Users Guide

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 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 trac 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 congurable in Dell
Networking OS. If you do not dene DR or BDR in 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.
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 ooded 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 ooded 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 ooded to areas where the
detailed next-hop information may not be available. An ABR oods 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 ooded 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 oods 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 dened by RFC2370.
For all LSA types, there are 20-byte LSA headers. One of the elds of the LSA header is the link-state ID.
Each router link is dened as one of four types: type 1, 2, 3, or 4. The LSA includes a link ID eld that identies, by the network
number and mask, the object this link connects to.
Depending on the type, the link ID has dierent meanings.
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Open Shortest Path First (OSPFv2)