Allied Telesis AT-iMG634 - R2 User's Guide download pdf (Page 309)

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Secondary IP addresses IP

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iMG/RG Software Reference Manual (IPNetwork Functions)

4.1.6.2 Similarities between virtual interfaces and real interfaces

A virtual interface is similar to a real interface:

• Virtual interfaces may be manipulated in the same way as real interfaces using the CLI.

• The IP stack will route between virtual interfaces and real interfaces in the same way that it routes between

real interfaces.

Note: Like real interfaces, virtual interfaces must have a unique subnet that does not overlap with other

interfaces. In order to have the router respond to more than one IP address on the same subnet,

secondary addresses must be used instead of virtual interfaces.

4.1.6.3 Differences between virtual interfaces and real interfaces

When the IP stack receives a packet from a transport that has associated virtual interfaces, the IP stack must

decide which interface the packet arrived on.

The source address of the incoming packet is compared with the subnet of each virtual interface on that trans-

port. If there is no match, the IP stack assumes that the packet arrived on the real interface.

The interface that the packet arrived on is important in two scenarios:

• When the Firewall is in use - different rules (such as policies, portfilters and validators) are configured

between different interfaces, so you need to know which interfaces the packet passes between.

• Some applications are written to only respond to traffic received on a specific interface. For example, DHCP

server.

Because the traffic for all virtual interfaces is received in the same way as the real interface, the only reasonable

way of selecting an interface is based on source address as described above. This means that:

• A virtual interface only receives packets with a source address matching its interface subnet, providing pack-

ets arrive via the real interface that the virtual interface is attached to.

• Packets that arrive with a source address that does not match a local subnet are deemed to have been

received on the real interface, even if the next hop would be reached through the virtual interface when

sending to that destination.

• Any packets from an unconfigured host, for example DHCP or BOOTP requests, are deemed to be received

on the real interface.

Note: Remember that the sender can spoof the source address of the packet; therefore security-related

decisions should not be based on the ability to distinguish between virtual interfaces on the same

transport.

4.1.7 Secondary IP addresses

Secondary IP addresses differ from virtual interfaces because there is no concept of a separate local subnet

associated with a secondary address.

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