Archives for category: ioshints

Multiple OSPF processes with mixed network statements can really mess things up! Fortunately, there is a better way to assign an interface into an OSPF area.

I’ve discussed overlapping OSPF network statements in one of my earlier posts. In the a “pre-IPv6-era” we were taught to use network statements in the OSPF stanza to assign the interfaces to a certain OSPF area. This configuration

interface Vlan100
  ip address 10.0.100.1 255.255.255.0
!
router ospf 1
  passive-interface default
  no passive-interface Vlan100
  network 10.0.100.0 0.0.0.255 area 1

…enables OSPF process on the interface Vlan100 and assigns that interface to OSPF area 1.
What happens if one uses multiple and overlapping networks statements in order to assign interfaces into different areas? Read the rest of this entry »

Most things in networking are not revolutionary. No need to reinvent the wheel.

After many years of continuous growth of your backbone network, you might decide to clean up the mess and renumber the network within one single IP prefix covering the whole area. If OSPF is your choice for the interior routing protocol for IPv4, this is the right time to think again about using the OSPF network statements. A single network statement per router is all you need to make all the backbone interfaces seen by the OSPF process.
Here, for example, all the backbone is covered by a single prefix 10.0.0.0/8:
interface FastEthernet0/0
 ip address 10.1.0.0 255.255.255.254
!
router ospf 4
 passive-interface default
 no passive-interface FastEthernet0/0
 ! all backbone links are derived from 10.0.0.0/8
 network 10.0.0.0 0.255.255.255 area 0
!

What happens if we add another interface within the same global 10.0.0.0/8 to the OSPF, but we are putting it into a different OSPF area. For this exception we have to use a more specific network statement, for example:
interface Vlan991
 ip address 10.4.4.4 255.255.255.254
!
router ospf 4
 ! adding Vlan991 to area 4 with a /32 network statement
 no passive-interface Vlan991
 network 10.4.4.4 0.0.0.0 area 4
!

Will it work or will it cause conflict with the “network 10.0.0.0 0.255.255.255 area 0” statement?
Read the rest of this entry »