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.

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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.

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.

How long is the LSA refresh interval
Defined in RFC 2328, the 1800-second interval cannot be changed.

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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