Owner's Manual

622 | Chapter 20. IPv4 Unicast Routing
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.