Election and functions of the IERS-DR and BDR
I. Problem Explanation
On an OSPF MA network, broadcast and NBMA networks have the following problems:
(1) In a network with n routers, a n*(n-1))/2 neighbor relationship is formed.
(2) The flooding and flooding of LSAs between neighbors are disordered. The same LSA is duplicated,
Such work efficiency is obviously low and consumes resources. How to solve this problem?
II. Functions of the DR and BDR
1) The designated router (DR) is responsible for establishing and maintaining adjacencies on the MA network and synchronizing LSAs.
(2) The DR and all other routers form an adjacency relationship and exchange link status information. Other routers do not directly exchange link state information, thereby greatly reducing resources consumed by adjacency relationship data and switching link state information in the MA network.
3) When the DR fails, the adjacency relationship between the DR and other routers becomes invalid, and the link state database cannot be synchronized. In this case, the DR needs to be re-elected, and the adjacency relationship with the non-DR router needs to be re-established, after LSA synchronization is complete, a backup designated router (BDR) is elected to prevent a single point of failure (DR). When the DR fails, the backup designated router (BDR) takes over the work of the DR.
3. DR and BDR election rules
DR/BDR election is based on interfaces.
The higher the DR priority of an interface, the higher the priority. If the DR priorities of interfaces are the same, the larger the value of router ID, the higher the priority.
When the DR priorities of interfaces are the same, the larger the value of router ID, the higher the priority.
The DR priority of the interface is 0, indicating that the interface does not participate in the election. The maximum value of router priority is not DR/BDR.
Adjacencies are further established between routers that have neighbor relationships on a P2P network or a P2MP network. On broadcast and NBMA networks, only neighbor relationships can be established between non-DR BDR routers. An adjacency relationship is established between a non-DR/BDR router and a DR/BDR router, and an adjacency relationship is established between the DR and the BDR.
Pay attention to the following points when selecting the DR and BDR:
1. A DR is elected only on a broadcast or NBMA interface. A DR is not elected on a P2P or P2MP interface.
2. A DR is a concept of a network segment and is used for router interfaces. A router may be a DR on an interface, or a BDR or DR Other on another interface.
3. If the DR and BDR are selected, a new router does not immediately become the DR on the network segment even if its DR priority is the largest.
4. The DR is not necessarily the router with the highest DR priority. Similarly, the BDR is not necessarily the router with the second highest DR priority.
