area border router in ospf

Answered Question
Aug 12th, 2009

Hi every body.

i have few questions about area border router.

My cisco book says an area border router is a router that connects to two different areas.

The other book says an area border router is the router that connects area 0 to other area say area 2.

The 2nd defination makes sense as as the area boder is responsible for inter areas communication. And inter area communication must use area 0 .

Just having one router to connect two different non zero areas does not serve any purpose as there will no commuincation possible between them.

My second question is about back bone router.

My book says a back bone is router with atleast one interface connected to area 0.

The other book says a back bone router is the router with all interfaces connected to area 0.

So i am confused.

Thanks a lot a have a good day.

I have this problem too.
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Correct Answer by Richard Burts about 7 years 5 months ago

One of the most important aspects of an ABR is that it generates summary advertisements (LSA type 3) to one area for prefixes that are in the other area.

Clearly in Cisco's implementation of OSPF it is a requirement that one of the areas be area 0 for the router to generate the summary LSAs. If you configure a router so that it has interfaces in 2 areas (perhaps area 1 and area 2) but no interface in area 0 then you will find that it has the entries for all routes in both areas in its routing table (and in its link state data base) but that it does not generate the type 3 summary LSAs. So in John's experiment R1 has all entries for area 1 and 2 (and 0 which it learned from R2) but it does not generate summary LSAs for area 1 and R0 would have only entries for area 2.

HTH

Rick

Correct Answer by John Blakley about 7 years 5 months ago

Just to add what everyone else said, I did this in GNS3:

R0 -> R1 -> R2

R2 had interfaces in area 0 and 1

R1 had interfaces in area 1 and 2

R0 had interfaces in area 2

R1 wasn't identified as an ABR, so it doesn't seem like it's a matter of having an interface in two different areas, but in fact the ABR does seem like it needs to have an interface in area 0 (backbone) in order to be an ABR.

HTH,

John

Correct Answer by Jon Marshall about 7 years 5 months ago

Sarah

Just to add to Jerry's post.

As Jerry said a backbone router must have at least one interface in Area 0 although a backbone router can actually have all it's interfaces in area 0.

So by definition an ABR is also a backbone router because they must both have at least one interface in area 0.

Jon

Correct Answer by Jerry Ye about 7 years 5 months ago

Hi Sarah,

This is the definition from the CCIE Routing and Switching Exam Quick Reference Sheets by Anthony Sequeira:

ABR - Connect one or more areas to the backbone; act as gateway for interarea traffic; seperate link-state database for each connected area.

Backbone Routers - At least one interface in the backbone area.

HTH,

jerry

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Correct Answer
Jerry Ye Wed, 08/12/2009 - 09:55

Hi Sarah,

This is the definition from the CCIE Routing and Switching Exam Quick Reference Sheets by Anthony Sequeira:

ABR - Connect one or more areas to the backbone; act as gateway for interarea traffic; seperate link-state database for each connected area.

Backbone Routers - At least one interface in the backbone area.

HTH,

jerry

Correct Answer
Jon Marshall Wed, 08/12/2009 - 09:59

Sarah

Just to add to Jerry's post.

As Jerry said a backbone router must have at least one interface in Area 0 although a backbone router can actually have all it's interfaces in area 0.

So by definition an ABR is also a backbone router because they must both have at least one interface in area 0.

Jon

Correct Answer
John Blakley Wed, 08/12/2009 - 10:48

Just to add what everyone else said, I did this in GNS3:

R0 -> R1 -> R2

R2 had interfaces in area 0 and 1

R1 had interfaces in area 1 and 2

R0 had interfaces in area 2

R1 wasn't identified as an ABR, so it doesn't seem like it's a matter of having an interface in two different areas, but in fact the ABR does seem like it needs to have an interface in area 0 (backbone) in order to be an ABR.

HTH,

John

Correct Answer
Richard Burts Wed, 08/12/2009 - 11:36

One of the most important aspects of an ABR is that it generates summary advertisements (LSA type 3) to one area for prefixes that are in the other area.

Clearly in Cisco's implementation of OSPF it is a requirement that one of the areas be area 0 for the router to generate the summary LSAs. If you configure a router so that it has interfaces in 2 areas (perhaps area 1 and area 2) but no interface in area 0 then you will find that it has the entries for all routes in both areas in its routing table (and in its link state data base) but that it does not generate the type 3 summary LSAs. So in John's experiment R1 has all entries for area 1 and 2 (and 0 which it learned from R2) but it does not generate summary LSAs for area 1 and R0 would have only entries for area 2.

HTH

Rick

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