OSPF and FA question

CosminCosmin
in CCIE Routing & Switching Technical

Hi all,

I encountered a strange scenario with redistribution, when a router, will receive a Type 5 LSA from another advertising router and the LSA will have the Forwarding Address equal with a local IP address of the receiving router. I notice that in this case, the routing bit is not set for that LSA.

I can only asume that SPF wil lignore external LSA originated by someone else, that have the FA equal with my ip address. 

Is my assumption corect? Couldnt find info about this on google and the RFC is a bit to long to read it now.

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  • ciprian_sciprian_s

    Hi,

    Following is an example on INE topology.

    R5 knows about 155.1.79.0/24 from ABR - R4 , tha's receiving the redistributed route from the ASBR - R6. The FA address is set to R7's vlan67 interface :

    R5#sh ip ospf database ext 155.1.79.0

     

                OSPF Router with ID (150.1.5.5) (Process ID 1)

     

                    Type-5 AS External Link States

      Routing Bit Set on this LSA in topology Base with MTID 0

      LS age: 1155

      Options: (No TOS-capability, DC, Upward)

      LS Type: AS External Link

      Link State ID: 155.1.79.0 (External Network Number )

      Advertising Router: 150.1.6.6

      LS Seq Number: 80000002

      Checksum: 0x3104

      Length: 36

      Network Mask: /24

            Metric Type: 2 (Larger than any link state path)

            MTID: 0 

            Metric: 1 

            Forward Address: 155.1.67.7

            External Route Tag: 7

     

    Next I enable OSPF SPF debugging on R5 and I create a loopback with an IP address equal to FA :

     

    R5# debug ip ospf spf

    OSPF SPF debugging is on

    OSPF SPF intra debugging is on

    OSPF SPF inter debugging is on

    OSPF SPF external debugging is on

    R5#

    R5#conf t

    Enter configuration commands, one per line.  End with CNTL/Z.

    R5(config)#int lo 67

    %LINK-3-UPDOWN: Interface Loopback67, changed state to up

    %LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback67, changed state to up

    R5(config-if)#ip add 155.1.67.7 255.255.255.0

    R5(config-if)#end

    R5#

    %SYS-5-CONFIG_I: Configured from console by console

    R5#

    OSPF-1 INTER: Check and generate summary LSA into all areas 

    OSPF-1 EXTER: Started Building Type 5 External Routes 

    OSPF-1 EXTER: Start processing Type 5 External LSA 155.1.79.0, mask 255.255.255.0

    OSPF-1 EXTER:  adv_rtr 150.1.6.6, age 1330, seq 0x80000002, metric 1, metric-type 2, fw-addr 155.1.67.7 

    OSPF-1 EXTER:    More specific non-OSPF route to forwarding address 

    OSPF-1 EXTER: Add forward address unreachable 155.1.67.7, allowed types Intra and Inter, to watched queue 

    R5#

    OSPF-1 EXTER: Deleted external route 155.1.79.0/255.255.255.0 

     

    RFC reflects the reason : 

        16.4.  Calculating AS external routes
    (3) If the forwarding address is non-zero, look up the forwarding address in the routing table.[24] The matching routing table entry must specify an intra-area or inter-area path; if no such path exists, do nothing with the LSA and consider the next in the list.


    As the forwarding address in neither "O" nor "O IA", it's connected, the LSA is not used.


    Hope it helps.
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  • CosminCosmin

    I see, i did knew that the FA must be learned via OSPF, but did not realise that if the FA is its own ip address will appear as Connected. :))) Great response, thanks.

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  • ciprian_sciprian_s

    You're welcome, always glad to help ! [:D]

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