DR/BDR is very important part of Broadcast and Non-Broad cast multi-access OSPF network types and it is needed in an Area in order to get LSA1 from OSPF routers and rely LSA2 to the OSPF routers about the network information for all OSPF routers, please know that the below information is my notes that i have been writing down from my study for CCIE.
DR/BDR on OSPF is determined per interface level so you can have for
each vlan/interface different DR/BDR from the other
You can increase priority for the DR by setting
#ip ospf priority 255 [the maximum is 255] and [0] means the router or the
interface will not participate in election.
Choosing the DR is by priority, highest router id, highest
loopback ip, highest physical interface ip
Choosing the DR/BDR is only on broadcast and non-broadcast network types on the OSPF
NOTE: preemption is not supported, so any device need to wait
for the DR to fail until it can take over.
NOTE: if no router declared itself as DR then the router will
say that I am the DR/BDR
You can check DR/BDR election process by #debug ip ospf adj [and shutdown the DR router and monitor the
debug messages]
*Jul 1 17:43:06.361: OSPF-1 ADJ Et0/0: Neighbor change event
*Jul 1 17:43:06.361: OSPF-1 ADJ Et0/0: DR/BDR election
*Jul 1 17:43:06.361: OSPF-1 ADJ Et0/0: Elect BDR 150.1.1.1
*Jul 1 17:43:06.361: OSPF-1 ADJ Et0/0: Elect DR 150.1.1.1
*Jul 1 17:43:06.361: OSPF-1 ADJ Et0/0: Elect BDR 0.0.0.0
*Jul 1 17:43:06.361: OSPF-1 ADJ Et0/0: Elect DR 150.1.1.1
On the hub/spoke the Full state will be with the DR only, so
it must be HUB configured as DR in order each spoke will form
full adjacency with
it.
Because spokes are not active to talk OSPF between each other
so the spoke will form full adjacency with DR and the BDR only.
If spoke becomes DR, the OSPF database will be broken and the
routing will be incomplete.
That’s why you need to make sure all spokes priority are
set to 0
NOTE: both DR/BDR will receive LSA1 in Area but only DR will
rely the information back to the rest of the network. So if R5 was BDR but not
DR, the OSPF DB will be broken
Because for example: R2 will send LSA1 to R5 and R4 but only
R4 will be able to reply the information to all the routers but it's not the
HUB so the process will fail.
NOTE: when you here the OSPF DB is broken, think about the DR
location in the Network.
No comments:
Post a Comment