Specifications

Overview of Dial Interfaces, Controllers, and Lines
Logical Interfaces
DC-8
Cisco IOS Dial Technologies Configuration Guide
Virtual Access Interfaces
A virtual access interface is a temporary interface that is spawned to terminate incoming PPP streams
that have no physical connections. PPP streams, Layer 2 Forwarding Protocol (L2F), and Layer 2 Tunnel
Protocol (L2TP) frames that come in on multiple B channels are reassembled on virtual access
interfaces. These access interfaces are constructs used to terminate packets.
Virtual access interfaces obtain their set of instructions from virtual interface templates. The attributes
configured in virtual templates are projected or cloned to a virtual access interfaces. Virtual access
interfaces are not directly user configurable. These interfaces are created dynamically and last only as
long as the tunnels or multilink sessions are active. After the sessions end, the virtual access interfaces
disappear.
Figure 6 shows how a virtual access interface functions to accommodate a multilink session event. Two
physical interfaces on two different access servers are participating in one multilink call from a remote
PC. However, each Cisco AS5300 access server has only one B channel available to receive a call. All
other channels are busy. Therefore all four packets are equally dispersed across two separate B channels
and two access servers. Each Cisco AS5300 access server receives only half the total packets. A virtual
access interface is dynamically spawned upstream on a Cisco 7206 backhaul router to receive the
multilink protocol, track the multilink frames, and reassemble the packets. The Cisco 7206 router is
configured to be the bundle master, which performs all packet assembly and reassembly for both
Cisco AS5300 access servers.
Figure 6 Virtual Access Interfaces Used for Multichassis Multilink Session Events
PC sending data over
a PPP packet stream
Cisco 1600
remote office
router
BRI
Fast
Ethernet
HSSI/ATM
Cisco AS5300.
One available B channel.
Receiving packets and
Cisco 7206 backhaul router.
Spawns all virtual access interfaces.
The dedicated bundlemaster.
Cisco AS5300.
One available B channel.
Receiving packets and
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PSTN/ISDN
ISDN network