Did a lab, got 100% on it, don't understand output

Jasiono

Ok I am doing subnetting labs right now and the basic Gist of it is this.

I am given a table in which I have 2 IP addresses that need to be entered into a router. One on s0/0/0 and the other on fa0/0.

Here is the table:
Serial0/0/0 - Address 172.16.133.233 - Mask 255.255.255.128
FastEthernet0/0 - Address 172.16.133.1 - Mask 255.255.255.128

I was told to figure out what subnets each reside in, so my conclusion was:
S0/0/0 will reside in 172.16.133.128/25
Fa0/0 will reside in 172.16.133.0/25

After that, I was told to configure the router with the addresses of each interface and verify that I have the correct answer by issuing the SHOW IP ROUTE command, which I did, and I saw that my connected routes where what I calculated.

All right, so I got a 100% on the lab, but I don't understand this portion of the output:

Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
+ - replicated route, % - next hop override


Gateway of last resort is not set


172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks
C 172.16.133.0/25 is directly connected, FastEthernet0/0
L 172.16.133.1/32 is directly connected, FastEthernet0/0
C 172.16.133.128/25 is directly connected, Serial0/0/0
L 172.16.133.233/32 is directly connected, Serial0/0/0

Why do my local addresses have a different subnet mask?

I have not done VLSM yet. I know what VLSM is about in a very high level overview, but I am not understanding this output. Is it something I should know by now even though I have not done VLSM yet? That is chapter 20 of the book and I just finished up chapter 14.

Hondabuff

Local address is the interface. /32 denotes 1 host. Easiest way to remember. Comes more into play later on when you start advertising routes via IGRP.

networker050184

It's a newer feature in IOS for IPv4 to show the local routes as a connected /32. It's not needed for the vast majority of the time.

Priston

read this

Local Host Routes Installed in the Routing Table on Cisco IOS and Cisco IOS-XR - Cisco

Jasiono

Ok so what I gather from all of your posts and the article provided, thanks for all of that by the way, is that everything local to the router will have a /32.


Seemed a bit confusing to me because I thought a subnet mask is there to segment the network portion from the host portion of the address.

I'll be fine as long as I remember /32 means local address. Seems easy enough to remember.

networker050184

A /32 doesn't necessarily mean local. Think of it in binary terms. It just means one address.

Jasiono

Got it now, yes.
Thanks for that clarification.
If it were a /31 there would be 2 addresses. The /32 basically means HEY THIS IS THE ADDRESS AND THIS IS THE ONE AND ONLY ONE

james43026

Entries marked as local in a routing table on newer versions of IOS, are used for CEF, this allows all local interfaces to have a CEF entry marked as "receive", this allows CEF and the layer 2 forwarding engine to recognize that a packet is destined for the local router, and the packet will be punted to the layer 3 forwarding engine AKA CPU for processing.

networker050184

CEF was able to function before this. It's really been implemented for MTR.

james43026

networker050184 wrote: »

CEF was able to function before this. It's really been implemented for MTR.

I do agree that it is used for MTR, and I forgot about that. But this was taken from Cisco as well.

"The result is that the router can always install a CEF entry for the configured IP address if it only searches for the corresponding entry in the routing table. This also prevents a misconfiguration where a route with a longer mask than the connected entry would be learned from another router, which causes traffic that is destined for the local IP address to be misdirected to a remote router."

This was taken directly from here

As well as

"[h=2]Conclusion[/h]It is normal for local host routes to be listed in the IPv4 and IPv6 routing table for IP addresses of the router's interfaces. Their purpose is to create a corresponding CEF entry as a receive entry so that the packets destined to this IP address can be processed by the router itself. These routes cannot be redistributed into any routing protocol."

My conclusion, while this helps MTR, the local host routes were created primarily for CEF.