Designated And Backup Designated Routers; Link-State Advertisements (Lsas) - Dell Z9500 Configuration Manual
Z-series core and aggregation switche
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An ABR can connect to many areas in an AS, and is considered a member of each area it connects to.
Autonomous System Border Router (ASBR)
The autonomous system border area router (ASBR) connects to more than one AS and exchanges information with the routers
in other ASs.
Generally, the ASBR connects to a non-interior gate protocol (IGP) such as BGP or uses static routes.
Internal Router (IR)
The internal router (IR) has adjacencies with ONLY routers in the same area, as Router E, M, and I shown in the example in the
Router
Types.
Designated and Backup Designated Routers
OSPF elects a designated router (DR) and a backup designated router (BDR). Among other things, the DR is responsible for
generating LSAs for the entire multiaccess network.
Designated routers allow a reduction in network traffic and in the size of the topological database.
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The DR maintains a complete topology table of the network and sends the updates to the other routers via multicast. All
routers in an area form a slave/master relationship with the DR. Every time a router sends an update, the router sends it to
the DR and BDR. The DR sends the update out to all other routers in the area.
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The BDR is the router that takes over if the DR fails.
Each router exchanges information with the DR and BDR. The DR and BDR relay the information to the other routers. On
broadcast network segments, the number of OSPF packets is further reduced by the DR and BDR sending such OSPF updates
to a multicast IP address that all OSPF routers on the network segment are listening on.
These router designations are not the same ad the router IDs described earlier. The DRs and BDRs are configurable in Dell
Networking OS. If you do not define DR or BDR, 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 link-state advertisement (LSA) communicates the router's local routing topology to all other local routers in the same area.
The LSA types supported by Dell Networking are defined as follows:
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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.
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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.
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Type 3: Summary LSA (OSPFv2), Inter-Area-Prefix LSA (OSPFv3) — 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), Inter-Area-Router LSA (OSPFv3) — 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.
Open Shortest Path First (OSPFv2 and OSPFv3)
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