ospf "all areas must be connected to area 0 rule"

trackit · February 2010

Hey!

Can somebody explain to me how is "all areas must be connected to area 0 rule" actually enforced?

thanks

EDIT: Also, what about "all inter-area traffic must pass through backbone" rule? In packet tracer i connected area 1 and area 2 to area 0, everything was by-the-book and ok. Then i connected area 1 and 2 directly together (ie linked one router in area 1 and one router in area 2 togehter and made previously an area 2 router an ABR between area 1 and 2). I then tracerouted packet from area 1 to area 2 and it went directly, not through backbone... im a little confused.

APA · February 2010

When linking the router previously only in area1 and the router previously only in area 2 you would have had to use a common area between them right?

Which area was it?

trackit · February 2010

for those routers common area became area 2... one router was still only in area 2 and another became ABR with interfaces in area 1 and area 2.

kryolla · February 2010

trackit wrote: »

for those routers common area became area 2... one router was still only in area 2 and another became ABR with interfaces in area 1 and area 2.

the router that was only in area 2 did it have routes to area 1. Also are you sure the router that has interfaces in area 1 and area 2 was an ABR. I don't think the ABR bit can be set without having an interface in area 0. Inter-area routes will be ignored unless it comes from area 0. This does not take into account the transit capability bit with virtual links.

trackit · February 2010

i though that if a router has interfaces in more than one area it automatically becomes ABR, i didnt know that one area must be area 0, ill investigate it. this lab i did was a quick mockup ip packet tracer, ill do a proper lab in gns3 and see what i get,

thanks

networker050184 · February 2010

trackit wrote: »

i though that if a router has interfaces in more than one area it automatically becomes ABR, i didnt know that one area must be area 0, ill investigate it. this lab i did was a quick mockup ip packet tracer, ill do a proper lab in gns3 and see what i get,

thanks

I wouldn't trust packet tracer for anything beyond the CCNA level. If you are testing advanced topics like this you will want real IOS.

EMcCaleb · February 2010

trackit wrote: »

Hey!

Can somebody explain to me how is "all areas must be connected to area 0 rule" actually enforced?

thanks

EDIT: Also, what about "all inter-area traffic must pass through backbone" rule? In packet tracer i connected area 1 and area 2 to area 0, everything was by-the-book and ok. Then i connected area 1 and 2 directly together (ie linked one router in area 1 and one router in area 2 togehter and made previously an area 2 router an ABR between area 1 and 2). I then tracerouted packet from area 1 to area 2 and it went directly, not through backbone... im a little confused.

Barring any configurations not mentioned (static routes, other routing protocols, policy routing, VL etc), that is not proper behavior.

ccie15672 · February 2010

..............................|
............AREA 1........... |.............AREA 2

Host A........R1..............R2..............R3.......Host B

[]

(e0)-O-(e1)

(e1)-O-(e0)

[]

(a) For a normal Area configuration, Host A will not be able to ping Host B.

(b) If R2 is injecting a default route into both A1 and A2 because both are either stubby or totally stubby, then Host A and B will be able to ping each other.

(c) If you place hosts on the R1/R2 and R2/R3 link and they have R2 as their default gateways, they will be able to ping each other. This is essentially the same as 'b'.

The rule isn't that user traffic can't be *forwarded* between areas... the OSPF RFC has nothing to say about how traffic is forwarded. The rule is that OSPF LSAs will not be passed directly between non-backbone areas.

**ignore the periods... I put them in there to line up the diagram...

trackit · February 2010

thanks for you replies guys!

And for anyone who is also a little confused about this topic i suggest to read draft-ietf-ospf-abr-behavior-00 - can be fixed using virtual links, this memo describes an alternative implementation of the OSPF ABR behavior, which allows the administrator to avoid it or, if virtual links are still used, to

the beginning of this draft really helped me to understand this topic better...

Basically router becomes ABR if it has interfaces in more than one area (regardless of area 0), but what enforces the area 0 rule is:

The inter-area routes are calculated by examining summary-LSAs. If the router has active attachments to multiple areas, only backbone summary-LSAs are examined.

trackit · February 2010

Actually it seems that Cisco and IBM routers have implemented little different approach to ABR than standard ospf

RFC 3509 - Alternative Implementations of OSPF Area Border Routers

so Kryolla was right, cisco routers will not be ABR just because they have interfaces in more than 1 area, one of the areas must be area 0.

ospf "all areas must be connected to area 0 rule"

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