IBM SC30-3681-08 Disadvantages of using ATM Virtual Interfaces

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 AVIssigniﬁcantly improves the conﬁguration options for protocols

such as IPX that are limited to one instance or address per ATMinterface. By

conﬁguring an appropriate number of AVIs,several IPX addresses can be

supported on each physical ATMinterface.

vThe ATMVirtual Interface feature is crucial for supporting multicast routing

protocols (such as MOSPF) over ATMnetworks.

In order for multicast to operate correctly, each logical subnet

must

conﬁgured 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 conﬁgured

on an interface and a source in one subnet sends a multicast packet to a

member in another subnet deﬁned 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

conﬁgured for multicast operation. However, by creating as manyAVIs as needed

(according to the number of subnets that are required to be conﬁgured on the

router), the number of physical ATMinterfaces will no longer limit the number of

subnets that can be conﬁgured on a router for correct multicast operation.

For example, the “one-armed” router cannot support multicast traffic over

interfaces other than ELANs without the AVIfeature, because incoming packets

will never be sent out the same interface and will be discarded instead.

vCreating multiple AVIson an ARI and conﬁguring each different protocol instance

(for example, each IP subnet) on a differentAVI on the same ARI, can improve

performance.

For example, when multiple subnets are conﬁgured 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 AVIsare created on that ARI and each IP subnet is conﬁgured on a

different AVI,every subnet can continue to use its existing MTU size without

consideration of other subnets conﬁgured on the same physical ATMinterface.

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 conﬁgured on a physical interface over different virtual

interfaces conﬁgured on the same physical interface. The per-interface protocol

lists get shortened, resulting in faster searches and reduced processing time.

Disadvantages of using ATM Virtual Interfaces

The disadvantages of using ATMVirtual Interfaces are:

vBecause AVIsdo 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 conﬁgured on a single ARI and theARI 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 allAVIs and

the single ARI itself.

ATM Virtual Interface Conﬁguration Concepts

Chapter21. UsingATM 275