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● Type 7: External LSA — Routers in an NSSA do not receive external LSAs from ABRs, but are allowed to send external
routing information for redistribution. They use Type 7 LSAs to tell the ABRs about these external routes, which the ABR
then translates to Type 5 external LSAs and floods as normal to the rest of the OSPF network.
● Type 9: Link Local LSA (OSPFv2) — For OSPFv2, this is a link-local "opaque" LSA as defined by RFC2370.
For all LSA types, there are 20-byte LSA headers. One of the fields of the LSA header is the link-state ID.
Each router link is defined as one of four types: type 1, 2, 3, or 4. The LSA includes a link ID field that identifies, by the network
number and mask, the object this link connects to.
Depending on the type, the link ID has different meanings.
● 1: point-to-point connection to another router/neighboring router.
● 2: connection to a transit network IP address of the DR.
● 3: connection to a stub network IP network/subnet number.
LSA Throttling
LSA throttling provides configurable interval timers to improve OSPF convergence times.
The default OSPF static timers (5 seconds for transmission, 1 second for acceptance) ensures sufficient time for sending and
resending LSAs and for system acceptance of arriving LSAs. However, some networks may require reduced intervals for LSA
transmission and acceptance. Throttling timers allow for this improved convergence times.
The LSA throttling timers are configured in milliseconds, with the interval time increasing exponentially until a maximum time has
been reached. If the maximum time is reached, the system, the system continues to transmit at the max-interval until twice the
max-interval time has passed. At that point, the system reverts to the start-interval timer and the cycle begins again.
When you configure the LSA throttle timers, syslog messages appear, indicating the interval times, as shown, for the transmit
timer (45000 ms) and arrival timer (1000 ms).
Mar 15 09:46:00: %STKUNIT0-M:CP %OSPF-4-LSA_BACKOFF: OSPF Process 10,Router lsa id
2.2.2.2 router-id 2.2.2.2 is backed off to transmit after 45000ms
Mar 15 09:46:06: %STKUNIT0-M:CP %OSPF-4-LSA_BACKOFF: OSPF Process 10,Router lsa id
3.3.3.3 rtrid 3.3.3.3 received before 1000ms time
Router Priority and Cost
Router priority and cost is the method the system uses to “rate” the routers.
For example, if not assigned, the system selects the router with the highest priority as the DR. The second highest priority is the
BDR.
● Priority is a numbered rating from 0 to 255. The higher the number, the higher the priority.
● Cost is a numbered rating from 1 to 65535. The higher the number, the greater the cost. The cost assigned reflects the cost
should the router fail. When a router fails and the cost is assessed, a new priority number results.
Open Shortest Path First (OSPFv2)
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