OSPF AS's and ASBR

Feel me on this one, techies...

I'm reading through the OSPF material in the BSCI ciscopress book by Teare and Paquet, and I don't understand how an ASBR can tell one AS from another. I don't see where the AS is defined anywhere. Here's what I mean.

In AS 1, we set up Area 0, connected to Area 1 and Area 2.

In Area 1 an ASBR connects to Area 0 from AS 2. It introduces some external routes from AS 2, which show up as O E1's in the route table.

Now the ABR that connects Area 1 to Area 0 in AS1 dies. What prevents (after dead time) the ASBR from considering Area 0 in AS2 from being it's backbone?

Preciate any thoughts,
Mike

Find more posts tagged with

SAVE $250 on Your CISCO Boot Camp — Use Code "EXAM26"

Exclusively for TechExams members for Infosec Boot Camps starting before April 30, 2026

View CISCO Boot Camps

Comments

MACattack

Can you revise the question it is not that clear. If I could remember there is no issues regarding AS. Is your lab like this
EXTERNAL network

r1 (ASBR)AS1(area1)----r2(AS1Area0AS2)---r3(area2AS2)

dtlokee

To have an ASBR you need an "external" routing domain that is being redistributed into OSPF. This could be from another routing protocol, or 2 different OSPF processes ("router ospf 1" and "router ospf 2" for example) which would have seperate area 0 for each process and would not share it as long as you configure the network statements so there is no overlap.

mikearama

dtlokee wrote:

To have an ASBR you need an "external" routing domain that is being redistributed into OSPF. This could be from another routing protocol, or 2 different OSPF processes ("router ospf 1" and "router ospf 2" for example) which would have seperate area 0 for each process and would not share it as long as you configure the network statements so there is no overlap.

But as I understand it from the book, the process number... the router ospf 1 and 2... are only locally significant, and don't even need to match on routers in the same area. If that's the case, it doesn't identify the AS.

I like your thinking though on the redistributed routes. Safe to say that once a router identifies routes on an int to be from a foreign AS, that it will always classify it so? Makes sense.

nice343

ASBR = Autonomous system Boundry router

ABR= area border router

ASBR performs redistribution of external routes

ABR connects to multiple OSPF areas

ABR performs summary for routes in a particular area

area 1 range "summary address"

ASBR performs summary of external routes

summary address "summary address"

dtlokee

When you have 2 ospf processes they will be treated that way, with seperate topology tables and neighbor tables, however they will share a common routing table.

so if you had 3 routers:

R1

R3

The segment between R1 and R2 is using 10.1.12.0/24 and the segment between R2 and R3 is using 10.1.23.0/24.

so on R1 you have :

Router ospf 1
network 10.1.12.0 0.0.0.255 area 0

On R2:

Router ospf 1
network 10.1.12.0 0.0.0.255 area 0

router ospf 2
network 10.1.23.0 0.0.0.255 area 0

On R3:

router ospf 1
network 10.1.23.0 0.0.0.255 area 0

This is how you can have 2 OSPF processes on the middle router. Routes from R1 will not be seen on R3 and routes from R3 will not bee seen on R1.

We could add:

R2:
router ospf 1
redistribute ospf 2 subnets

router ospf 2
redistribute ospf 1 subnets

Now the router is an ASBR between 2 OSPF domains.

mikearama

Excellent... got it now. Much obliged.