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

6 thoughts on “Fun with OSPF and my first blogpost

  1. Good post, thanks for sharing!
    So to summarize your findings, when there’s redistribution in the ospf process, the LSA T1 have their E bit set which then is a flag for the ABR to flag T4 LSAs pointing to the ASBR.

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