Netgear XCM8810 Chassis User Manual - Page 622

NETGEAR 8800 User Manual ARP ARP operates on the interface and responds to every request coming from either the primary or secondary subnet. When multiple subnets are configured on a VLAN and an ARP request is generated by the switch over that VLAN, the source IP address of the ARP request must be a local IP address of the subnet to which the destination IP address (which is being ARPed) belongs. For example, if a switch multinets the subnets 10.0.0.0/24 and 20.0.0.0/24 (with VLAN IP addresses of 10.0.0.1 and 20.0.0.1), and generates an ARP request for the IP address 10.0.0.2, then the source IP address in the ARP packet is set to 10.0.0.1 and not to 20.0.0.1. Route Manager The Route Manager installs a route corresponding to each of the secondary interfaces. The route origin is direct, is treated as a regular IP route, and can be used for IP data traffic forwarding. These routes can also be redistributed into the various routing protocol domains if you configure route redistribution. IRDP Some functional changes are required in Internet Router Discovery Protocol (IRDP) to support IP multinetting. When IRDP is enabled on a Layer 3 VLAN, XCM8800 periodically sends ICMP router advertisement messages through each subnet (primary and secondary) and responds to ICMP router solicitation messages based on the source IP address of the soliciting host. Unicast Routing Protocols Unicast routing protocols treat each IP network as an interface. The interface corresponding to the primary subnet is the active interface, and the interfaces corresponding to the secondary subnet are passive subnets. For example, in the case of Open Shortest Path First (OSPF), the system treats each network as an interface, and hello messages are not sent out or received over the non-primary interface. In this way, the router link state advertisement (LSA) includes information to advertise that the primary network is a transit network and the secondary networks are stub networks, thereby preventing any traffic from being routed from a source in the secondary network. Interface-based routing protocols (for example, OSPF) can be configured on per VLAN basis. A routing protocol cannot be configured on an individual primary or secondary interface. Configuring a protocol parameter on a VLAN automatically configures the parameter on all its associated primary and secondary interfaces. The same logic applies to configuring IP forwarding, for example, on a VLAN. Routing protocols in the multinetted environment advertise the secondary subnets to their peers in their protocol exchange process. For example, for OSPF the secondary subnets are advertised as stub networks in router LSAs. RIP also advertises secondary subnets to its peers residing on the primary subnet. 622 | Chapter 20. IPv4 Unicast Routing