notes: DR BDR

- Will be elected on broadcast and NMBA networks

Addressing:

- All DROther routers send updates to the destination multicast address AllDRouters (224.0.0.6) (0100.5E00.0006).
- All DR/BDR routers send updates to the destination multicast address AllSPFRouters (224.0.0.5) (0100.5E00.0005).
- The concept behind the DR is that the broadcast link itself is considered a "pseudonode"
- The cost from an attached router to the pseudonode is the outgoing cost of that router's interface to the broadcast link, but the cost from the pseudonode to any attached router is 0.
- The DR is a property of a router's interface, not the entire router.
- On broadcast segments, traffic doesn't flow through the DR, only updates are sent to the DR and BDR.
- The DR/BDR must have layer2 connectivity to all neighbors.

DR

- some networks need this
- one router " in charge" of database distribution.
- no other router will have the authority for updates.
- routes dont get propagated if wrong router elected.
- key in frame relay networks.

Note: when DR has not been chosen, DR listes as 0.0.0.0 - unelected DR. which is bad to see on network types that requires the DR, on the other hand on the network types that dont require a DR this is ok.




Router interface priority:

- Influences the election process between DR and BDR, but will not override an active DR or BDR.
- OSPF elections do not support pre-emption.
- Highest priority value wins. The default priority on Cisco routers are 1.
- Routers with a priority of 0 are ineligible to become the DR or BDR.
- The Priority can be changed on a per multi-access-interface basis with the command "ip ospf priority".

Note:  in a hub and spoke environment | spoke must have a priority 0, and hub must be the DR.


Router-ID

- Could be used as a tie-breaker when router priorities are equal.
- Is the highest loopback IP in a ‘UP’ state. If no loopbacks are configured, it is the highest interface IP in a ‘UP’ state.
- Can be statically set. must be assigned first before any other ospf config
- identifies an OSPF neighbor
- uses the highest IP address of all loopbacks. if no loopback uses the highest IP address.
- used for virtual-link commands
- must be unique, Note: if two shares the same RID they will not form adjacencies.
note: when RID is change the ff also changes
1. LSA changes
2. reachability
3. virtual links
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COMMANDS
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- Highest router priority wins the DR/BDR election. (Default=1, Ineligible=0)

interface e0
ip ospf priority {priority}
router-id {id}


CCIE lab scenario:

Hub: IP 1.1.1.1
spoke1: IP 2.2.2.2
spoke2: IP 3.3.3.3, highest IP: 223.255.255.255

initially Hub router is the DR, then aspoke 2 was  added later with a new interface with IP 223.255.255.255.0. when hub router reloads R3 becomes the new DR.

on all routers not allowed to use priority command. Hub must be the DR.

solution:  RID is not based on IP address it is just a 32 bit number so you can use 224.0.0.1 or higher as an RID.