Load Balancing

Load Balancing

When two equal cost paths exist for a destination, an IP unicast routing protocol, such as OSPF, will load-balance unicast traffic over the two links. Load-balancing, without additional configuration, is not possible with multicast routing protocols. The reason that load-balancing does not occur for multicast traffic over equal cost links is because of the selection of the RPF interface. Only one RPF interface can be selected for a multicast source and therefore all multicast traffic must flow over that link. Multicast traffic flowing on the other link will be rejected because it does not arrive on the RPF interface (see Figure 9-6).

Figure 9-6: Multicast traffic is only accepted on one link.

In order to achieve multicast load-balancing, we need to configure a tunnel between routers A and B in Figure 9-6. All multicast traffic will flow across the tunnel and the unicast routing protocols will load-balance across the actual physical links (see Figure 9-7). Load-balancing occurs because we are encapsulating the multicast traffic in unicast IP packets. Multicasting needs to be disabled on the physical interfaces and enabled on the tunnel interface.

Figure 9-7: Load-balancing multicast traffic using a tunnel.

The configurations for routers A and B are listed below:

Router A

interface ethernet 0

ip address 172.16.2.1 255.255.255.0

interface serial 0

ip address 172.16.1.1 255.255.255.252

bandwidth 200

clock rate 200000

interface serial 1

ip address 172.16.1.5 255.255.255.252

bandwidth 200

clock rate 200000

interface tunnel 0

ip unnumbered ethernet 0

ip pim dense-mode (or sparse or sparse-dense mode)

tunnel source ethernet 0

tunnel destination 172.16.3.1

Router B

interface ethernet 0

ip address 172.16.3.1 255.255.255.0

interface serial 0

ip address 172.16.1.2 255.255.255.252

bandwidth 200

interface serial 1

ip address 172.16.1.6 255.255.255.252

bandwidth 200

interface tunnel 0

ip unnumbered ethernet 0

ip pim dense-mode (or sparse or sparse-dense mode)

tunnel source ethernet 0

tunnel destination 172.16.2.1

Load-balancing will now occur over the two serial links, but the mechanisms will be different, depending on whether the routers are process-switching or fast-switching. For process-switching, the load-balancing occurs with each packet using a round-robin method. Also, the packet counts on each link will be the same. For fast-switching, load-balancing occurs with each multicast flow because an (S,G) flow will be assigned to one of the physical interfaces.