Reference Guide

for the router (for example, the ASBR where the Type 5 advertisement originated. The link-state ID for
Type 4 LSAs is the router ID of the described ASBR).
• Type 5: LSA — These LSAs contain information imported into OSPF from other routing processes.
They are flooded to all areas, except stub areas. The link-state ID of the Type 5 LSA is the external
network number.
• 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.
• 4: virtual link neighboring router ID.
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
Virtual Links
In the case in which an area cannot be directly connected to Area 0, configure a virtual link between that
area and Area 0.
The two endpoints of a virtual link are ABRs, and you must configure the virtual link in both routers. The
common nonbackbone area to which the two routers belong is called a transit area. A virtual link
specifies the transit area and the router ID of the other virtual endpoint (the other ABR).
660
Open Shortest Path First (OSPFv2)