Configuring Transparent Bridging
Transparent and SRT Bridging Configuration Task List
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Cisco IOS Bridging and IBM Networking Configuration Guide
To configure a VLAN on a transparently bridged network, use the following commands beginning in
global configuration mode:
Transparently bridged VLANs are supported in conjunction with only the IEEE Spanning-Tree Protocol.
When you logically segment a transparently bridged network into VLANs, each VLAN computes its
own spanning-tree topology. Configuring each VLAN to compute its own spanning-tree topology
provides much greater stability than running a single spanning tree throughout. Traffic bridged within
one VLAN is unaffected by physical topology changes occurring within another VLAN.
Note The current implementation of SDE encapsulation is not recommended for serial or
Ethernet media.
Routing between ISL VLANs
Our VLAN Routing implementation is designed to operate across all router platforms. However, the
Inter-Switch Link (ISL) VLAN trunking protocol currently is defined on 100 BaseTX/FX Fast Ethernet
interfaces only and therefore is appropriate to the Cisco 7000 and higher-end platforms only. The IEEE
802.10 protocol can run over any LAN or HDLC serial interface. VLAN traffic is fast switched. The
actual format of these VLAN encapsulations are detailed in the IEEE Standard 802.10-1992 Secure Data
Exchange and in the Inter-Switch Link (ISL) Protocol Specification.
Our VLAN Routing implementation treats the ISL and 802.10 protocols as encapsulation types. On a
physical router interface that receives and transmits VLAN packets, you can select an arbitrary
subinterface and map it to the particular VLAN “color” embedded within the VLAN header. This
mapping allows you to selectively control how LAN traffic is routed or switched outside of its own
VLAN domain. In the VLAN routing paradigm, a switched VLAN corresponds to a single routed subnet,
and the network address is assigned to the subinterface.
To route a received VLAN packet the Cisco IOS software VLAN switching code first extracts the VLAN
ID from the packet header (this is a 10-bit field in the case of ISL and a 4-byte entity known as the
security association identifier in the case of IEEE 802.10), then demultiplexes the VLAN ID value into
a subinterface of the receiving port. If the VLAN color does not resolve to a subinterface, the Cisco IOS
software can transparently bridge the foreign packet natively (without modifying the VLAN header) on
the condition that the Cisco IOS software is configured to bridge on the subinterface itself. For VLAN
packets that bear an ID corresponding to a configured subinterface, received packets are then classified
by protocol type before running the appropriate protocol specific fast switching engine. If the
subinterface is assigned to a bridge group then non-routed packets are de-encapsulated before they are
bridged. This is termed “fall-back bridging” and is most appropriate for nonroutable traffic types.
In Figure 10, Router A provides inter-VLAN connectivity between multiple Cisco switching platforms
where there are three distinct virtual topologies present. For example, for VLAN 300 across the two
Catalyst 1200A segments, traffic originating on LAN interface 1 is “tagged” with a VLAN ID of 300 as
it is switched onto the FDDI ring. This ID allows the remote Catalyst 1200A to make an intelligent
Command Purpose
Step1 interface type
slot/port.subinterface-number
Specifies a subinterface.
Step2 encapsulation sde said Specifies the IEEE 802.10 Security data exchange security association
identifier (in other words, specifies the “color”).
Step3 bridge-group bridge-group Associates the subinterface with an existing bridge group.