What is the purpose of setting the DN bit in an OSPF LSA

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If a Provider Edge (PE) device advertises Type-3, 5, and 7 link-state advertisements (LSAs) to Customer Edge (CE) devices through area 0, the optional high-order bit of these LSAs must be set and called the DN bit. If these LSAs are advertised through an area other than area 0, the DN bit can be set or not set.

The DN bit is used to prevent routing loops. A PE ignores any LSA whose DN bit is set. This prevents a routing loop caused when a PE learns from the CE the LSA generated by another PE in CE dual-homing scenarios.

PE sets the DN-bit of Type 3, 5, and 7 LSAs and checks the DN-bit of Type 3, 5, and 7 LSAs.

Other related questions:
What is the purpose of a tag in an ASE-LSA and NSSA-LSA
In OSPF VPN multiple instances, a link-state advertisement (LSA) tag is used to prevent routing loops between PEs. In OSPF public network instances, the tag field does not prevent routing loops but is used to set the filtering conditions of routing policies. The rules of setting the tags of ASE-LSAs and NSSA-LSAs are as follows: In OSPF multiple instances, the route tag is set through the route-tag command. In OSPF public networks, the route tag is set through the default tag command. In either of the preceding two cases, you can run the import-route tag command to set the tag.

What is the purpose of an OSPF route tag
OSPF route tags are used in only Virtual Private Network (VPN) scenarios to prevent Type 5 link-state advertisement (LSA) loops in Customer Edge (CE) dual-homing networks. When OSPF detects that the route tag of a Type 5 LSA is the same as a route tag on the Provider Edge (PE) router, this route is ignored. When a CE router is connected to two PEs, PE1 sends the Type 5 LSA generated based on the redistributed Border Gateway Protocol (BGP) route. The CE then forwards this LSA to PE 2. Because an OSPF route has a higher priority than a BGP route to the CE, PE 2 replaces the BGP route with the OSPF route. Thus, a routing loop occurs. With a route tag configured, when the PE detects that the route tag of the LSA is the same as that of its route tag, the PE ignores the LSA, thereby avoiding routing loops. The default route tag is calculated based on the AS numbers in the BGP. If BGP is not configured, the default value of route tag is 0.

Why configure DN bits for OSPF on S series switches
When a PE sends Type 3, Type 5, and Type7 LSAs to a CE through Area 0, the highest optional bit of these LSAs must be reset and is called the DN bit. If the PE sends the LSAs not through Area 0, the highest optional bit of these LSAs can be reset or not. The DN bit is used to prevent routing loops. A PE does not process the received LSA that contains a DN bit. This prevents the peer PE from learning the same LSA from the CE on a CE dual-homing network, avoiding loops. PEs reset and check the DN bit of Type 3, Type 5, and Type7 LSAs.

What does the TOS field in an OSPF LSA indicate
The TOS field in an OSPF LSA was designed for QoS routing. Packets with different TOS values are transmitted on links with different cost values, enabling TOS-based routing. That is, IP packets with the same destination but different TOS values are forwarded on differentiated routes. This application, however, is canceled in RFC 2328. The OSPF devices support only TOS value 0. That is, the route is based only on destination IP address.

Set the interval at which OSPF LSAs are retransmitted on S series switches
An OSPF-enabled S series switch sends an acknowledgement packet after receiving an LSA packet. If a device does not receive any acknowledgement packet, it retransmits the LSA to its peer device. The link-state retransmit interval parameter specifies the interval at which the LSA is retransmitted. The default value is 5, in seconds. You can run the ospf timer retransmit interval command in the interface view to reset a value.

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