Concept Guide

• 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 oods as normal to the rest of the OSPF network.
• Type 8: Link LSA (OSPFv3) — This LSA carries the IPv6 address information of the local links.
• Type 9: Link Local LSA (OSPFv2), Intra-Area-Prex LSA (OSPFv3) — For OSPFv2, this is a link-local "opaque" LSA as dened by
RFC2370. For OSPFv3, this LSA carries the IPv6 prexes of the router and network links.
• Type 11 - Grace LSA (OSPFv3) — For OSPFv3 only, this LSA is a link-local “opaque” LSA sent by a restarting OSPFv3 router during a
graceful restart.
For all LSA types, there are 20-byte LSA headers. One of the elds of the LSA header is the link-state ID.
Each router link is dened as one of four types: type 1, 2, 3, or 4. The LSA includes a link ID eld that identies, by the network number and
mask, the object this link connects to.
Depending on the type, the link ID has dierent 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 congurable interval timers to improve OSPF convergence times.
The default OSPF static timers (5 seconds for transmission, 1 second for acceptance) ensures sucient 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 congured 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 congure the LSA throttle timers, syslog messages appear, indicating the interval times, as shown below for the transmit timer
(45000ms) and arrival timer (1000ms).
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
NOTE
: The sequence numbers are reset when previously cleared routes that are waiting for the LSA throttle timer to expire are
re-enabled.
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 0 to 255. The higher the number, the higher the priority.
• Cost is a numbered rating 1 to 65535. The higher the number, the greater the cost. The cost assigned reects 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 and OSPFv3)
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