Cisco Systems BC-109 User Manual
Configuring Source-Route Bridging
This chapter describes source-route bridging (SRB) configuration tasks. For a discussion of remote
source-route bridging (RSRB) configuration tasks, refer to the “Configuring Remote Source-Route
Bridging” chapter in this publication.
For a complete description of the SRB commands mentioned in this chapter, refer to the
“Source-Route Bridging Commands” chapter in the Bridging and IBM Networking Command
Reference. To locate documentation of other commands that appear in this chapter, use the command
reference master index or search online.
SRB Configuration Task List
Perform the tasks in the following sections to configure SRB:
• Configure Source-Route Bridging
• Configure Bridging of Routed Protocols
• Configure Translation between SRB and Transparent Bridging Environments
• Configure NetBIOS Support
• Configure LNM Support
• Secure the SRB Network
• Tune the SRB Network
• Establish SRB Interoperability with Specific Token Ring Implementations
• Monitor and Maintain the SRB Network
See the end of this chapter for “SRB Configuration Examples.”
Warning The Cisco IOS software issues a warning if a duplicate bridge definition exists in a router. You
must remove an old bridge definition before adding a new bridge definition to a router configuration.
Configuring Source-Route Bridging BC-109
Configure Source-Route Bridging
Configure Source-Route Bridging
Our implementation of source-route bridging enables you to connect two or more Token Ring
networks using either Token Ring or Fiber Distributed Data Interface (FDDI) media.
The Cisco IOS software offers the ability to encapsulate source-route bridging traffic using RFC
1490 Bridged 802.5 encapsulation. This encapsulation provides SRB over Frame Relay
functionality.
You can configure the Cisco IOS software for source-route bridging by performing the tasks in one
of the first three sections and, optionally, the tasks in the last section:
• Configure a Dual-Port Bridge
• Configure a Multiport Bridge Using a Virtual Ring
• Configure SRB over FDDI
• Configure Fast-Switching SRB over FDDI
• Configure SRB over Frame Relay
• Enable the Forwarding and Blocking of Spanning-Tree Explorers
• Enable the Automatic Spanning-Tree Function
• Limit the Maximum SRB Hops
Configure a Dual-Port Bridge
A dual-port bridge is the simplest source-route bridging configuration. When configured as a
dual-port bridge, the access server or router serves to connect two Token Ring LANs. One LAN is
connected through one port (Token Ring interface), and the other LAN is connected through the
other port (also a Token Ring interface). Figure 45 shows a dual-port bridge.
Figure 45 Dual-Port Bridge
Token
Ring
129
To configure a dual-port bridge that connects two Token Rings, you must enable source-route
bridging on each of the Token Ring interfaces that connect to the two Token Rings. To enable
source-route bridging, use the following command in interface configuration mode for each of the
Token Ring interfaces:
Command Purpose
source-bridge local-ring bridge-number target-ring Enable local source-route bridging on
T0
T1
Token
Ring
130
S1102a
a Token Ring interface.
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Note Ring numbers need to be unique across interfaces and networks, so that when you enable
source-route bridging over an interface the local and target rings are defined. Each node on the
network will know if it is the target of explorer packets sent on the network.
Bridging and IBM Networking Configuration Guide
Configure a Multiport Bridge Using a Virtual Ring
A dual-port bridge is a limitation imposed by IBM Token Ring chips; the chips can process only two
ring numbers. If you have a router with two or more Token Ring interfaces, you can work around the
two-ring number limitation. You can configure your router as multiple dual-port bridges or as a
multiport bridge using a virtual ring.
You can define several separate dual-port bridges in the same router. However, the routers on the
LANs cannot have any-to-any connectivity; that is, they cannot connect to every other router on the
bridged LANs. Only the routers connected to the dual-port bridge can communicate with one
another. Figure 46 shows two separate dual-port bridges (T0-T2 and T1-T3) configured on the same
router.
Figure 46 Multiple Dual-Port Bridges
Token
Ring
T1
Token
Ring
T2
T3
Token
Ring
T0
Token
Ring
S2323
To configure multiple dual-port source-route bridges, use the following command in interface
configuration mode for each Token Ring interface that is part of a dual-port bridge:
Command Purpose
source-bridge local-ring bridge-number target-ring Enable local source-route bridging on
a Token Ring interface.
If you want your network to use only SRB, you can connect as many routers as you need via Token
Rings. Remember, source-route bridging requires you to bridge only Token Ring media.
Configure a Multiport Bridge Using a Virtual Ring
A better solution for overcoming the two-ring number limitation of IBM Token Ring chips is to
configure a multiport bridge using a virtual ring. A virtual ring on a multiport bridge allows the
router to interconnect three or more LANs with any-to-any connectivity; that is, connectivity
between any of the routers on each of the three LANs is allowed. A virtual ring creates a logical
Token Ring internal to the Cisco IOS software, which causes all the Token Rings connected to the
router to be treated as if they are all on the same Token Ring. The virtual ring is called a ring group.
Figure 47 shows a multiport bridge using a virtual ring.
Configuring Source-Route Bridging BC-111
Configure Source-Route Bridging
Figure 47 Multiport Bridge Using a Virtual Ring
T2
Token
Ring
To take advantage of this virtual ring feature, each Token Ring interface on the router must be
configured to belong to the same ring group. For information about configuring a multiport bridge
using a virtual ring, see the “Configure a Multiport Bridge Using a Virtual Ring” section later in this
chapter.
To configure a source-route bridge to have more than two network interfaces, you must perform the
following tasks:
Token
Ring
T1
Ring
group
T3
Token
Ring
T0
Token
Ring
S2324
Step 1 Define a ring group.
Step 2 Enable source-route-bridging and assign a ring group to a Token Ring interface.
Once you have completed these tasks, the router acts as a multiport bridge not as a dual-port bridge.
Note Ring numbers need to be unique across interfaces and networks.
Define a Ring Group in SRB Context
Because all IBM Token Ring chips can process only two ring numbers, we have implemented the
concept of a ring group or virtual ring. A ring group is a collection of Token Ring interfaces in one
or more routers that share the same ring number. This ring number is used just like a physical ring
number, showing up in any route descriptors contained in packets being bridged. Within the context
of a multiport bridge that uses SRB rather than RSRB, the ring group resides in the same router. See
the “Configuring Remote Source-Route Bridging” chapter to compare ring groups in the SRB and
RSRB context.
A ring group must be assigned a ring number that is unique throughout the network. It is possible to
assign different Token Ring interfaces on the same router to different ring groups, if, for example,
you plan to administer them as interfaces in separate domains.
To define or remove a ring group, use one of the following commands in global configuration mode:
Command Purpose
source-bridge ring-group ring-group
[virtual-mac-address]
no source-bridge ring-group ring-group
[virtual-mac-address]
Define a ring group.
Remove a ring group.
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Bridging and IBM Networking Configuration Guide
Enable SRB and Assign a Ring Group to an Interface
After you have defined a ring group, you must assign that ring group to those interfaces you plan to
include in that ring group. An interface can only be assigned to one ring group. To enable any-to-any
connectivity among the end stations connected through this multiport bridge, you must assign the
same target ring number to all Token Ring interfaces on the router.
To enable SRB and assign a ring group to an interface, use the following command in interface
configuration mode:
Command Purpose
source-bridge local-ring bridge-number target-ring Enable source-route bridging and
Configure SRB over FDDI
Cisco’s implementation of SRB expands the basic functionality to allow autonomous switching of
SRB network traffic for FDDI interfaces, adding counters to SRB accounting statistics, and
implementing process-level switching of SRB over FDDI. This functionality provides a significant
increase in performance for Token Rings interconnected across an FDDI backbone (see Figure 48).
Configure SRB over FDDI
assign a ring group to a Token Ring
interface.
SRB over FDDI is supported on the Cisco 4000-M, Cisco 4500-M, Cisco 4700-M, Cisco 7000
series, Cisco 7200 series, and Cisco 7500 routers.
Figure 48 Autonomous FDDI SRB
Token
Ring
FDDI
Virtual
ring
Token
Ring
S3227
To configure autonomous FDDI SRB, use the following commands, beginning in global
configuration mode:
Step Command Purpose
1 interface fddi slot/port Configure an FDDI interface.
2 source-bridge local-ring bridge-number
target-ring
3 source-bridge route-cache cbus Enable autonomous switching.
Enable source-route bridging.
Configuring Source-Route Bridging BC-113
Configure Source-Route Bridging
Configure Fast-Switching SRB over FDDI
Fast-Switching SRB over FDDI enhances performance. For example, if you want to use access-lists,
fast-switching SRB over FDDI provides fast performance and access-list filters capability.
To configure fast-switching SRB over FDDI, use the following commands, beginning in global
configuration mode:
Step Command Purpose
1 interface fddi slot/port Configure an FDDI interface.
2 source-bridge local-ring bridge-number
target-ring
3 source-bridge spanning Enable source-bridge spanning.
4 source-bridge route-cache Enable fast-switching.
5 multiring protocol-keyword Enable the collection and use of RIF
Configure SRB over Frame Relay
Cisco IOS software offers the ability to encapsulate source-route bridging traffic using RFC 1490
Bridged 802.5 encapsulation. This provides SRB over Frame Relay functionality that is
interoperable with other vendors’ implementations of SRB over Frame Relay and with some
vendors’ implementations of FRAS BAN.
Enable source-route bridging.
information.
Note In the initial release, SRB over Frame Relay does not support the Cisco IOS software proxy
explorer, automatic spanning-tree, or LAN Network Manager functions.
To configure SRB over Frame Relay, use the following commands in interface configuration mode:
Step Command Purpose
1 interface serial number Specify the serial port.
2 encapsulation frame-relay Enable Frame Relay encapsulation.
3 interface serial slot/port.subinterface-number
point-to-point
4 frame-relay interface-dlci dlci ietf Configure a DLCI number for the
5 source-bridge source-ring-number bridge-number
target-ring-number conserve-ring
Configure a Frame Relay
point-to-point subinterface.
point-to-point subinterface.
Assign a ring number to the Frame
Relay permanent virtual circuit.
Enable the Forwarding and Blocking of Spanning-Tree Explorers
When trying to determine the location of remote destinations on a source-route bridge, the source
device will need to send explorer packets. Explorer packets are used to collect routing information
field (RIF) information. The source device can send spanning-tree explorers or all-routes explorers.
Note that some older IBM devices generate only all-routes explorer packets, but many newer IBM
devices are capable of generating spanning-tree explorer packets.
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A spanning-tree explorer packet is an explorer packet that is sent to a defined group of nodes that
comprise a statically configured spanning tree in the network. In contrast, an all-routes explorer
packet is an explorer packet that is sent to every node in the network on every path.
Bridging and IBM Networking Configuration Guide
Enable the Automatic Spanning-Tree Function
Forwarding all-routes explorer packets is the default. However, in complicated source-route bridging
topologies, using this default can generate an exponentially large number of explorers that are
traversing the network. The number of explorer packets becomes quite large because duplicate
explorer packets are sent across the network to every node on every path. Eventually each explorer
packet will reach the destination device. The destination device will respond to each of these
explorer packets. It is from these responses that the source device will collect the RIF and determine
which route it will use to communicate with the destination device. Usually, the route contained in
the first returned response will be used.
The number of explorer packets traversing the network can be reduced by sending spanning-tree
explorer packets. Spanning-tree explorer packets are sent to specific nodes; that is, to only the nodes
on the spanning tree, not to all nodes in the network. You must manually configure the spanning-tree
topology over which the spanning-tree explorers are sent. You do this by configuring which
interfaces on the routers will forward spanning-tree explorers and which interfaces will block them.
To enable forwarding of spanning-tree explorers on an outgoing interface, use the following
command in interface configuration mode:
Command Purpose
source-bridge spanning Enable the forwarding of
spanning-tree explorer packets on an
interface.
Note While enabling the forwarding of spanning-tree explorer packets is not an absolute
requirement, it is strongly recommended in complex topologies. Configuring an interface to block
or forward spanning-tree explorers has no effect on how that interface handles all-routes explorer
packets. All-routes explorers can always traverse the network.
To block forwarding of spanning tree explorers on an outgoing interface, use the following command
in interface configuration mode:
Command Purpose
no source-bridge spanning Block spanning-tree explorer packets
Enable the Automatic Spanning-Tree Function
The automatic spanning-tree function supports automatic resolution of spanning trees in SRB
networks, which provides a single path for spanning explorer frames to traverse from a given node
in the network to another. Spanning explorer frames have a single-route broadcast indicator set in
the routing information field. Port identifiers consist of ring numbers and bridge numbers associated
with the ports. The spanning-tree algorithm for SRB does not support Topology Change Notification
bridge protocol data unit (BDPU).
Note Although the automatic spanning-tree function can be configured with source-route
translational bridging (SR/TLB), the SRB domain and transparent bridging domain have separate
spanning trees. Each Token Ring interface can belong to only one spanning tree. Only one bridge
group can run the automatic spanning-tree function at a time.
on an interface.
Configuring Source-Route Bridging BC-115
Configure Source-Route Bridging
To create a bridge group that runs an automatic spanning-tree function compatible with the IBM
SRB spanning-tree implementation, use the following command in global configuration mode:
Command Purpose
bridge bridge-group protocol ibm Create a bridge group that runs the
To enable the automatic spanning-tree function for a specified group of bridged interfaces, use the
following command in interface configuration mode:
Command Purpose
source-bridge spanning bridge-group Enable the automatic spanning-tree
To assign a path cost for a specified interface, use the following command in interface configuration
mode:
Command Purpose
source-bridge spanning bridge-group path-cost path-cost Assign a path cost for a specified
automatic spanning-tree function.
function on a group of bridged
interfaces.
group of bridged interfaces.
Note Ports running IEEE and IBM protocols form a spanning tree together on the LAN, but they
do not mix in the router itself. Make sure the configurations are correct and that each LAN runs only
one protocol.
See the end of this chapter for an example of source-route bridging with the automatic spanning-tree
function enabled.
Limit the Maximum SRB Hops
You can minimize explorer storms if you limit the maximum number of source-route bridge hops.
For example, if the largest number of hops in the best route between two end stations is six, it might
be appropriate to limit the maximum source-route bridging hops to six to eliminate unnecessary
traffic. This setting affects spanning-tree explorers and all-routes explorers sent from source devices.
To limit the number of SRB hops, use one of the following commands in interface configuration
mode:
Command Purpose
source-bridge max-hops count Control the forwarding or blocking of
source-bridge max-in-hops count Control the forwarding or blocking of
source-bridge max-out-hops count Control the forwarding or blocking of
all-routes explorer frames received
on this interface.
spanning-tree explorer frames
received on this interface.
spanning-tree explorer frames sent
from this interface.
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Bridging and IBM Networking Configuration Guide
Configure Bridging of Routed Protocols
Source-route bridges use Media Access Control (MAC) information, specifically the information
contained in the RIF, to bridge packets. A RIF contains a series of ring and bridge numbers that
represent the possible paths the source node might use to send packets to the destination. Each ring
number in the RIF represents a single Token Ring in the source-route bridged network and is
designated by a unique 12-bit ring number. Each bridge number represents a bridge that is between
two Token Rings in the SRB network and is designated by a unique 4-bit bridge number. The
information in a RIF is derived from explorer packets traversing the source-route bridged network.
Without the RIF information, a packet could not be bridged across a source-route bridged network.
Unlike source-route bridges, Level 3 routers use protocol-specific information (for example, Novell
Internetwork Packet Exchange (IPX) or Xerox Network Systems (XNS) headers) rather than MAC
information to route datagrams. As a result, the Cisco IOS software default for routed protocols is
to not collect RIF information and to not be able to bridge routed protocols. However, if you want
the software to bridge routed protocols across a source-route bridged network, the software must be
able to collect and use RIF information to bridge packets across a source-route bridged network. You
can configure the software to append RIF information to routed protocols so that routed protocols
can be bridged. Figure 49 shows a network topology in which you would want to use this feature.
Figure 49 Topology for Bridging Routed Protocols across a Source-Route Bridged
Network
Configure Bridging of Routed Protocols
Cisco routed network Cisco routed network
To configure the Cisco IOS software to bridge routed protocols, you must perform the task in the
first section, and optionally, one or both of the tasks in the other sections as follows:
• Enable Use of the RIF
• Configure a Static RIF Entry
• Configure the RIF Timeout Interval
Enable Use of the RIF
You can configure the Cisco IOS software so that it will append RIF information to the routed
protocols. This allows routed protocols to be bridged across a source-route bridged network. The
routed protocols that you can bridge are as follows:
• Apollo Domain
• AppleTalk
Source-route bridged
Token Ring network
S2328
• ISO Connectionless Network Service (CLNS)
• DECnet
• IP
• IPX
Configuring Source-Route Bridging BC-117
Configure Bridging of Routed Protocols
• VINES
• XNS
Enable use of the RIF only on Token Ring interfaces on the router.
To configure the Cisco IOS software to append RIF information, use the following command in
interface configuration mode:
Command Purpose
multiring {protocol-keyword [all-routes | spanning] | all |
other}
For an example of how to configure the software to bridge routed protocols, see the “SRB and
Routing Certain Protocols Example” section later in this chapter.
Configure a Static RIF Entry
If a Token Ring host does not support the use of IEEE 802.2 TEST or XID datagrams as explorer
packets, you might need to add static information to the RIF cache of the router.
To configure a static RIF entry, use the following command in global configuration mode:
Enable collection and use of RIF
information.
Command Purpose
rif mac-address rif-string
ring}
{interface-name | ring-group
Configure the RIF Timeout Interval
RIF information that can be used to bridge routed protocols is maintained in a cache whose entries
are aged.
Note The rif validate enable commands have no effect on remote entries learned over RSRB.
To configure the number of minutes an inactive RIF entry is kept in the cache, use the following
commands in global configuration mode:
Step Command Purpose
1 rif timeout minutes Specify the number of minutes an inactive
2 rif validate-enable Enable RIF validation for entries learned on
3 rif validate-enable-age Enable RIF validation on an SRB that is
4 rif validate-enable-route-cache Enable synchronization of the RIF cache with
Enter static source-route information
into the RIF cache.
RIF entry is kept.
an interface (Token Ring or FDDI).
malfunctioning.
the protocol route cache.
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Bridging and IBM Networking Configuration Guide
Configure Translation between SRB and Transparent Bridging Environments
Configure Translation between SRB and Transparent Bridging
Environments
Source-route translational bridging (SR/TLB) is a Cisco IOS software feature that allows you to
combine SRB and transparent bridging networks without the need to convert all of your existing
source-route bridges to source-route transparent (SRT) nodes. As such, it provides a cost-effective
connectivity path between Ethernets and Token Rings, for example.
When a router is configured for SR/TLB, the router operates in fast-switching mode by default,
causing packets to be processed in the interrupt handler when the packets first arrive, rather than
queuing them for scheduled processing. You can also use the no source-bridge transparent
fastswitch command to disable fast-switched SR/TLB, causing the router to handle packets by
process switching. For more information on disabling fast-switched SR/TLB, refer to the “Disable
Fast-Switched SR/TLB” section in this chapter.
Note When you are translationally bridging, you will have to route routed protocols and
translationally bridge all others, such as local-area transport (LAT).
Overview of SR/TLB
You can bridge packets between an SRB domain and a transparent bridging domain. Using this
feature, a software “bridge” is created between a specified virtual ring group and a transparent bridge
group. To the source-route station, this bridge looks like a standard source-route bridge. There is a
ring number and a bridge number associated with a ring that actually represents the entire transparent
bridging domain. To the transparent bridging station, the bridge represents just another port in the
bridge group.
When bridging from the SRB (typically, Token Ring) domain to the transparent bridging (typically,
Ethernet) domain, the source-route fields of the frames are removed. The RIFs are cached for use by
subsequent return traffic.
When bridging from the transparent bridging domain to the SRB domain, the router checks the
packet to see if it has a multicast or broadcast destination or a unicast (single host) destination. If it
is multicast, the packet is sent as a spanning-tree explorer. If it is a unicast destination, the router
looks up the path to the destination in the RIF cache. If a path is found, it will be used; otherwise,
the router will send the packet as a spanning-tree explorer.
An example of a simple SR/TLB topology is shown in Figure 50.
Configuring Source-Route Bridging BC-119
Configure Translation between SRB and Transparent Bridging Environments
Figure 50 Example of a Simple SR/TLB Topology
Transparent
bridging "ring"
Transparent
Source-route
bridged domain
Token
Ring
Frames lose RIFs in this direction
Router running
SR/TLB
bridging domain
Frames gain RIFs in this direction
Note The Spanning-Tree Protocol messages used to prevent loops in the transparent bridging
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domain are not passed between the SRB domain and the transparent bridging domain. Therefore,
you must not set up multiple paths between the SRB and transparent bridging domains.
The following notes and caveats apply to all uses of SR/TLB:
• Multiple paths cannot exist between the source-route bridged domain and the transparent bridged
domain. Such paths can lead to data loops in the network, because the spanning-tree packets used
to avoid these loops in transparent bridging networks do not traverse the SRB network.
• Some devices, notably PS/2s under certain configurations running OS/2 Extended Edition
Version 1.3, do not correctly implement the “largest frame” processing on RIFs received from
remote source-route bridged hosts. The maximum Ethernet frame size is smaller than that
allowed for Token Ring. As such, bridges allowing for communication between Ethernet and
Token Ring will tell the Token Ring hosts, through the RIF on frames destined to the Token Ring,
that hosts on the Ethernet cannot receive frames larger than a specified maximum, typically
1472 bytes. Some machines ignore this run-time limit specification and send frames larger than
the Ethernet can accept. The router and any other Token Ring/Ethernet bridge has no choice but
to drop these frames. To allow such hosts to successfully communicate across or to an Ethernet,
you must configure their maximum frame sizes manually. For the PS/2, this can be done through
Communications Manager.
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• Any access filters applied on any frames apply to the frames as they appear on the media to which
the interface with the access filter applies. This is important because in the most common use of
SR/TLB (Ethernet and Token Ring connectivity), the bit ordering of the MAC addresses in the
frame is swapped. Refer to the SR/TLB examples in the “SRB Configuration Examples” section
of this chapter.
Caution Bridging between dissimilar media presents several problems that can prevent communication
from occurring. These problems include bit order translation (or usage of MAC addresses as data), maximum
transmission unit (MTU) differences, frame status differences, and multicast address usage. Some or all of
these problems might be present in a multimedia bridged LAN and prevent communication from taking place.
Because of differences in the way end nodes implement Token Ring, these problems are most prevalent when
bridging between Token Rings and Ethernets or between Token Ring and FDDI LANs.
Bridging and IBM Networking Configuration Guide
Enable Bridging between Transparent Bridging and SRB
We currently know that problems occur with the following protocols when bridged between Token
Ring and other media: Novell IPX, DECnet Phase IV, AppleTalk, VINES, XNS, and IP. Further,
problems can occur with the Novell IPX and XNS protocols when bridged between FDDI and other
media. We recommend that these protocols be routed whenever possible.
To enable SR/TLB, you must perform the task in the following section:
• Enable Bridging between Transparent Bridging and SRB
In addition, you can also perform the tasks in the following sections:
• Disable Fast-Switched SR/TLB
• Enable Translation Compatibility with IBM 8209 Bridges
• Enable Token Ring LLC2-to-Ethernet Conversion
Enable Bridging between Transparent Bridging and SRB
Before enabling bridging, you must have completely configured your router using multiport SRB
and transparent bridging. Once you have done this, establish bridging between transparent bridging
and source-route bridging by using the following command in global configuration mode:
Command Purpose
source-bridge transparent ring-group pseudo-ring
bridge-number tb-group [oui]
Enable bridging between transparent
bridging and SRB.
Disable Fast-Switched SR/TLB
To disable fast-switched SR/TLB and cause the router to handle packets by process switching, use
the following command in global configuration mode:
Command Purpose
no source-bridge transparent ring-group fastswitch Disable fast-switched SR/TLB.
Enable Translation Compatibility with IBM 8209 Bridges
To transfer data between IBM 8209 Ethernet/Token Ring bridges and routers running the SR/TLB
software (to create a Token Ring backbone to connect Ethernets), use the following command on
each Token Ring interface in interface configuration mode:
Command Purpose
ethernet-transit-oui [90-compatible | standard | cisco] Move data between IBM 8209
Ethernet/Token Ring bridges and
routers running translational bridging
software.
Configuring Source-Route Bridging BC-121
Configure Translation between SRB and Transparent Bridging Environments
Enable Token Ring LLC2-to-Ethernet Conversion
The Cisco IOS software supports the following types of Token Ring-to-Ethernet frame conversions
using Logical Link Control, type 2 (LLC2) Protocol:
• Token Ring LLC2 to Ethernet Type II (0x80d5 processing)
• Token Ring LLC2 to Ethernet 802.3 LLC2 (standard)
For most non-IBM hosts, Token Ring LLC2 frames can be translated in a straightforward manner
into Ethernet 802.3 LLC2 frames. This is the default conversion in the Cisco IOS software.
However, many Ethernet-attached IBM devices use nonstandard encapsulation of LLC2 on Ethernet.
Such IBM devices, including PS/2s running OS/2 Extended Edition and RT-PCs, do not place their
LLC2 data inside an 802.3 format frame, but rather place it into an Ethernet Type 2 frame whose
type is specified as 0x80d5. This nonstandard format is called 0x80d5, named after the type of frame.
This format is also sometimes called RT-PC Ethernet format because these frames were first widely
seen on the RT-PC. Hosts using this nonstandard 0x80d5 format cannot read the standard Token Ring
LLC2 to Ethernet 802.2 LLC frames.
To enable Token Ring LLC2 to Ethernet LLC2 conversion, you can perform one or both of the
following tasks:
• Enable 0x80d5 Processing
• Enable Standard Token Ring LLC2-to-Ethernet LLC2 Conversion
Enable 0x80d5 Processing
You can change the Cisco IOS software’s default translation behavior of translating Token Ring LLC
to Ethernet 802.3 LLC to translate Token Ring LLC2 frames into Ethernet 0x80d5 format frames.
To enable this nonstandard conversion, use the following command in global configuration mode:
Command Purpose
source-bridge enable-80d5 Change the Ethernet/Token Ring translation behavior to translate Token
Ring LLC2 frames into Ethernet 0x80d5 format frames.
Enable Standard Token Ring LLC2-to-Ethernet LLC2 Conversion
After you change the translation behavior to perform Token Ring LLC2 frames into Ethernet 0x80d5
format frames, some of the non-IBM hosts in your network topology might use the standard Token
Ring conversion of Token Ring LLC2 to 802.3 LLC2 frames. If this is the case, you can change the
translation method of those hosts to use the standard translation method on a per-DSAP basis. The
translation method for all the IBM hosts would still remain as Token Ring LLC2 to Ethernet 0x80d5
translation.
To define non-IBM hosts in your network topology to use the standard translation method while the
IBM hosts use the nonstandard method, use the following command in global configuration mode:
Command Purpose
source-bridge sap-80d5 dsap Allow some other devices to use normal LLC2/IEEE 802.3 translation on
a per-DSAP basis.
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Bridging and IBM Networking Configuration Guide
Configure NetBIOS Support
NetBIOS is a nonroutable protocol that was originally designed to transmit messages between
stations, typically IBM PCs, on a Token Ring network. NetBIOS allows messages to be exchanged
between the stations using a name rather than a station address. Each station knows its name and is
responsible for knowing the names of other stations on the network.
Note In addition to this type of NetBIOS, which runs over LLC2, we have implemented another
type of NetBIOS that runs over IPX. For information on the IPX type of NetBIOS, refer to the
chapter “Configuring Novell IPX” in the Network Protocols Configuration Guide, Pa r t 2 .
NetBIOS name caching allows the Cisco IOS software to maintain a cache of NetBIOS names,
which avoids the high overhead of transmitting many of the broadcasts used between client and
server NetBIOS PCs (IBM PCs or PS/2s) in an SRB environment.
When NetBIOS name caching is enabled, the software performs the following actions:
• Notices when any hosts send a series of duplicated “query” frames and reduces them to one frame
per period. The time period is configurable.
• Keeps a cache of mappings between NetBIOS server and client names and their MAC addresses.
By watching NAME_QUERY and NAME_RECOGNIZED request and response traffic between
clients and servers, the Cisco IOS software can forward broadcast requests sent by clients to find
servers (and by servers in reply to their clients) directly to their needed destinations, rather than
forwarding them for broadcast across the entire bridged network.
Configure NetBIOS Support
The software will time out the entries in the NetBIOS name cache after a specific interval of their
initial storage. The timeout value is a user-configurable value. You can configure the timeout value
for a particular Token Ring if the NetBIOS name cache is enabled on the interface connecting to that
Token Ring. In addition, you can configure static name cache entries that never time out for
frequently accessed servers whose locations or paths typically do not change. Static RIF entries are
also specified for such hosts.
Generally, NetBIOS name caching is most useful when a large amount of NetBIOS broadcast traffic
creates bottlenecks on WAN media connecting distant locations, and the WAN media is
overwhelmed with this traffic. However, when two high-speed LAN segments are directly
interconnected, the packet savings of NetBIOS name caching is probably not worth the processor
overhead associated with it.
Note NetBIOS name caching is not recommended to be turned on in backbone routers, particularly
if you have it enabled in all the routers connected to the backbone. NetBIOS caching should be
distributed among multiple routers. NetBIOS name caching can be used only between Cisco routers
that are running Software Release 9.1 or later.
To enable NetBIOS name caching, you must perform the tasks in the following sections:
• Enable the Proxy Explorers Feature on the Appropriate Interface
• Specify Timeout and Enable NetBIOS Name Caching
In addition, you can configure NetBIOS name caching as described in the following sections:
• Configure the NetBIOS Cache Name Length
• Enable NetBIOS Proxying
Configuring Source-Route Bridging BC-123
Configure NetBIOS Support
• Create Static Entries in the NetBIOS Name Cache
• Specify Dead-Time Intervals for NetBIOS Packets
Enable the Proxy Explorers Feature on the Appropriate Interface
In order to enable NetBIOS name caching on an interface, the proxy explorers feature must first be
enabled on that interface. This feature must either be enabled for response to all explorer packets or
for response to NetBIOS packets only.
To determine whether the proxy explorers feature has been enabled, use the following command in
EXEC mode:
Command Purpose
show startup-config Determine whether or not the proxy
explorers feature has been enabled.
To determine whether proxy explorers has been configured for response to all explorer packets, look
in the configuration file for the source-bridge proxy-explorer entry for the appropriate interface.
For example, if the appropriate interface is Token Ring 0, look for an entry similar to the following:
interface tokenring 0
source-bridge proxy-explorer
If that entry does not exist, look for the source-bridge proxy-netbios-only entry for the appropriate
interface.
If neither entry exists, proxy explorers has not yet been enabled for the appropriate interface. To
enable proxy explorers for response to all explorer packets, refer to the section “Configure Proxy
Explorers” later in this chapter.
Otherwise, enable proxy explorers only for the NetBIOS name caching function by using the
following command in global configuration mode:
Command Purpose
source-bridge proxy-netbios-only Enable use of proxy explorers only
for the NetBIOS name caching
function and not for their general
local response to explorers.
Specify Timeout and Enable NetBIOS Name Caching
After you have ensured that the proxy explorers feature has been enabled for the appropriate
interface, you can specify a cache timeout and enable NetBIOS name caching. To do this, use the
following commands:
Step Command Purpose
1 netbios name-cache timeout minutes Specify the timeout for entries in the
NetBIOS name cache.
2 netbios enable-name-cache Enable NetBIOS name caching for
the appropriate interfaces.
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Bridging and IBM Networking Configuration Guide
Configure the NetBIOS Cache Name Length
To specify how many characters of the NetBIOS type name that the name cache will validate, enter
the following command in global configuration mode:
Command Purpose
netbios name-cache name-len length Specify the number of characters of
Enable NetBIOS Proxying
The Cisco IOS software can act as a proxy and send NetBIOS datagram type frames. To enable this
capability, use the following global configuration command:
Command Purpose
netbios name-cache proxy-datagram seconds Enable NetBIOS proxying.
To define the validation time when the software is acting as a proxy for NetBIOS NAME_QUERY
command or for explorer frames, use the following global configuration command:
Configure the NetBIOS Cache Name Length
the NetBIOS type name to cache.
Command Purpose
rif validate-age seconds Define validation time.
Create Static Entries in the NetBIOS Name Cache
If the router communicates with one or more NetBIOS stations on a regular basis, adding static
entries to the NetBIOS name cache for these stations can reduce network traffic and overhead. You
can define a static NetBIOS name cache entry that associates the server with the NetBIOS name and
the MAC address. If the router acts as a NetBIOS server, you can specify that the static NetBIOS
name cache is available locally through a particular interface. If a remote router acts as the NetBIOS
server, you can specify that the NetBIOS name cache is available remotely. To do this, use one of the
following commands in global configuration mode:
Command Purpose
netbios name-cache mac-address netbios-name
interface-name
netbios name-cache mac-address netbios-name
ring-group group-number
If you have defined a NetBIOS name cache entry, you must also define a RIF entry. For an example
of how to configure a static NetBIOS entry, see the “NetBIOS Support with a Static NetBIOS Cache
Entry Example” section later in this chapter.
Define a static NetBIOS name cache entry
and specify that it is available locally
through a particular interface.
Define a static NetBIOS name cache entry
and specify that it is available remotely.
Specify Dead-Time Intervals for NetBIOS Packets
When NetBIOS name caching is enabled and default parameters are set on the router (as well as the
NetBIOS name server and the NetBIOS name client), approximately 20 broadcast packets per logon
are kept on the local ring where they are generated. The broadcast packets are of the type
ADD_NAME_QUERY, ADD_GROUP_NAME, and STATUS_QUERY.
Configuring Source-Route Bridging BC-125
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