360 | Open Shortest Path First (OSPFv2)
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These router designations are not the same as the router IDs described earlier. The DR and BDR are
configurable in FTOS. If no DR or BDR is defined in FTOS, the system assigns them. OSPF looks at the
priority of the routers on the segment to determine which routers are the DR and BDR. The router with the
highest priority is elected the DR. If there is a tie, the router with the higher router ID takes precedence.
After the DR is elected, the BDR is elected the same way. A router with a router priority set to zero cannot
become the DR or BDR.
Link-State Advertisements (LSAs)
A LSA communicates the router’s local routing topology to all other local routers in the same area.
The LSA types supported by Dell Force10 are defined as follows:
• 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 together 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 flood the information for the router (for example, the Autonomous
System Border Router [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 - External 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
• Routers in a 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.
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