What happens if the ABR or ASBR in OSPF goes down

dppagc

Question as above.

chowzen

Hi dppagc,

Could you provide more information to your question ?

I would assume that there is more to it, as the answer is quite simple - the ABR/ASBR is connecting a current sector to another. It will go "dark" - ie. that segment where the ABR/ASBR served will be cut off from the rest of the topology(if there is no redundancy or alternative paths).

Several other things will happen, but it is wholly too much information to just post (will end up extracting OCG pages on the roles and functions of ABRs and ASBRs)

joetest

Simple answer: The networks behind each interface will be separated and becomes unreachable to each other.

dppagc

What i mean is: is there any redundancy mechanism if the abr or asbr goes down?

pevangel

Yes, another abr or asbr.

gorebrush

Well when the 1st ABR goes down - what do you think happens? Any ideas of your own initially?

OfWolfAndMan

Simple answer: The traffic in an area doesn't leave unless there is redundancy. That's why you should implement more than one ABR into an area.

Now, I have a question for you. Let's say you have area 1 connected to area 0. There are two routers, R2 and R3, connecting the two areas to one router in the backbone, router 4 (You have lo0 configured as 4.4.4.4). Both links in area 1 to the two AB routers (We are assuming these links are both coming from the one router only in Area 1, R1) are FastEthernet links, but the two links from the ABRs to router 4 are not the same. One is a serial link and one is a fastethernet link. You have area 1 configured as a totally stubby area. When you run a trace route from R1 to 4.4.4.4, which path does it take?

shednik

To make it very simple.

• With redundancy and equal cost paths the all traffic will just switch over to the other route since both would already be in the route table

• With redundancy and no equal cost paths the routers will rerun the spf algorithm on the current database and install the other route

• Without redundancy they routes will be removed from the table, instantly if the abr/asbr sends a graceful shutdown or it could take up to 40 seconds if the dead timers are at the default.

HTH

-Joe