Discovering the Network with OSPF

Discovering the Network with OSPF
The moment OSPF is enabled on a router and networks are added to the OSPF process, the
router will try to discover the OSPF neighbors on the connected links that support or simulate
160 Chapter 5  OSPF Operation in a Single Area
broadcasts. Here is a sample of which OSPF events transpire when the interface is added to an
OSPF process:
RouterA(config-router)#network 172.16.10.5 0.0.0.0 area 0
RouterA(config-router)#
OSPF: Interface Serial0 going Up
OSPF: Tried to build Router LSA within MinLSInterval
OSPF: Tried to build Router LSA within MinLSInterval^Z
RouterA#
OSPF: rcv. v:2 t:1 l:44 rid:172.16.20.1
aid:0.0.0.0 chk:3B91 aut:0 auk: from Serial0
OSPF: rcv. v:2 t:2 l:32 rid:172.16.20.1
aid:0.0.0.0 chk:2ECF aut:0 auk: from Serial0
OSPF: Rcv DBD from 172.16.20.1 on Serial0 seq 0x71A opt 0x2 flag
➥0x7 len 32 state INIT
OSPF: 2 Way Communication to 172.16.20.1 on Serial0, state 2WAY
OSPF: Send DBD to 172.16.20.1 on Serial0 seq 0x2E opt 0x2 flag 0x7 len 32
OSPF: First DBD and we are not SLAVE
OSPF: rcv. v:2 t:2 l:52 rid:172.16.20.1
aid:0.0.0.0 chk:A641 aut:0 auk: from Serial0
OSPF: Rcv DBD from 172.16.20.1 on Serial0 seq 0x2E opt 0x2 flag
➥0x2 len 52 state EXSTART
OSPF: NBR Negotiation Done. We are the MASTER
OSPF: Send DBD to 172.16.20.1 on Serial0 seq 0x2F opt 0x2 flag 0x3 len 52
OSPF: Database request to 172.16.20.1
OSPF: rcv. v:2 t:2 l:32 rid:172.16.20.1
aid:0.0.0.0 chk:35C1 aut:0 auk: from Serial0
OSPF: rcv. v:2 t:3 l:36 rid:172.16.20.1
aid:0.0.0.0 chk:5A1 aut:0 auk: from Serial0
OSPF: Rcv DBD from 172.16.20.1 on Serial0 seq 0x2F opt 0x2 flag
➥0x0 len 32 state EXCHANGE
OSPF: Send DBD to 172.16.20.1 on Serial0 seq 0x30 opt 0x2 flag 0x1 len 32
OSPF: rcv. v:2 t:4 l:64 rid:172.16.20.1
aid:0.0.0.0 chk:F4EA aut:0 auk: from Serial0
OSPF: rcv. v:2 t:2 l:32 rid:172.16.20.1
aid:0.0.0.0 chk:35C0 aut:0 auk: from Serial0
OSPF: Rcv DBD from 172.16.20.1 on Serial0 seq 0x30 opt 0x2 flag
0x0 len 32 state EXCHANGE
OSPF: Exchange Done with 172.16.20.1 on Serial0
OSPF: Synchronized with 172.16.20.1 on Serial0, state FULL
Configuring OSPF 161
This simple debug output describes exactly what we talked about earlier in this chapter
regarding LSA exchanges and the state of adjacent OSPF neighbors. The state information was
underlined for your convenience.
We used the OSPF debugging commands to produce this output. The configuration commands
consisted of two simple OSPF commands:
router ospf 1 This command starts the OSPF process on RouterA. The number 1 indicates the
OSPF process ID. The OSPF process ID is significant only to the router on which it is configured.
Using a different process ID on the same router generates a separate OSPF routing process,
which does not share routing information with any other OSPF process. This is not desirable for
basic router configurations, but is invaluable for VPN service providers in keeping customer
traffic separated.
network 172.16.10.5 0.0.0.0 area 0 This command adds the link or links associated with
172.16.10.5 to the OSPF routing process. The wildcard mask indicates that only this single IP
address is going to be included in the routing process. Area 0 indicates that the interface with
the address 172.16.10.5 is assigned to Area 0.
The generic IOS syntax for the commands is router ospf process-id and network
ip-address wildcard-mask area area-id, respectively.
This would be a good time to explain what a wildcard mask is. The wildcard mask used for
OSPF is the same type of wildcard mask used in access lists. The 0 bits signify the bits that must
be an exact match and the 1 bit represents the “don’t care” bits. For instance, if I entered the
command network 172.168.24.0 0.0.0.3 area 0, the interface on the router with the IP
address of 172.168.24.1 or 172.168.24.2 would have OSPF started on it, and the network
attached to this interface, in this case 172.168.24.0/30, would be advertised in this router’s LSA
to its neighbors.
Be aware that entering the mask in standard subnet mask format will result in
OSPF’s converting what you enter into the corresponding wildcard mask by
subtracting each octet’s value from 255. While this works in the real world, it is
technically incorrect when you are asked to implement the correct syntax. To
help get used to inverted wildcard masks, you can use this technique to begin
with a standard mask and convert it to a wildcard mask by subtracting each
octet’s value from 255. Note that due to the flexibility of the wildcard mask and
the lack of restriction from mixing 1s and 0s (unlike with subnet masks), this
technique may yield only a starting point, but it’s still helpful until you become
more familiar with wildcard masks.