Information Technology Reference
In-Depth Information
Ta b l e 1 1- 3
Major LSA Types
Type
Description
Internal
router
Any router whose interfaces all belong to the same OSPF area. These routers
keep only one link-state database.
ABR
Routers that are connected to more than one area. These routers maintain a
link-state database for each area they belong to. These routers generate sum-
mary LSAs.
ASBR
Routers that inject external LSAs into the OSPF database (redistribution).
These external routes are learned via either other routing protocols or static
routes.
Backbone
router
Routers with at least one interface attached to Area 0.
Tip:
An OSPF router can be an ABR, an ASBR, and a backbone router at the same time.
The router is an ABR if it has an interface on Area 0 and another interface in another area.
The router is a backbone router if it has one or more interfaces in Area 0. The router is an
ASBR if it redistributes external routes into the OSPF network.
OSPF DRs
On multiaccess networks (such as Ethernet), some routers get selected as DRs. The pur-
pose of the DR is to collect LSAs for the multiaccess network and to forward the LSA to
all non-DR routers; this arrangement reduces the amount of LSA traffic generated. A
router can be the DR for one multiaccess network and not the DR in another attached
multiaccess network.
The DR also floods the network LSAs to the rest of the area. OSPF also selects a BDR; it
takes over the function of the DR if the DR fails. Both the DR and BDR become adjacent
to all routers in the multiaccess network. All routers that are not DR and BDR are some-
times called DRothers. These routers are only adjacent to the DR and BDR. The DR gener-
ates a Type 2 (network) LSA, which advertises all other routers on the multiaccess
segment. This allows the DRothers routers to get the Type 1 LSAs. OSPF routers multicast
LSAs only to adjacent routers. DRothers multicast packets to the DR and BDR using the
multicast address 224.0.0.6 (ALLDRouters). The DR floods updates using ALLSPFRouters
(224.0.0.5).
DR and BDR selection is based on an OSPF DR interface priority. The default value is 1,
and the highest priority determines the DR. In a tie, OSPF uses the numerically highest
router ID. The router ID is the IP address of the configured loopback interface. The router
ID is the highest configured loopback address, or if the loopback is not configured, it's the
highest physical address. Routers with a priority of 0 are not considered for DR/BDR se-
lection. The dotted lines in Figure 11-4 show the adjacencies in the network.
