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Beginner

Routing Table - Understanding

I have the below route in my routing table. 

The route table shows 17.0.0.0/20 is subnetted, but it also says it has specific OSPF route for 17.42.192.0 in the table.

 

But there is no subnet mask associated to the route 17.46.192.0. So would like to know how to read the route for 17.46.192.0

What is the meaning of this 17.46.192.0 ?

Which subnet is covered ?

 

      17.0.0.0/20 is subnetted, 1 subnets
O E2     17.46.192.0 [110/20] via 10.127.36.10, 7w0d, GigabitEthernet1/1
                     [110/20] via 10.127.36.2, 7w0d, GigabitEthernet2/1

3 REPLIES 3
Highlighted
Hall of Fame Master

Re: Routing Table - Understanding

Hello @RS19 ,

first of all , prefix 17.46.192.0 is not a subnet of 17.0.0.0/20 last IP address in 17.0.0.0/20 is 17.0.15.255.

Said this, noting that the prefix 17.46.192.0 is learned in OSPF as O E2 there are two ways to discover the prefix length network mask:

a)

show ip route 17.46.192.0

 

b)

show ip ospf database external 17.46.192.0

 

both of these two commands should give you the same prefix length / subnet mask.

 

Hope to help

Giuseppe

 

 

Highlighted
Beginner

Re: Routing Table - Understanding

Thanks . Will check the output you mentioned.

 

I will explain what I am trying to do.

There are some routes which are learned in my L3 switch.

For those routes I am planning to apply ACL to a particular VLAN so that access is restricted.

 

To know the routes for which access needs to be blocked, i was looking at the routes which was learned.

That was the output i shared earlier.

Now I am not sure if I need to block 17.0.0.0/20 or 172.46.192.0 (But not sure what will be the subnet of this)

 

      17.0.0.0/20 is subnetted, 1 subnets
O E2     17.46.192.0 [110/20] via 10.127.36.10, 7w0d, GigabitEthernet1/1
                     [110/20] via 10.127.36.2, 7w0d, GigabitEthernet2/1

Highlighted
Collaborator

Re: Routing Table - Understanding

 

do not block 17.0.0.0/20 or any other prefixed that is followed by is subnetted. this is just router grouping or categorized routes into groups (arranging table).

but block 172.46.192.0

Giuseppe gave you commands how to find out length/mask.  Easiest one is sh ip route x.x.x.x   other one, Note external keyword is due to E2

 

 

Regards, ML
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