IP forwarding

[SAM_24]

Hi,

Let us assume IP FWD is enabled on my servers and I have two ip addressess ( different subnet, let us say subnet A and subnet B ) configured. Subnet B is down. Host A wants to send data to host B using subnet B.
Sub A
Host A <-------------------> Host B
Sub B
<------------------->

What happens now? Because Sub B is down the host A will route the packets using Sub A? Is this the expected behaviour? Or host A should say host B is not reachable by Sub B network? Help.

Thanks.

[Ron Kinner]

Usual networking answer:

It depends.

What do the routing tables of the forwarding host look like?

IF it uses static routing and it has a default route back through A then you will have a routing loop and the packet will be passed backed and forth forever (actually until the ttl drops to 0 when the packet will be discarded.)
If it doesn't have a default route then it may drop the packet on the floor or send back a network unreachable message depending on how it is configured.

If it is using a dynamic routing table then it may actually figure out another way to get there but if this were the case and Host A was also involved with the dynamic routing then Host A would know the packet couldn't get there and wouldn't even bother sending it to the forwarder.

Ron

[U.SivaKumar_2]

Hi,
let's start analysing.
When you add two network card in the server and
assign two IP addresses belonging to two different subnets. Two route entries for these
subnets with the respective interface IP addresses as the gateway will added by default.
For example if you have ip addresses 10.0.0.1 and 20.0.0.1 assigned to your server. These routes will be added to your server automatically.
network 10.0.0.0 gateway 10.0.0.1
network 20.0.0.0 gateway 20.0.0.1
you can add redundant route for 10.0.0.0 . But the first entry for 10.0.0.0 in routing table
will only active.
Now assume that your lan cable is unplugged from 10.0.0.1 card.

1)If Dead gateway detection is present , server will be still able to get ICMP reply from 10.0.0.1 so will not switchover to redundant route.
2)If Dead gateway detection is not present
server will stick to the primary gateway and
will return network unreachable messages.

SO I don't think your plan will succeed.

regards,
U.SivaKumar