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Area Border Router (ABR)
Within an AS, an area border router (ABR) connects one or more areas to the backbone.
The ABR keeps a copy of the link-state database for every area it connects to, so it may keep multiple copies of the link state
database. 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.
An ABR can connect to many areas in an AS, and is considered a member of each area it connects to.
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 previous
example.
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.
● 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.
● 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 the Dell
Networking OS. If you do not define DR or BDR in the Dell Networking OS, 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:
● 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), 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: 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: 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 8: Link LSA (OSPFv3) — This LSA carries the IPv6 address information of the local links.
● Type 9: Link Local LSA (OSPFv2), Intra-Area-Prefix LSA (OSPFv3) — For OSPFv2, this is a link-local "opaque" LSA
as defined by RFC2370. For OSPFv3, this LSA carries the IPv6 prefixes 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.
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Open Shortest Path First (OSPFv2 and OSPFv3)