Routing Bit Set on this LSA

EdTheLad

Just thought i'd post my findings.
If you read about the routing bit you will learn its cisco proprietary and locally significant. It's useful for troubleshooting
when multiple summary or external LSA's contain the same prefix, it will show which LSA is preferred by ospf.
Now, while investigating this i was seeing discrepancies on which LSA's had this bit set, finally i figured it out.
For type 1 LSA's, this bit is present for ABRs and ASBRs within the area, if the router is purely internal to an area generating just type 1 and 2 lsa's, there will be no mention of the routing bit in that routers type 1 LSA.
It's a small piece of info that could come in handy.

gorebrush

Hmm how did you come across this? I read some of these threads, and it's just stuff I never encounter but worry I need to know it for the lab....

EdTheLad

Just been labbing ospf and noticed this in the database and then wondered why it's present sometimes and not others, afew google searches later and i was still wondering if it was used on type 1 lsa's. Then i saw it on a couple of type 1's and not on others. It brought up another point i previously overlooked, i always thought when an ABR received a type 5 lsa it would look at the advertising router and generate a type 4 into neighboring area. This is incorrect, once you enter the redistribute command under ospf, the router is now an ASBR, a type 1 lsa is then sent out all ospf interfaces with the asbr flag set. The ABR see's this flag and hence the ABR generates a type 4 lsa for the asbr neighbor. So if you have redistribute configured and you don't actually send any type 5 lsa's , the type 4's will be generated anyway by the ABR.

EdTheLad

Though i noticed something different, but first post still stands true.