📄 local source-route bridging example.htm
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<title>Cisco - Local Source-Route Bridging Examples</title>
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<h1>Local Source-Route Bridging Examples</h1>
<hr>
<h2>Introduction</h2>
<p>Local source-route bridging, or SRB, is an IBM feature that
bridges packets from one interface on a router to another
interface on the same router. There are two basic scenarios for
using it: bridging only two rings together, and either bridging
more than two rings or combining SRB with other IBM features such
as RSRB, SDLLC, and QLLC. </p>
<p>The configurations depicted here are partial configurations -
they portray only that part of the configuration required for
source-route bridging support, and none of the IP or other
protocol support. In addition, depending on your topology and
traffic type, you may need to use the <b>source-bridge spanning</b>
interface command or the IBM Automatic Spanning Tree support
available in IOS 10.2 or later. See the document on IBM Spanning
Tree to determine whether this is a requirement for you. </p>
<h2>Simple Source Route Bridging - Two Local Rings</h2>
<pre>
Token T0 ---------- T1 Token
Ring 1------| ROUTER |------Ring 2
----------
</pre>
<p>With this topology, simple SRB is the best solution. In order
to use simple SRB, each of these rings must have a ring number.
If there are any other source-route bridges on these rings, then
ring numbers have already been assigned, and those defined to the
routers must agree with the other bridges on the rings.
Otherwise, any unique ring number can be used. </p>
<h3>Example #1 - simple SRB, no other SR bridges in the
environment</h3>
<pre>
interface tokenring 0
source-bridge 1 1 2
<b>source-bridge spanning</b>
interface tokenring 1
source-bridge 2 1 1
<b>source-bridge spanning</b>
</pre>
<p>In this example, since there are no other Source-Route Bridges
in the network, we can pick our own ring numbers. We have
assigned ring number 1 to the ring attached to interface Token
Ring 0, and ring number 2 to the ring attached to interface Token
Ring 1. The middle number is the bridge number - this must be
defined to be the same number on both interfaces. We are telling
interface Token Ring 0 that it is attached to ring 1, and it is
to send source-route bridged traffic over bridge 1 to ring 2.
Similarly, interface Token Ring 1 is attached to ring 2, and it
is to send source-route bridged traffic over bridge 1 to ring 1. </p>
<p>If the source ring on one interface did not match the
destination ring on the other, or if the bridge numbers were not
set to the same value, no source-route bridged traffic would flow
between these two interfaces. </p>
<h3>Example #2 - simple SRB, other SR bridges in the environment</h3>
<p>In this case, we have to do some planning before we can
configure the router interface. Let's assume that there is
another Cisco router doing SRB on the same ring as interface
Token Ring 0. Further, there's an IBM bridge on the same ring as
interface Token Ring 1. The other Cisco router is defined to be
on ring 100, and the IBM bridge is defined to be on ring 200. The
router needs to be configured as follows: </p>
<pre>
interface tokenring 0
source-bridge 100 1 512
<b>source-bridge spanning</b>
interface tokenring 1
source-bridge 512 1 100
<b>source-bridge spanning</b>
</pre>
<p>Even though the IBM bridge claims that the ring is 200, we are
configuring the router as if it resides on ring 512. While Cisco
routers define the ring as a decimal value, IBM bridges use hex
values to represent their rings. So in order to match the
definition on the IBM, use ox {decimal} and the conversion will
be done auromatically. </p>
<p>If there are other source-route bridging devices besides Cisco
routers and IBM bridges, you will need to determine whether those
devices use decimal or hexadecimal values to represent ring
numbers in order to determine how to configure the Cisco router
interfaces. </p>
<p>Before installing any source-route bridging device between two
existing bridged environments, you should always check to make
sure that you will not have duplicate ring numbers within your
source-route bridging network. In this case, ring numbers need to
be changed before the installation of the new router/bridge. It
is possible that bridge number 1 was in use in both the
source-route bridging network behind the existing Cisco and the
network behind the IBM bridge. </p>
<h2>Source-Route Bridging with a Ring-group</h2>
<p>With the previous example, there is another configuration
option that involves creating a ring-group, also known as a
virtual ring. Here's a configuration example: </p>
<pre>
source-bridge ring-group 4095
interface tokenring 0
source-bridge 1 1 4095
<b>source-bridge spanning</b>
interface tokenring 1
source-bridge 2 1 4095
<b>source-bridge spanning</b>
</pre>
<p>The net effect of this configuration seems the same as that of
the first configuration sample - interface Token Ring 0 is
defined to be on ring 1, interface Token Ring 1 is defined to be
on ring 2, and traffic is bridged between these two interfaces.
However, there is a slight difference, in that end stations think
they are sending traffic from ring 1 to ring 4095 (the virtual
ring), from which it is sent to ring 2. The traffic takes an
extra hop onto the virtual ring before reaching its destination. </p>
<p>Even though there is a limit (of seven) to the number of hops
source-route bridged traffic can take, sometimes it is helpful to
force the extra hop. Consider the following scenario: </p>
<pre>
Token
Ring 2
|
|T1
Token T0 ---------- T2 Token
Ring 1------| ROUTER |------Ring 3
----------
</pre>
<p>Assume we wanted to bridge traffic from any ring to any other
ring. Trying to extend the prior configuration, you might think
that the configuration would look like this: </p>
<pre>
interface tokenring 0 * INCORRECT configuration *
source-bridge 1 1 2 * INCORRECT configuration *
source-bridge 1 1 3 * INCORRECT configuration *
interface tokenring 1 * INCORRECT configuration *
source-bridge 2 1 1 * INCORRECT configuration *
source-bridge 2 1 3 * INCORRECT configuration *
interface tokenring 2 * INCORRECT configuration *
source-bridge 3 1 1 * INCORRECT configuration *
source-bridge 3 1 2 * INCORRECT configuration *
</pre>
<p>However, each interface supports only one source-bridge
command - attempting to configure a second one will simply
overwrite the first. The virtual ring gives us a mechanism to get
around this limitation: </p>
<pre>
source-bridge ring-group 4095</pre>
<pre> interface tokenring 0
source-bridge 1 1 4095
<b> source-bridge spanning</b> </pre>
<pre> interface tokenring 1
source-bridge 2 1 4095 <b>
source-bridge spanning</b> </pre>
<pre> interface tokenring 2
source-bridge 3 1 4095
<b> source-bridge spanning</b> </pre>
<p>This will do the trick - each ring will be able to bridge
traffic to the other two interfaces. Even if you are only
bridging two interfaces together, use the virtual ring if you
ever expect to be adding additional interfaces on the router to
the source-route bridged network, or if you plan on adding remote
source-route bridging to the router. </p>
<p>In the case above, the bridge numbers do not have to match,
unlike simple SRB. This would work just as well if the router
were configured like this: </p>
<pre>
source-bridge ring-group 4095</pre>
<pre> interface tokenring 0
source-bridge 1 1 4095
<b> source-bridge spanning</b> </pre>
<pre> interface tokenring 1
source-bridge 2 4 4095
<b> source-bridge spanning</b> </pre>
<pre> interface tokenring 2
source-bridge 3 9 4095
<b> source-bridge spanning</b> </pre>
<p>Any number of Token Ring interfaces on a router can be
source-route bridged together in this manner. The configuration
above would simply be extended as follows: </p>
<pre>
interface tokenring 3
source-bridge 4 1 4095
<b>source-bridge spanning</b></pre>
<pre> interface tokenring 4
source-bridge 5 1 4095
<b> source-bridge spanning</b> </pre>
<pre> [...and so on] </pre>
<p>Now take a look at this environment, where two separate Token
Ring interfaces are on the same ring for redundancy. </p>
<pre>
T0 ---------- T2
Token ------| ROUTER |------Token
Ring 1------| | Ring 2
T1 ----------
</pre>
<p>This can be supported. The key thing to remember is that in
the case of parallel bridges - two separate bridges connecting
ring x and ring y - each of the bridges must have a unique bridge
number. In order to support this environment, the router
configuration could be as follows: </p>
<pre>
source-bridge ring-group 4095</pre>
<pre> interface tokenring 0
source-bridge 1 1 4095
<b> source-bridge spanning</b> </pre>
<pre> interface tokenring 1
source-bridge 1 2 4095 <b>
source-bridge spanning</b> </pre>
<pre> interface tokenring 2
source-bridge 2 1 4095
<b> source-bridge spanning</b> </pre>
<hr>
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