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Configure a Multiport Bridge Using a Virtual Ring
Configuring Source-Route Bridging BC-111
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
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:
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.
Command Purpose
source-bridge local-ring bridge-number target-ring Enable local source-route bridging on
a Token Ring interface.
S2323
T1
T0
T2
T3
Token
Ring
Token
Ring
Token
Ring
Token
Ring