Understanding Virtual LANs
Let’s take a look at some technologies
that are essential for VLAN implementations.
Cisco developed the Inter-Switch Link, or
ISL, mechanism to support high-speed trunking between switches
and switches, routers, or servers in Fast Ethernet environments.
Cisco’s Inter-Switch Link protocol (ISL) enables VLAN
traffic to cross LAN segments. ISL is used for interconnecting
multiple switches and maintaining VLAN information as traffic
goes between switches. ISL uses “packet tagging”
to send VLAN packets between devices on the network without
impacting switching performance or requiring the use and exchange
of complex filtering tables. Each packet is tagged depending
on the VLAN to which it belongs.
The benefits of packet tagging include manageable broadcast
domains that span the campus; bandwidth management functions
such as load distribution across redundant backbone links
and control over spanning tree domains; and a substantial
cost reduction in the number of physical switch and router
ports required to configure multiple VLANs.
The ISL protocol enables in excess of 1000 VLANs concurrently
without requiring any fragmentation or re assembly of the
Additionally, ISL wraps a 48-byte “envelope” around
the packet that handles processing, priority, and quality-of-service,
or QoS, features. ISL is not limited to Fast Ethernet/Ethernet
packet sizes (1518 bytes) and can even accommodate large packet
sizes up to 16000 bytes — which is appropriate for Token
Ring. It is important to understand that ISL (and 802.1q—a
format used by some other vendors, for that matter) are both
just packet-tagging formats. Neither sets up a standard for
While Cisco was first to market with its
revolutionary packet tagging schemes for Fast Ethernet and
FDDI, they are proprietary solutions. Other vendors implemented
their own unique methods for sharing VLAN information across
the network. As a result, a standards body was created within
the IEEE to provide one common VLAN communication standard.
This ultimately benefits customers using switches from various
vendors in the marketplace.
Within the 802.1Q standard, packet tagging is the exchange
vehicle for VLAN information.
Because ISL is so widely deployed in our installed customer
base, Cisco will continue to support both ISL and 802.1Q.
It is important to note that Cisco’s dual mode support
of both methods will be implemented via hardware ASICs, which
will provide tremendous performance.
VLAN Standard Implementation
This diagram illustrates a typical customer
implementation of the 802.1Q VLAN standard. This scenario
is based upon a customer network composed of two separate
campuses based on different vendors’ technology (Cisco
and vendor X).
If the customer already has Cisco switches deployed, it can
maintain its use of ISL. Also, it can maintain its use of
the VLAN trunking scheme used by vendor X. However, the new
joined network must use the 802.1Q standard to share VLAN
information between switches within the campus.