3Com 4007, Руководство по внедрению, Страница: 469 / 684

OSPF Interfaces 469
Priority You assign the interface priority to an OSPF router to determine its status
as a designated router. A router can function in one of three ways:
■
Designated router (DR) — The router that has the highest priority
value, unless a designated router already exists on the network
segment.
■
Backup designated router (BDR) — The router that has a lower
priority than the DR; the BDR takes over DR functions if the DR fails.
■
Not a designated router — Any router that is given a priority of 0 or
that is not elected DR or BDR. Priority 0 routers can never be elected as
a DR or a BDR.
Using Priority to Select a Designated Router
Each OSPF area on a broadcast or nonbroadcast multiaccess network that
has at least two attached routers requires a designated router and a
backup designated router. The designated router (DR) forms adjacencies
to all other routers in the area. If for any reason the DR fails, the backup
designated router takes over as the designated router.
To configure a router to be chosen as a designated router, you must
understand how the designated router is elected:
■
The routing interface that has the highest routing priority within an
area is elected as the designated router using the Hello protocol.
■
In case of a tie — two or more routers having the same highest
routing priority — the router with the highest router ID is chosen as
designated router.
■
After a designated router is chosen, the same process is used to elect a
backup-designated router, with the existing designated router
excluded from the election.
Therefore, to designate a router to be elected as the designated router for
an area, configure the router with a higher router interface priority than
the other routers within the same area. If you want to prevent certain
routers within an area from serving as a designated router or backup
designated router, configure those routers with a router priority of “0,”
because interfaces with a router priority of “0” are not eligible for
designated router and backup designated router election.