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Directly connected, via Null-0

Hello,

I have a question on "directly connected, via Null0". I am searching the web, but also wanted to get your inputs.

On a router i have ospf running and under ospf summary address of 166.177.234.0 is configured.

Now, let's say when i do " sh ip route" for 166.177.234.190 i get directly connected to Null-0.

My understanding is if i am getting the Null-0 for 166.177.234.190 then i cannot reach it. I know if summary-address is defined then in the routing table we would get Null-0 as to prevent looping.

Can someone please elaborate on this point. Or direct me to a website. Will appreciate it much

Regards,

Hassan..

router ospf 1

summary-address 166.177.234.0 255.255.255.0

1 REPLY
Hall of Fame Super Silver

Re: Directly connected, via Null-0

Hassan

It seems that you have provided the answer to your own question. With OSPF as with some other protocols when you configure a summary address the IOS creates a route for the summary address pointed at null0. The purpose for this is to prevent looping as you have said.

If we look more closely at the logic it is like this: you will be advertising the summary route to your neighbors, for example 166.177.234.0/24 which tells your neighbors that if they need to get to that network that they should forward to you and you can get to the destination. Lets assume that your router has learned these subnets from its OSPF neighbors 166.177.234.0/26, 166.177.234.64/26, and 166.177.234.192/26 but it does not have a route to 166.177.234.128/26 (which includes the 166.177.234.190 address). If a neighbor forwards to your router a packet with destination 166.177.234.190 what should you do? Lets also assume that you have learned your default route from that neighbor. If you forward toward your default route it would be forwarding back to the neighbor who sent you the packet and a loop is created. So OSPF creates the null0 route so that if you do not know how to forward to the destination address you will discard the packet.

HTH

Rick

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