Users Guide

Link-State Advertisements (LSAs)
A link-state advertisement (LSA) communicates the router’s local routing topology to all other local routers in
the same area.
Dell Networking supports the following LSA types:
• Type 1: Router LSA — The router lists links to other routers or networks in the same area. Type 1 LSAs
are flooded across their own area only. The link-state ID of the Type 1 LSA is the originating router ID.
• Type 2: Network LSA — The DR in an area lists which routers are joined within the area. Type 2 LSAs are
flooded across their own area only. The link-state ID of the Type 2 LSA is the IP interface address of the
DR.
• Type 3: Summary LSA (OSPFv2) — An ABR takes information it has learned on one of its attached areas
and can summarize it before sending it out on other areas it is connected to. The link-state ID of the
Type 3 LSA is the destination network number.
• Type 4: AS Border Router Summary LSA (OSPFv2) — In some cases, Type 5 External LSAs are flooded to
areas where the detailed next-hop information may not be available. An ABR floods the information 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.
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.
Open Shortest Path First (OSPFv2) 737