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Totally stubby again

cdad2000

Hey Everyone,

Totally subby does not accept any external networks(type 5 LSA). Does that include default routes? Or would that be a stub area?

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Comments

dtlokee

Totally stub does not accept Type 3 or type 5 LSAs which means anything from another area is filtered out. The ABR of the totally stubby area will generate a default route so the routers in the totally stubby area have a path to the rest of the network.

aueddonline

stub areas don't except type 4,5 LSAs

totally stub don't except type 3,4,5 with the exception of a default route from the ABR

cdad2000

OK, so the ABR will generate a default route to pass packets to and from totally and stub area. But, only stub accepts external routes.

cisco_trooper

cdad2000 wrote:

OK, so the ABR will generate a default route to pass packets to and from totally and stub area. But, only stub accepts external routes.

"A stub area is an area into which AS External LSAs are not flooded. And if type 5 LSAs are
not known inside an area, type 4 LSAs are unnecessary; these LSAs are also blocked. ABRs
at the edge of a stub area use Network Summary (type 3) LSAs to advertise a single default
route (destination 0.0.0.0) into the area. Any destination that the internal routers cannot
match to an intra- or inter-area route will match the default route. Because the default
route is carried in type 3 LSAs, it will not be advertised outside of the area."

"Totally stubby areas use a default route to reach not only destinations external to the
autonomous system but also all destinations external to the area. The ABR of a totally
stubby area will block not only AS External LSAs but also all Summary LSAs—with the
exception of a single type 3 LSA to advertise the default route."

This subject matter can be really confusing and hard to remember until you start doing it. For me, I had to read the Chapters 5 thru 8 in BSCI twice. Finally I just read Chapter 8 in Routing TCP/IP by Doyle, created a massive lab in dynamips and went to town on it. It took a while to get comfortable with this topic.

Good Luck

fonque

the cisco website has alot of good writeups on this topic.

http://www.cisco.com/warp/public/104/nssa.html
http://www.cisco.com/warp/public/104/8.html

this one really helped me
http://www.cisco.com/warp/public/104/21.html

there are more that go into gritty detail on what all of the LSA types are and how they are generated.