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DES-7200 Configuration Guide Chapter 2 Configuring BGP IP VPN
2-65
DN bit
DN bit is a loop detection technique when OSPF protocol is run between PE and CE. In certain
circumstance, running OSPF between PE and CE may cause loops. For example, when multiple
PEs are connected to one VPN site, if one PE advertises the VPN route learned to the VPN site,
and such route is further advertised to another PE by running OSPF protocol inside the VPN site
and then broadcasted, routing loop may take place.
As shown below: The route from 192.168.10.0/24 is advertised by PE1 to PE2 and PE3. CE2
advertises the route to CE3 through OSPF protocol. The route is then advertised to PE3 and
redistributed to the BGP protocol of PE3. PE3 selects the protocol redistributed by OSPF and
converts this route into VPN-IPV4 route before advertisement, thus causing routing loop.