Disadvantages of using ATM Virtual Interfaces

The disadvantages of using ATM Virtual Interfaces are:

v Because AVIs do not have any physical resources of their own, each virtual interface may have fewer Virtual Connections (VCs) than a single physical interface. The available resources (in this example VCs) are partitioned among the different virtual interfaces con®gured on a single ARI and the ARI itself.

In the current implementation, resource allocation is on demand. Each physical ATM interface has a pool of resources that are available for use by all AVIs and the single ARI itself.

ATM Virtual Interface Con®guration Concepts

interfaces that can be created depends on the data packet size for the interfaces and is limited to a maximum number of 253 per router.

The use of AVIs signi®cantly improves the con®guration options for protocols such as IPX that are limited to one instance or address per ATM interface. By con®guring an appropriate number of AVIs, several IPX addresses can be supported on each physical ATM interface.

vThe ATM Virtual Interface feature is crucial for supporting multicast routing protocols (such as MOSPF) over ATM networks.

In order for multicast to operate correctly, each logical subnet must be con®gured on a different interface because multicast routing protocols typically function in such a way that a packet coming in from a router interface will never be sent out over the same interface. Thus, if more than one subnet is con®gured on an interface and a source in one subnet sends a multicast packet to a member in another subnet de®ned on the same interface, this member will never receive the packet.

By creating an individual virtual interface for each subnet, packet multicasting can be performed successfully. Typically, the number of ATM interfaces on a router will be limited, in turn limiting the number of subnets that can be correctly con®gured for multicast operation. However, by creating as many AVIs as needed (according to the number of subnets that are required to be con®gured on the router), the number of physical ATM interfaces will no longer limit the number of subnets that can be con®gured on a router for correct multicast operation.

For example, the ªone-armedº router cannot support multicast traffic over interfaces other than ELANs without the AVI feature, because incoming packets will never be sent out the same interface and will be discarded instead.

vCreating multiple AVIs on an ARI and con®guring each different protocol instance (for example, each IP subnet) on a different AVI on the same ARI, can improve performance.

For example, when multiple subnets are con®gured on a single physical ATM interface, the interface will have to reduce the maximum transmission unit or MTU (the maximum packet size that can be sent or received over that interface) to the smallest MTU of all subnets sharing the same interface. However, if multiple AVIs are created on that ARI and each IP subnet is con®gured on a different AVI, every subnet can continue to use its existing MTU size without consideration of other subnets con®gured on the same physical ATM interface. This avoids possible reduction in throughput and delays due to packet fragmentation and reassembly caused by MTU size reduction.

Another performance improvement can be achieved by distributing the number of protocol addresses con®gured on a physical interface over different virtual interfaces con®gured on the same physical interface. The per-interface protocol lists get shortened, resulting in faster searches and reduced processing time.