Tracing a route in EIGRP and null0

[mahesh18]

Hi Everyone,

Happy New Year!

I am checking if routing is there from source to Destination IP 172.23.7.110?

During tracing i reach one device along the path and did

sh ip route vrf RC   172.23.7.110

Routing entry for 172.23.0.0/16

  Known via "eigrp 52", distance 5, metric 2816, type internal

  Redistributing via eigrp 52

  Routing Descriptor Blocks:

  * directly connected, via Null0

      Route metric is 2816, traffic share count is 1

      Total delay is 10 microseconds, minimum bandwidth is 1000000 Kbit

      Reliability 255/255, minimum MTU 1500 bytes

      Loading 1/255, Hops 0

What should i do on this device so i can reach 172.23.7.110 from here?

Is it ok to put static route on this vrf RC?

Regards

Mahesh

[Jon Marshall]

Mahesh

Impossible to say whether you should or shouldn't add a route without understanding your topology.

The null route is for 172.23.0.0/16 and is presumably put in the routing table because you have configured a summary address under EIGRP ?

If so you usually configure a summary address on the router that has the more specific routing information so that it only drops traffic if it cannot find a more specific route.

Jon

[mahesh18]

Hi Jon,

Router is configured with VRF

I checked the interfaces which were running VRF RC.

It does not show me summary address command  under any interface?

Is there anyother way i can verify that Router is advertising the summary route?

Regards

Mahesh

[Reza Sharifi]

Hi Mahesh,

Also, look under the interface.  Should look something like this:

ip summary-address eigrp autonomous-system-number ip-address mask

HTH

Reza

[mahesh18]

Hi Reza,

I checked interface config it is not there.

Regards

Mahesh

[Jon Marshall]

Mahesh

You may have a static route configured and redistributing into EIGRP.

Can you see that in the configuration ?

Jon

[Richard Burts]

Mahesh

A simple way to find out what is the situation would be to do

show run | include 172.23.0.0

That would show whether it is a summary address, or a static route, or whatever.

HTH

Rick

[mahesh18]

Hi Rick

I did

show run | include 172.23.0.0
  network 172.23.0.0
  network 172.23.0.0
  network 172.23.0.0
  network 172.23.0.0
  network 172.23.0.0
 

Thats only info i find.

Regards

Mahesh

[mahesh18]

Hi Jon

I did sh ip route vrf RC

D       172.23.0.0/16 is a summary, 7w0d, Null0
 

config

router eigrp 52
 auto-summary
 !
 address-family ipv4 vrf RC
  redistribute connected
  redistribute static
  network 10.0.0.0
  network 172.23.0.0
  auto-summary
  autonomous-system 52
 exit-address-family
 

show run | include 172.23.0.0
  network 172.23.0.0
  network 172.23.0.0
  network 172.23.0.0
  network 172.23.0.0
  network 172.23.0.0
 

Regards

Mahesh

[Jon Marshall · Accepted solution]

Mahesh

You have "auto-summary" under your EIGRP configuration and this is what is creating the summary route in your routing table.

The key point remains though. That router should have the more specific routes also but it doesn't appear to have one for the subnet you are trying to get to.

You need to work out why that isn't the case ie. what is the next hop router supposed to be and why isn't that router advertising that subnet ?

Jon

[mahesh18]

Hi Jon,

Thanks a lot i know now how to proceed further.

Best Regards

Mahesh

[Richard Burts · Accepted solution]

Mahesh

The additional information that you posted for Jon does give us the reason for what you are observing. Under router eigrp you have configured auto-summary.  Your configuration suggests that there are some interfaces in network 10.0.0.0. With auto-summary enabled when EIGRP is going to advertise out an interface in network 10.0.0.0 it will create a summary address for 172.23.0.0 and specify null 0 as the next hop.

Now that we understand where the summary route is coming from we can now begin to think about your other question - should you add a route for the address that you are trying to reach. As Jon has said we really need more information to be able to answer that question.

- should this router be able to reach that address?

- if so then how should this router learn that address?

- is the router where that address is located advertising that subnet?

- should the router where that address is located be advertising that subnet?

- is some device along the path dropping the advertisement or filtering out the advertisement?

When there are answers for those questions it will be easier to know whether you should add a route on this router.

HTH

Rick

[mahesh18]

Hi Rick,

Thanks a lot for great and  detailed explanation.

I will find answers to your questions and proceed further.

Best Regards

Mahesh

[Richard Burts]

Mahesh

This has been a very interesting one to work out. I am glad that we figured out how the summary route was created. Now you just need to work out the details of whether that address should really be reachable from that router, and if so how to create the optimum routing logic. Sometimes a static route will be the optimum solution, but frequently a better solution may be a dynamic advertisement.

HTH

Rick