no auto summary, what does it do?

I'm going though Todd's book and working through the examples.

I have R3 which is connected to a wireless PC on the network 172.16.10.0.

R3 Shows this route

R3(config-router)#do show ip route
Codes: C - connected, S - static, I - IGRP, 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, E - EGP
       i - IS-IS, 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

Gateway of last resort is 10.1.5.1 to network 0.0.0.0

     10.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
D       10.0.0.0/8 is a summary, 00:00:07, Null0
D       10.1.1.0/24 [90/25628160] via 10.1.5.1, 00:00:07, FastEthernet0/0
D       10.1.2.0/24 [90/20514560] via 10.1.5.1, 00:00:07, FastEthernet0/0
D       10.1.3.0/24 [90/20514560] via 10.1.5.1, 00:00:07, FastEthernet0/0
D       10.1.4.0/24 [90/20514560] via 10.1.5.1, 00:00:07, FastEthernet0/0
C       10.1.5.0/24 is directly connected, FastEthernet0/0
     172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
D       172.16.0.0/16 is a summary, 00:00:07, Null0
C       172.16.10.0/24 is directly connected, Dot11Radio0/3/0
D    192.168.10.0/24 [90/20517120] via 10.1.5.1, 00:00:07, FastEthernet0/0
D    192.168.20.0/24 [90/20517120] via 10.1.5.1, 00:00:07, FastEthernet0/0
D    192.168.30.0/24 [90/20517120] via 10.1.5.1, 00:00:07, FastEthernet0/0
D    192.168.40.0/24 [90/20517120] via 10.1.5.1, 00:00:07, FastEthernet0/0
S*   0.0.0.0/0 [1/0] via 10.1.5.1

R3 is connected to a Corp router:

Corp(config-router)#do show ip route
Codes: C - connected, S - static, I - IGRP, 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, E - EGP
       i - IS-IS, 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

Gateway of last resort is not set

     10.0.0.0/24 is subnetted, 5 subnets
C       10.1.1.0 is directly connected, Vlan1
C       10.1.2.0 is directly connected, Serial0/3/0
C       10.1.3.0 is directly connected, Serial0/1/0
C       10.1.4.0 is directly connected, Serial0/2/0
C       10.1.5.0 is directly connected, FastEthernet0/0
     172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
D       172.16.0.0/16 [90/28160] via 10.1.5.2, 00:00:04, FastEthernet0/0
S       172.16.10.0/24 [150/0] via 10.1.5.2
D    192.168.10.0/24 [90/20514560] via 10.1.3.2, 00:00:04, Serial0/1/0
                     [90/20514560] via 10.1.2.2, 00:00:04, Serial0/3/0
D    192.168.20.0/24 [90/20514560] via 10.1.3.2, 00:00:04, Serial0/1/0
                     [90/20514560] via 10.1.2.2, 00:00:04, Serial0/3/0
D    192.168.30.0/24 [90/20514560] via 10.1.4.2, 00:00:04, Serial0/2/0
D    192.168.40.0/24 [90/20514560] via 10.1.4.2, 00:00:04, Serial0/2/0
Corp(config-router)#

When the command 'no auto-summary' is used on R3 and Corp routers the 172.16. networks goes to a single line.

Corp Router

     10.0.0.0/24 is subnetted, 5 subnets
C       10.1.1.0 is directly connected, Vlan1
C       10.1.2.0 is directly connected, Serial0/3/0
C       10.1.3.0 is directly connected, Serial0/1/0
C       10.1.4.0 is directly connected, Serial0/2/0
C       10.1.5.0 is directly connected, FastEthernet0/0
     172.16.0.0/24 is subnetted, 1 subnets
D       172.16.10.0 [90/28160] via 10.1.5.2, 00:00:04, FastEthernet0/0
D    192.168.10.0/24 [90/20514560] via 10.1.3.2, 00:00:31, Serial0/1/0
                     [90/20514560] via 10.1.2.2, 00:00:31, Serial0/3/0
D    192.168.20.0/24 [90/20514560] via 10.1.3.2, 00:00:31, Serial0/1/0
                     [90/20514560] via 10.1.2.2, 00:00:31, Serial0/3/0
D    192.168.30.0/24 [90/20514560] via 10.1.4.2, 00:00:31, Serial0/2/0
D    192.168.40.0/24 [90/20514560] via 10.1.4.2, 00:00:31, Serial0/2/0
Corp(config-router)#

Gateway of last resort is 10.1.5.1 to network 0.0.0.0

     10.0.0.0/24 is subnetted, 5 subnets
D       10.1.1.0 [90/25628160] via 10.1.5.1, 00:01:00, FastEthernet0/0
D       10.1.2.0 [90/20514560] via 10.1.5.1, 00:01:00, FastEthernet0/0
D       10.1.3.0 [90/20514560] via 10.1.5.1, 00:01:00, FastEthernet0/0
D       10.1.4.0 [90/20514560] via 10.1.5.1, 00:01:00, FastEthernet0/0
C       10.1.5.0 is directly connected, FastEthernet0/0
     172.16.0.0/24 is subnetted, 1 subnets
C       172.16.10.0 is directly connected, Dot11Radio0/3/0
D    192.168.10.0/24 [90/20517120] via 10.1.5.1, 00:01:00, FastEthernet0/0
D    192.168.20.0/24 [90/20517120] via 10.1.5.1, 00:01:00, FastEthernet0/0
D    192.168.30.0/24 [90/20517120] via 10.1.5.1, 00:01:00, FastEthernet0/0
D    192.168.40.0/24 [90/20517120] via 10.1.5.1, 00:01:00, FastEthernet0/0
S*   0.0.0.0/0 [1/0] via 10.1.5.1
R3(config-router)#

Todd explains the following '172.16.10.0 is a better match that 172.16.0.0 and the Corp had a static route with a AD of 150 to R3. with the longest match of 172.16.10.0.

However R3 was advertising with a summary that 172.16.0.0 was directly connected, so the routing tables inserted both. Once auto-summary was turned off the route of 172.16.10.0 with a lower AD was advertised and the static route disappeared from the routing table.'

I don't understand the second paragraph, R3 is advertising 172.16.10.0 not 172.16.0.0.

Also does longest match takes presence over lowest AD, or vice versa?

Would really appreciate any advice.

Thanks

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Comments

lon21

anyone, please?

Forsaken_GA

lon21 wrote: »

However R3 was advertising with a summary that 172.16.0.0 was directly connected, so the routing tables inserted both. Once auto-summary was turned off the route of 172.16.10.0 with a lower AD was advertised and the static route disappeared from the routing table.'

I don't understand the second paragraph, R3 is advertising 172.16.10.0 not 172.16.0.0.

R3 was advertising 172.16.0.0/16 while auto summary was turned on. So the Corp router had a route from EIGRP for 172.16.0.0/16, and a static route which pointed to 172.16.10.0/24 at an AD of 150.

Once R3 turned off auto-summary, instead of sending 172.16.0.0/16, it sent the real route, 172.16.10.0/24. Well you already had that route in Corp's routing table as a static route. So what happens? The static route has an AD of 150, the EIGRP route has an AD of 90. 90 is lower, so it wins.

Also does longest match takes presence over lowest AD, or vice versa?

Yes, longest match takes precedence, but in this case, you have two matching routes, so the longest prefix is a tie. So AD counts.

instant000

DISCLAIMER: I can't exactly follow you (not sure of what book or page you're referring to), but I'm hoping my explanation is something that you can follow. I have Lammle CCNA sixth edition, but when glancing through, didn't see what you referred to in your post.

Todd explains the following '172.16.10.0 is a better match that 172.16.0.0 and the Corp had a static route with a AD of 150 to R3. with the longest match of 172.16.10.0.

However R3 was advertising with a summary that 172.16.0.0 was directly connected, so the routing tables inserted both. Once auto-summary was turned off the route of 172.16.10.0 with a lower AD was advertised and the static route disappeared from the routing table.'

I don't understand the second paragraph, R3 is advertising 172.16.10.0 not 172.16.0.0.

Read these:

Enhanced Interior Gateway Routing Protocol - Cisco Systems

Cisco IOS IP Command Reference, Volume 2 of 4: Routing Protocols, Release 12.3 T - IP Routing Protocol Commands: A [Cisco IOS Software Releases 12.3 T] - Cisco Systems

Basically, the automatically summarized route made its own entry into the routing table.

I'll break it down a bit further.
Auto-summarization means to automatically summarize on the classful boundary.
For a class B, that would be summarizing at the /16 point.
So, a router with the network 172.16.10.0/24 on it, but configured for auto-summary, would additionally advertise the route for 172.16.0.0/16.

Likewise, a router configured with autosummary and network 10.1.1.0/24, would additionally advertise 10.0.0.0/8. These are two different network entries for the routing table, as you can see by your own output above.

Look up, you also have the 10.0.0.0/8 route up there, before you turn off the auto-summary.

As you can note, all your 192.168.X.0 networks use the default /24 mask, so there is no need for an automatically summarized route for those to appear. If you had used masks longer than /24, they would have been automatically summarized also.

Also does longest match takes presence over lowest AD, or vice versa?

Now, read this:

Route Selection in Cisco Routers - Cisco Systems

Longest prefix match is used by the router to decide which route, of the many routes it has, will it use to forward the traffic. That is, the router could have all these networks in the routing table: (looks weird, but humor me, anyway ... the basic thought here is that all of these are valid, different routes ... summarization would be useful, if you were trying to minimize some routing tables and wear and tear on your router.)

10.0.0.0/8
10.1.0.0/16
10.1.1.0/23
10.1.1.0/24
10.1.1.0/29
10.1.1.0/30

And, it gets a packet for 10.1.1.16, which network would it make sense to use to send the information? (Answer: the 10.1.1.0/24 network, as it matches the longest prefix for that address, and should have that target host. ... If you do the subnet math on the /29 and /30 networks, the .16 address won't be a valid host address.)

Hope this helps!

lon21

OK, this is what I've learnt, reason both routes were in the routing table was due to having different prefix lengths.

When the no auto-summary cmd was excuteded both routes had the same prefix length therefore one with lower AD was chosen in the routing table and other discarded.

The more I learn, the more I love Cisco.

Thanks for your help.