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IP addresses Reachability

Posted on 2001-06-19
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Last Modified: 2012-08-13
Consider the following network.


        |--------|     |      |----------|
      A |        | B   |    C |          | D
--------|   R1   |-----|------|    R2    |----------
    IP1 |        |IP2  |   IP3|          | IP4
        |--------|     |      |----------|

Let R1 and R2 be the two routers.
A, B, C and D are the interfaces and
IP1, IP2, IP3 and IP4 are the corresponding IP address.
IP2 and IP3 are on the same subnet (same LAN).

Now assuming that interface A is down.
RIP/OSPF is running on R1 and R2 on all interfaces.

Now the question is :======

Should R1 advertise IP1 to R2 eventhough the interface A is down.

Or in otherwords If we ping IP1 from R2 should it succeed?
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Question by:rameshnaik
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by:ToniVR
ID: 6206617
Please asure me that this ain't a homework question, because it seems to be one.
And if you look into the specifications of RIP and OSPF, you'll find the answer.
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by:mikecr
ID: 6207294
You need to figure out which interfaces your going to pass routing updates to. Once you've done that, you've answered your own question.
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by:lrmoore
ID: 6208870
no and no
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by:rkumar115
ID: 6215043
You are running two protocols RIP and OSPF , and each protocol behaves in a different way when a link goes down.

But the actual concept is that the interface which goes down won't be accessible  by other router .I will give you the concept not involving the RIP or OSPF protocol.


R1 should not Advertise the failed interface IP address to any of the attached interfaces.
Routers maintain the routes in a routing table, The routing table contains all the routes which can be reached by the router.  For Example the Cisco router shows the output like this when you give a command show ip route.

 10.0.0.0/24 is subnetted, 1 subnets
C       10.1.1.0 is directly connected, Ethernet0
C    192.168.10.0/24 is directly connected, Loopback0
     192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C       192.168.1.0/24 is directly connected, Async1
C       192.168.1.2/32 is directly connected, Async1
O E2 192.168.20.0/24 [110/20] via 192.168.1.2, 01:28:24, Async1              


If the route becomes unreachable the router removes it from the routing table in our case  it is R1 and stops advertizing it through its interfaces.


Since the route is not advertized other routers will drop the route from there routing tables.

So in our case the router A won't advertize the IP1 address and Router B won't be able to ping because the Router A stopped Advertizing the route and its routing table doesnot contain any route to that particular destination.


Hope this explains your doubt.
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Author Comment

by:rameshnaik
ID: 6217022
I got what you are trying to say.Routing Protocols will not advertise the failed interface to their peers. But in Router R2 if i add a static route for IP1 with IP2 as NextHop what will happen? Will i be able to ping the failed interface then.
If Yes, then my question is if a router receives a packet to one of its interfaces which is down, then should it respond to the sender?
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by:rkumar115
ID: 6217866
The Router R1 will remove the route from the routing table
when the link fails ,what i mean is , the interface won't be there in the routing table as C (connected).
Now if we add a static route from R2 to R1 and then give a ping to the interface which failed----

You won't be able to ping . I will explain you how it happens..
When you give a ping command on the Router R2 , It will look in the routing table to find how to reach that IP address. Since you have given a static route it will find out the interface through which it can reach the Destination IP ,in our case destination IP address is  IP1 and it determines the interface to be C.

Now the ICMP ping packet reaches the Router R1 through the interface B, But there is no route in the routing table for IP1 in Router R1 . Hence the Router R1 drops the Packet.

Hence the Ping fails.


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by:rkumar115
ID: 6217867
The Router R1 will remove the route from the routing table
when the link fails ,what i mean is , the interface won't be there in the routing table as C (connected).
Now if we add a static route from R2 to R1 and then give a ping to the interface which failed----

You won't be able to ping . I will explain you how it happens..
When you give a ping command on the Router R2 , It will look in the routing table to find how to reach that IP address. Since you have given a static route it will find out the interface through which it can reach the Destination IP ,in our case destination IP address is  IP1 and it determines the interface to be C.

Now the ICMP ping packet reaches the Router R1 through the interface B, But there is no route in the routing table for IP1 in Router R1 . Hence the Router R1 drops the Packet.



Hence the Ping fails.


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Accepted Solution

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rkumar115 earned 200 total points
ID: 6217873
The Router R1 will remove the route from the routing table
when the link fails ,what i mean is , the interface won't be there in the routing table as C (connected).
Now if we add a static route from R2 to R1 and then give a ping to the interface which failed----

You won't be able to ping . I will explain you how it happens..
When you give a ping command on the Router R2 , It will look in the routing table to find how to reach that IP address. Since you have given a static route it will find out the interface through which it can reach the Destination IP ,in our case destination IP address is  IP1 and it determines the interface to be C.

Now the ICMP ping packet reaches the Router R1 through the interface B, But there is no route in the routing table for IP1 in Router R1 . Hence the Router R1 drops the Packet.



Hence the Ping fails.


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Author Comment

by:rameshnaik
ID: 6218253
Thanks rkumar. I got it.
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