Fun with OSPF and my first blogpost

Hello there guys , this is my first blogpost on this blog , i’ll try to write everything about R&S and SP stuff when i’ll have time and muse ūüôā Stay tuned with my CCIE Journey.

Recently i had a great conversation with my twitter friend @icemarkom about OSPF LSA T4 propagation he showed me how i was wrong ,¬† so this article will be interesting for people who still thinks ( like i was thinking before ) that LSA T5 is neccessary on ASBR’s to Advertise T4 LSA through ABR.

Well okay , When router becomes ASBR it will set E flag on LSA T1 LSU , Some people think that ABR’s need to receive Type 5 LSA to generate Type 4 LSA into another area , As i was thinking before the conversation. ūüôā

Here is the little scenario ,

Run ospf with simple configuration ,R1#

router ospf 10
 network area 1
router ospf 10
 network area 1
 network area 0

router ospf 10
 network area 1
 network area 0

Here is the Simple LSA Type 1 LSU From R1 to R2 without any redistribution , We see Flag s 0x00 .Image

What about if we redistribute connected or¬† routing protocol routes into ospf¬† with routmap which doesn’t match anything?!
Let’s do it

R1(config)#ip access-list standard nothing
R1(config-std-nacl)#deny any
R1(config)#route-map nothing
R1(config-route-map)#match ip address nothing
R1(config-route-map)#router ospf 10
R1(config-router)#redistribute connected subnets route-map nothing

Let’s look at LSU from R1 to R2 , Now we see Flag is 0x02 which means here is something called E-bit


Now R2 is advertising Type 4 LSA to Area 0 with LSID

Router#sh ip ospf database asbr-summary

            OSPF Router with ID ( (Process ID 1)

        Summary ASB Link States (Area 0)

  Routing Bit Set on this LSA
  LS age: 156
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(AS Boundary Router)
  Link State ID: (AS Boundary Router address)
  Advertising Router:
  LS Seq Number: 80000001
  Checksum: 0x20BE
  Length: 28
  Network Mask: /0
    TOS: 0     Metric: 1

What about R1’s or R2’s LSDB? there is not anything about external T5 or T7 LSA.

R1#show ip ospf database external

OSPF Router with ID ( (Process ID 10)
R2#show ip ospf database external

OSPF Router with ID ( (Process ID 10)

I said it is so stupid decision to send LSU with E-bit without any external route , but as marko said when you are redistributing from any routing protocol or connected route it doesn’t matter this will create or not Type 5 external LSA’s into LSDB , ASBR Always set E-bit when there is redistribution statement under ospf proccess ,¬†So ospf avoid SPF calculation when Type 5 LSA is removed or¬† Added Into LSDB¬† during redistribution , SPF Calc happens when Type 1 or Type 2 LSA’s are changed , Every other lsa is used for interarea communication and uses Distance vector logic.

That’s all , special thanks to marko for clearing somethings for me! ,

Hope this helps ūüôā