Unwanted multicast packets from more than one VLAN

When two routers exist in a network for multicast network configurations such as in the following figure, the non-DR (Switch A) receives multicast packets that are forwarded to the user from DR (Switch B). At this time, Switch A creates negative cache entries and discards the hardware for the unnecessary packets.

The following figure illustrates the discarding of multicast packets when multiple VLANs are set.

Figure 15-31: Disposing of multicast packets when multiple VLAN are set

When the ip pim multiple-negative-cache configuration command is not set, one negative cache entry is created for one multicast communication (S, G) of each global network or VRF. If multiple VLANs are set in the same port, only negative cache entries that consider the first VLAN that received a packet as a receiving interface will be created. Other VLANs will not create negative cache entries even if the same multicast packet arrives.

For this reason, although multicast packets can be discarded in the VLAN that created the negative cache entry, if a large number of multicast packets of wrong-incoming interface or cache-misshit is received in another VLAN, packet congestion occurs in the CPU processing.

When the ip pim multiple-negative-cache configuration command is set, two or more negative cache entries are created for each multicast communication (S, G). Because negative cache entries can be created in each VLAN, packet congestion in the CPU processing can be prevented even if a large number of multicast packets of wrong-incoming interface or cache-misshit is received.

(a) Recommended configuration

We recommend network configurations based on PIM-SM for hierarchical network configurations and those with redundant routes. However, special care is needed for the placement of the rendezvous point. The following figure shows a recommended network configuration for PIM-SM.

Figure 15-32: PIM-SM recommended networking configuration

(b) Configuration that requires attention

The following configuration needs special attention:

• In configurations such as the one shown in the figure below, in which two or more routers connected directly to the sender exist on the same network, when choosing one as the rendezvous point, make sure that the DR and the rendezvous point are the same.

  If a switch other than the rendezvous point is used as the DR, the load on Switches A and B will increase because PIM-Register messages are sent from the DR to the rendezvous point. Also, when multicast packets within the PIM-Register messages are forwarded, packet loss might occur on the rendezvous point. Note that if the rendezvous point is the DR, encapsulation is not performed using PIM-Register messages.

  Figure 15-33: Configuration requiring attention (connection between multiple routers and senders)

(c) Inappropriate configuration

Do not use PIM-SM for configurations such as the following.

• Configuration in which a recipient exists between the sender and rendezvous point

  When multicast communication with group 1 is performed from the server in the following configuration, forwarding via the rendezvous point cannot be performed efficiently.

  Figure 15-34: Non-adaptive configuration (when there is a receiver between the sender and the rendezvous point)

(a) Configuration that requires attention

The following configuration needs special attention:

• Multiple PIM-SSM routers are running on the same line as the multicast group (receiver)

  To operate PIM-SSM in IGMPv2 or IGMPv3(EXCLUDE configuration, set the configuration command ip pim ssm and ip igmp ssm-map static for all routers on the same line.

  Figure 15-35: Configuration requiring attention (connection between multiple routers and hosts)

(a) Configuration that requires attention

The following configuration needs special attention:

• When building a redundant configuration with two or more VRF border routers in a IPv4 multicast extranet using PIM-SM VRF gateway, set one of VRF border routers as the rendezvous point.

  If the rendezvous point is not a VRF border router, each VRF border router forwards the same IPv4 multicast packets until the shortest path delivery tree is generated.

  Figure 15-36: Configuration requiring attention (when a redundant configuration is constructed with two or more VRF border routers)