What does the TOS field in an OSPF LSA indicate

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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.

Other related questions:
What does the OSPF LSA refresh interval mean
When an OSPF link state advertisement (LSA) age reaches the link-state refresh time (1800 seconds), the OSPF updates the LSAs for advertisement.

Common LSA types of OSPF
Common LSA types of OSPF are as follows: Type 1 LSA: router LSA. Generated by each router, describes the router's link status and cost, and advertised within the area to which it belongs. Type 2 LSA: network LSA. Describes the link status of all routers on the local network segment. Network-LSAs are generated by a designated router (DR) and advertised within the area to which the DR belongs. Type 3 LSA: summary LSA. Generated by ABR, describes all routes in the area, and advertised to other relevant areas. Type 4 LSA: ASBR-summary LSA. An ASBR-summary-LSA describes routes to the ASBR in an area. The routes are advertised to all areas except the area to which the ASBR belongs. Type 5 LSA: AS-external-LSA. Describes AS external routes, which are advertised to all areas except stub areas and NSSAs. AS-external-LSAs are generated by an ASBR. Type 7 LSA: NSSA AS-external-LSA. Describes AS external routes. NSSA-LSAs are generated by an ASBR and advertised only within NSSAs. The original OSPF packet coding is not Type Length Value (TLV)-based. For the extension of OSPF functions, only the LSA types of OSPF can be extended. Type 6 LSAs are Group-Membership-LSAs used to identify multicast group membership in the Multicast Open Shortest Path First (MOSPF) protocol. Type 6 LSAs are not supported on the firewall. Type 8 LSAs are External-Attributes-LSAs used to redistribute Border Gateway Protocol (BGP) routes into OSPF and reserve the BGP autonomous system (AS) path information. Type 8 LSAs are not supported on the firewall. RFC 2370 defines an important LSA type, namely, Opaque LSA, which allows for TLV-like structures. OSPF applications, such as OSPF traffic engineering, are based on the Opaque LSA extension abilities: Type 9 LSAs are Opaque LSAs that are advertised within the local link only; Type 10 LSAs are Opaque LSAs that are advertised within the local area only; Type 11 LSAs, similar to Type 5 LSAs, are Opaque LSAs that are advertised within the local AS.

OSPF LSA update interval
On S series switches supporting OSPF, OSPF updates and advertises a generated LSA every 1800 seconds (LSA link status update interval). The interval 1800s is defined by RFC 2328 and cannot be modified.

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

In what case does an OSPF LSDB have a large number of LSAs that cannot be aged
When the available memory of a switch is low, the SOCK first send packets in high-priority queues to the CPU. When the available memory is very low, the SOCK sends only Hello packets to the CPU. As a result, LSAs in the OSPF LSDB cannot be updated. When the age of LSAs reaches 3600s, OSPF cannot calculate routes. Because ACK packets are not sent to the CPU, LSAs in the LSDB cannot be aged. To address this problem, you can adjust the memory usage threshold, delete unwanted sub-interfaces, and expand memory capacity.

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