OSPF AS's and ASBR

mikearama

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

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

---

R2

---

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.