Users Guide

Designated
router
Maintains a complete topology table of the network and sends 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 to all other routers in the area.
Backup
designated
router
Router that takes over if the DR fails.
Each router exchanges information with the DR and BDR. The DR and BDR relay information to other routers. On broadcast
network segments, the number of OSPF packets reduces by the DR sending OSPF updates to a multicast IP address that all
OSPF routers on the network segment are listening on.
DRs and BDRs are configurable. If you do not define the DR or BDR, OS10 assigns them per the protocol. To determine which
routers are the DR and BDR, OSPF looks at the priority of the routers on the segment. The default router priority is 1. The
router with the highest priority is elected 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 a DR or BDR.
Link-state advertisements
A link-state advertisement (LSA) communicates the router’s routing topology to all other routers in the network.
Type 1—Router
LSA
Router lists links to other routers or networks in the same area. Type 1 LSAs flood across their own area
only. The link-state ID of the Type 1 LSA is the originating router ID.
Type 2—Network
LSA
DR in an area lists which routers are joined within the area. Type 2 LSAs flood 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)
ABR takes information it has learned on one of its attached areas and summarizes it before sending it out
on other areas it connects to. The link-state ID of the Type 3 LSA is the destination network's IP address.
Type 4—AS
Border Router
Summary LSA
(OSPFv2), Inter-
Area-Router LSA
(OSPFv3)
In some cases, Type 5 External LSAs flood to areas where the detailed next-hop information may not be
available because it may be using a different routing protocol. The ABR floods the information for the
router, the ASBR where the Type 5 originated. The link-state ID for Type 4 LSAs is the router ID of the
described ASBR.
Type 5—AS-
External LSA
LSAs contain information imported into OSPF from other routing processes. Type 5 LSAs flood to all
areas except stub areas. The link-state ID of the Type 5 LSA is the external network number.
Type 7—NSSA-
External LSA
(OSPFv2), LSA
(OSPFv3)
Routers in an NSSA do not receive external LSAs from ABRs but 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)
Type 8 LSA carries the IPv6 address information of the local links.
Type 9—Link-
Local Opaque
LSA (OSPFv2),
Intra-Area Prefix
LSA (OSPFv3)
Link-local opaque LSA as defined by RFC2370 for OSPFv2. Intra-Area-Prefix LSA carries the IPv6
prefixes of the router and network links for OSPFv3.
Type 11—Grace
LSA (OSPFv3)
Link-local opaque LSA for OSPFv3 only is sent during a graceful restart by an OSPFv3 router.
The LSA header is common to LSA types. Its size is 20 bytes. 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 the object this link
connects to, by the network number and mask. Depending on the type, the link ID has different meanings.
1
Point-to-point connection to another router or neighboring router
2 Connection to a transit network IP address of the DR
3 Connection to a stub network IP network or subnet number
Layer 3 935