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This article mainly shows you "what is OSPF LSA", the content is easy to understand, clear, hope to help you solve your doubts, the following let the editor lead you to study and learn this article "what is OSPF LSA?"

LSA detailed explanation

LSA Typ

LSA1: router LSA, which is generated by each router, lists all the links or interfaces of the router and indicates its outbound cost

If a router is a node on an ABR or ASBR or virtual link, it can be identified by LSA1

LSA2: network LSA, in each BMA, DR is generated, depicting all the Router connected to a BMA network.

Note: LAN network, use LSA2 to locate a (multiplex) network point, and use the network number of LSA2 as the entry installed in the routing table

And peer-to-peer networks, which are all done by LSA1.

In addition: ① 's typical LSA2, even if BMA traverses P-to-P 's network, will still flood the entire area.

② LSA2 DR is responsible for sending a link-state ID that is DR interface IP, containing all Router in all BMA

③ LSA1 and 2 can carry multiple IP prefixes with one LSA header, while LSA3, 4, and 5 can only carry one IP prefix.

LSA3: network Summary LSA

ABR sends a summary of the network to another area, and the border router packages all the lowest cost links to the destination to another area.

Note: if there is a virtual link in ①, the ABR of the unconnected Area0 in the virtual link will also package all the link information into LSA3 and send it to the Area0 through the virtual link, so if there is address summarization, you need to do it on both sides of the virtual link at the same time.

② OSPF has a rule that ABR ignores LSA3/4 received from non-backbone areas. It will only put it into Database and will not be added to the SPF algorithm.

LSA4:ASBR Summary LSA (show ip ospf database asbr-summary)

Sent by ABR, advertise a host address with a mask of 0 to reach an ASBR. In essence, LSA4 can be understood as an interregional LSA1 to reach ASBR.

Note: when LSA 3 and 4 crosses the area, it will be reborn (modify the originating router and link cost), but not LSA 5, it will propagate throughout the OSPF domain.

In addition: LSA4 should not appear in NSSA/Stub areas. LSA3/LSA4 belongs to the same type of summary in OSPF link-state advertisements, so you can think of LSA4 as a special LSA3, so where LSA3 is allowed to pass, LSA4 should also be allowed to pass, just because the trigger condition of LSA4 (LSA5 passing through ABR) has not been met, so it cannot be seen.

The condition for generating LSA4: A. ABR received the E bit identified by LSA1 in this area.

B. ABR received a LSA4 from another ABR

LSA5: autonomous system external LSA (show ip ospf database external)

ASBR originates to advertise the default route outside the AS, and flooding the default route throughout the AS can also be introduced as an external route

LSA6: group member LSA, LSA used by Multicast OSPF

LSA7: for the ASBR declared external network in the NSSA area, the address marked ASBR on the forward address

Note: ① determines that FWD has an order: new lo > old lo > new physical port > old physical port.

In addition, zero FWD is cleared at 7 to 5 at ABR=ASBR in NSSA area.

Inherit FWD from 7 to 5 when ABR ≠ ASBR

On the ABR in the NSSA region, this LSA7 is converted to LSA5 by default (specified by the P bit)

Area 1 nssa translate type7 suppress-fa

Clear forward address when LSA7 is converted to LSA5 (no LSA4 is generated)

② when there are multiple ASBR connected to the same area, only the ASBR with larger RID will publish LSA5, while when there are multiple ABR in NSSA area, only the router with higher RID will do the work of LSA7 to 5.

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