Table Of Contents

Step 8. Setting all router priorities to zero

Goals

The goal of this step is to demonstrate what happens when all routers on a multi-access network have priority 0.

A router with OSPF interface priority 0 cannot become DR or BDR. If all routers on a multi-access network have priority 0, no DR or BDR will be elected, and:

• No Network LSA will be generated for that network.

• The network will not be properly recognized in the OSPF topology.

This effectively disables OSPF routing for that network segment.

Configuration

This configuration is based on Step 6. All routers have their priorities set to 0 for the multi-access interface.

The configuration in omnetpp.ini is the following:

[Config Step8]
description = "Setting all router ids to zero"
extends = Step6

# when the router id of a multi-access interface is zero then no Wait timer is started.
# this means that no DR/BDR election will be performed and no Network LSA is exchanged.
# this also means that the Ethernet network will not be recognized by any routers.

*.R*.ospf.ospfConfig = xmldoc("ASConfig_zero_priority.xml")

*.visualizer.routingTableVisualizer.destinationFilter = "R*"
*.visualizer.routingTableVisualizer.nodeFilter = "R* and not *.N*"

The OSPF configuration:

<?xml version="1.0"?>
<OSPFASConfig xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="OSPF.xsd">

  <Router name="R1" RFC1583Compatible="true">
    <BroadcastInterface ifName="eth0" />
    <BroadcastInterface ifName="eth1" routerPriority="0" />
  </Router>
  
  <Router name="R2" RFC1583Compatible="true">
    <BroadcastInterface ifName="eth0" />
    <BroadcastInterface ifName="eth1" routerPriority="0" />
  </Router>
  
  <Router name="R3" RFC1583Compatible="true">
    <BroadcastInterface ifName="eth0" routerPriority="0" />
    <BroadcastInterface ifName="eth1" />
  </Router>
  
  <Router name="R4" RFC1583Compatible="true">
    <BroadcastInterface ifName="eth0" routerPriority="0" />
    <BroadcastInterface ifName="eth1" />
  </Router>
  
  <Router name="R5" RFC1583Compatible="true">
    <BroadcastInterface ifName="eth0" routerPriority="0" />
    <BroadcastInterface ifName="eth1" />
  </Router>
  
  <Router name="R6" RFC1583Compatible="true">
    <BroadcastInterface ifName="eth0" routerPriority="0" />
    <BroadcastInterface ifName="eth1" />
  </Router>

</OSPFASConfig>

Results

When all routers have priority 0:

1. The DR/BDR election completes, but no router is elected as DR or BDR.

2. No Network LSA is generated for the Ethernet LAN between the routers.

3. The OSPF topology is incomplete - the multi-access network is not visible to the routing protocol.

4. Routing between networks may fail or use suboptimal paths since the multi-access segment is not properly integrated into the OSPF topology.

This demonstrates the critical role of the DR in multi-access networks and shows what happens when the DR/BDR election mechanism is effectively disabled.

Sources: omnetpp.ini, Network2.ned, ASConfig_zero_priority.xml

Discussion

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Table Of Contents

• Step 8. Setting all router priorities to zero
  • Goals
    • Configuration
    • Results
  • Discussion

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Step 7. Influencing OSPF DR/BDR election

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Step 9. High-priority OSPF router joins after OSPF DR/BDR election