OSPF area zero hour, nine a.m...

With OSPF, area 0 is the backbone area, more than one area is not a requirement, but multiple areas in the network must connect to it.  Ethernet is a multi-access type network; the serial connections below are PPP.   In point-to-point networks, DR and BDR elections are unecessary.  In multi-access networks they are...


r2620_01#sh ip ospf neigh

Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           1   FULL/BDR        00:00:32    192.168.1.200   FastEthernet0/
0
100.0.0.1         1   FULL/DR         00:00:39    192.168.1.100   FastEthernet0/
0
3.3.3.3           0   FULL/  -        00:00:39    10.0.30.2       Serial0/1
2.2.2.2           0   FULL/  -        00:00:33    10.0.20.2       Serial0/0
r2620_01#

so if we lose the ethernet connection to 100.0.0.1 what happens... 
first of course

r2620_01#
Dec 15 08:28:24.087: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEtherne
t0/0, changed state to down
Dec 15 08:28:24.091: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on FastEthernet0/0 f
rom FULL to DOWN, Neighbor Down: Interface down or detached
Dec 15 08:28:24.095: %OSPF-5-ADJCHG: Process 1, Nbr 100.0.0.1 on FastEthernet0/0
 from FULL to DOWN, Neighbor Down: Interface down or detached
r2620_01#

then
r2620_01#sh ip ospf neigh

Neighbor ID     Pri   State           Dead Time   Address         Interface
3.3.3.3           0   FULL/  -        00:00:34    10.0.30.2       Serial0/1
2.2.2.2           0   FULL/  -        00:00:38    10.0.20.2       Serial0/0
r2620_01#

true to form no election takes place... let's bring it back...
r2620_01#
Dec 15 08:42:45.863: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEtherne
t0/0, changed state to up
r2620_01#

then
Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           1   INIT/DROTHER    00:00:38    192.168.1.200   FastEthernet0/
0
3.3.3.3           0   FULL/  -        00:00:35    10.0.30.2       Serial0/1
2.2.2.2           0   FULL/  -        00:00:39    10.0.20.2       Serial0/0
r2620_01#
Dec 15 08:43:25.879: %OSPF-5-ADJCHG: Process 1, Nbr 100.0.0.1 on FastEthernet0/0
 from LOADING to FULL, Loading Done
Dec 15 08:43:25.883: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on FastEthernet0/0 f
rom LOADING to FULL, Loading Done
r2620_01#

and post election after the smoke settles...

r2620_01#sh ip ospf neigh

Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           1   FULL/BDR        00:00:35    192.168.1.200   FastEthernet0/
0
100.0.0.1         1   FULL/DR         00:00:32    192.168.1.100   FastEthernet0/
0
3.3.3.3           0   FULL/  -        00:00:32    10.0.30.2       Serial0/1
2.2.2.2           0   FULL/  -        00:00:36    10.0.20.2       Serial0/0
r2620_01#

now we'll put s0/1's network  into a different area, area 1, on both sides
router ospf 1
 log-adjacency-changes
 network 1.1.1.1 0.0.0.0 area 0
 network 10.0.20.0 0.0.0.255 area 0
 network 10.0.30.0 0.0.0.255 area 0
 network 192.168.1.0 0.0.0.255 area 0

r2620_01(config)#router ospf 1
r2620_01(config-router)#netw 10.0.30.0 0.0.0.255 area 1
r2620_01(config-router)#
Dec 15 08:50:37.531: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on Serial0/1 from FU
LL to DOWN, Neighbor Down: Interface down or detached
r2620_01(config-router)#
Dec 15 08:50:37.535: %OSPF-6-AREACHG: 10.0.30.0/24 changed from area 0 to area1

then change the other side to area 1
r2620_03(config-router)#netw 10.0.30.0 0.0.0.255 area 1
r2620_03(config-router)#
*Mar  5 06:52:08.733: %OSPF-6-AREACHG: 10.0.30.0/24 changed from area 0 to area
1
r2620_03(config-router)#
*Mar  5 06:52:08.757: %OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1 on Serial0/0 from L
OADING to FULL, Loading Done
r2620_03(config-router)# 

then back to our router r2620_01

r2620_01(config-router)#
Dec 15 08:52:57.727: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on Serial0/1 from LO
ADING to FULL, Loading Done
r2620_01(config-router)#

now we have multiple areas and the result is:

r2620_01#sh ip ospf neigh

Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           1   FULL/BDR        00:00:34    192.168.1.200   FastEthernet0/
0
100.0.0.1         1   FULL/DR         00:00:32    192.168.1.100   FastEthernet0/
0
2.2.2.2           0   FULL/  -        00:00:36    10.0.20.2       Serial0/0
3.3.3.3           0   FULL/  -        00:00:35    10.0.30.2       Serial0/1
r2620_01#

a re-election, where multi-access wins...