Routing Bit not set in LSA

[sukamal.das@bt.com]

Hi All,

As per below logs there are no Routing bit sets in Type 5 LSA but one best route installed in routing table.

I want to know that the routing bit is optional to mention in LSA or is there any special condition to have it.

else how could it can be understand that which route has been installed in routing table without checking the routing table.

R1#sh ip ospf database external 192.168.1.0

            OSPF Router with ID (1.1.1.1) (Process ID 1)

                Type-5 AS External Link States

  LS age: 631
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 192.168.1.0 (External Network Number )
  Advertising Router: 4.4.4.4
  LS Seq Number: 8000000B
  Checksum: 0x498E
  Length: 36
  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 88
        Forward Address: 0.0.0.0
        External Route Tag: 0

  LS age: 823
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 192.168.1.0 (External Network Number )
  Advertising Router: 5.5.5.5
  LS Seq Number: 8000000A
  Checksum: 0x2DA7
  Length: 36
  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 88
        Forward Address: 0.0.0.0
        External Route Tag: 0

********************************

R1#sh ip route 192.168.1.0
Routing entry for 192.168.1.0/24
  Known via "ospf 1", distance 110, metric 88, type extern 2, forward metric 20
  Last update from 10.1.12.2 on Ethernet0/1, 00:13:03 ago
  Routing Descriptor Blocks:
  * 10.1.12.2, from 4.4.4.4, 00:13:03 ago, via Ethernet0/1
      Route metric is 88, traffic share count is 1

********************************

Can some one help me in understanding this better.

Thanks in advance.

Sukamal Das

[Deepak Kumar]

Hi,

You shared very less information so we can't comment on this.

Please visit this document: https://drive.google.com/file/d/17BProJiVrhHuZ-hcIDnL5c2wyT3HVh5M/view

Regards,

Deepak Kumar

[paul driver]

Hello
TBH I am not quite sure why you don’t see the routing bit set especially from R1's perspective?

The Routing bit as I understand it is basically the router validating the reachability to a destination network advertised by the OSPF LSA, however it doesn't mean the route to that destination detailed in the LSA would be preferred, an LSA without the routing bit set could be the preferred path.

Looking at your topology I would have to expected to see this " Routing Bit Set" on both external LSAs on R1, as that rtr as valid paths to that 192.168.1.0 via it ospf peers R2 and R3.

From R2 perspective, you should expect to see two received external LSA's for network 192,168,1,0 however only " Routing Bit Set" towards R6 via 4.4.4.4 because R4 has the best path towards R6 for 192.168.1.0

From R3 perspective, two received external LSA's however only " Routing Bit Set" towards R6 via 5.5.5.5 because R5 has the best path towards R6 for 192.168.1.0

From R4 perspective, again two external LSA's however only the " Routing Bit Set" towards R6 via  5.5.5.5 would be seen for network 192.168.1.0  if I am correct this would be because not only would other LSA be self originating for network 192.168.1.0 it would prefer its eigrp path.

The same logic would also apply from R5 perspective.

I guess If I am incorrect in the above explanation in anyway I am sure others correct me, Ill look forward to any other feedback.

[sukamal.das@bt.com]

Hi Paul,

Thanks for your valuable response.

As per my topology there are to ASBRs one is 4.4.4.4 and other is 5.5.5.5 now to reach the external network the lowest cost of the link is towards ASBR 4.4.4.4, and in routing table it is reflecting.

My questing is if the best cost ASBR is 4.4.4.4 then why OSPF is not putting the "Routing bit set to the LSA" for this ASBR is Type 5 LSA out put.

Note: It is lab in GNS 3 and the IOS is IOU image "Cisco IOS Software, Linux Software (I86BI_LINUX-ADVENTERPRISEK9-M), Version 15.5(2)T,"

[paul driver]

Hello

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sukamal.das@bt.com wrote:

Hi Paul,

Thanks for your valuable response.

As per my topology there are to ASBRs one is 4.4.4.4 and other is 5.5.5.5 now to reach the external network the lowest cost of the link is towards ASBR 4.4.4.4, and in routing table it is reflecting.

My questing is if the best cost ASBR is 4.4.4.4 then why OSPF is not putting the "Routing bit set to the LSA" for this ASBR is Type 5 LSA out put.

Note: It is lab in GNS 3 and the IOS is IOU image "Cisco IOS Software, Linux Software (I86BI_LINUX-ADVENTERPRISEK9-M), Version 15.5(2)T,"

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As stated I would expect not to see the routing bit set on the asbr as it has self originated one of the lsa’s and its eigrp route would be preferred but as its self originated external lsa  is propergated throughput the ospf domain I would have them expected the routing bit to become set especially by the time it’s reached R1

it could be @Kevin Rivest @has a point regards the software you are using but I wasn’t aware of any change to the workings regards the ospf routing bit so I cannot confirm if this is the case or not or maybe I am just missing something regards this process !

[Kevin Rivest]

Having the routing bit set in the LSA just means that it is possible to reach the destination via the advertising router for the LSA. It does not specify whether that specific LSA is the best available path or currently in use. An example of when you would not see the routing bit set is if the router or path is not reachable due to a network mismatch on a link.

Also as far as why your output does not say Routing Bit Set on this LSA is, I am guessing, because of the version of software you are using on your router. Some versions default the assumption that the routing bit is set when displaying the LSDB and will only show Routing Bit is not set on this LSA if the LSA is not reachable, and nothing if it is reachable. Others assume the opposite and will show nothing if the LSA is unreachable and Routing Bit Set on this LSA if it is reachable. It look like you have the first.

Hope that answers your question.

[paul driver]

Hello @Kevin Rivest 

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@Kevin Rivest wrote:

Also as far as why your output does not say Routing Bit Set on this LSA is, I am guessing, because of the version of software you are using on your router. Some versions default the assumption that the routing bit is set when displaying the LSDB and will only show Routing Bit is not set on this LSA if the LSA is not reachable, and nothing if it is reachable. Others assume the opposite and will show nothing if the LSA is unreachable and Routing Bit Set on this LSA if it is reachable. It look like you have the first.

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Kevin would you know what software version this stems?

As for the OP, if you look indeed the "routing bit" is not set however the same route detailed in that LSA actually gets installed in the rib, so to me doesn't seem correct.