Multihoming Policy

If a system is configured with more than one IP interface, then the system might have multiple equivalent routes to a given destination, forcing the system to choose which IP interface to send a packet on. Similarly, a packet may arrive on one IP interface, but be destined to an IP address that is hosted on another IP interface. The system's behavior in such situations is determined by the selected multihoming policy.

The multihoming policy value controls the system policy for accepting and transmitting IP packets when multiple IP interfaces are simultaneously enabled. The value of the multihoming policy can be loose (default), adaptive, or strict.

Table 2-10 Multihoming Policies

Policy Description

Policy	Description
`Loose`	Do not enforce any binding between an IP packet and the IP interface used to send or receive the packet. An IP packet will be accepted on an IP interface as long as its destination IP address is up on the appliance. An IP packet will be transmitted over the IP interface tied to the route that most specifically matches an IP packet's destination address, without any regard for the IP addresses hosted on that IP interface. If no eligible routes exist, drop the packet.
`Adaptive`	Identical to `Loose`, except prefer routes with a gateway address on the same subnet as the packet's source IP address. An IP packet will be accepted on an IP interface as long as its destination IP address is up on the appliance. An IP packet will be transmitted over the IP interface tied to the route that most specifically matches an IP packet's destination address. If multiple routes are equally specific, prefer routes that have a gateway address on the same subnet as the packet's source address. If no eligible routes exist, drop the packet.
`Strict`	Require a strict binding between an IP packet and the IP interface used to send or receive it. An IP packet will be accepted on an IP interface as long as its destination IP address is up on that IP interface. An IP packet will only be transmitted over an IP interface if its source IP address is up on that IP interface. To enforce this, when matching against the available routes, the appliance will ignore any routes that have gateway addresses on a different subnet from the packet's source address. If no eligible routes remain, drop the packet.

Loose

Do not enforce any binding between an IP packet and the IP interface used to send or receive the packet.

An IP packet will be accepted on an IP interface as long as its destination IP address is up on the appliance.
An IP packet will be transmitted over the IP interface tied to the route that most specifically matches an IP packet's destination address, without any regard for the IP addresses hosted on that IP interface.

If no eligible routes exist, drop the packet.

Adaptive

Identical to Loose, except prefer routes with a gateway address on the same subnet as the packet's source IP address.

An IP packet will be accepted on an IP interface as long as its destination IP address is up on the appliance.
An IP packet will be transmitted over the IP interface tied to the route that most specifically matches an IP packet's destination address. If multiple routes are equally specific, prefer routes that have a gateway address on the same subnet as the packet's source address.

If no eligible routes exist, drop the packet.

Strict

Require a strict binding between an IP packet and the IP interface used to send or receive it.

An IP packet will be accepted on an IP interface as long as its destination IP address is up on that IP interface.
An IP packet will only be transmitted over an IP interface if its source IP address is up on that IP interface. To enforce this, when matching against the available routes, the appliance will ignore any routes that have gateway addresses on a different subnet from the packet's source address.

If no eligible routes remain, drop the packet.

When selecting the multihoming policy, a key consideration is whether any of the appliance's IP interfaces will be dedicated to administration (for example, for dedicated BUI access) and thus accessed over a separate administration network. In particular, if a default route is created to provide remote access to the administration network, and a separate default route is created to provide remote access to storage protocols, then the default system policy of Loose might cause the administrative default route to be used for storage traffic. By switching the policy to Adaptive or Strict, the appliance will consider the IP address associated with the request as part of selecting the route for the reply. If no route can be found on the same IP interface, the Adaptive policy will cause the system to use any available route, whereas the Strict policy will cause the system to drop the packet.