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scottsee
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Classfull routing table.

Sat Jan 22, 2011 2:53 pm

I'm not understanding what I'm seeing.

What I expect to see is the routing table doing this:

Implementation of no ip classless

1 - Router handles a packet destined to 172.16.3.1
2 - Router examines its routing table to find a level 1 ultimate route - none found
3 - Router examines its routing table, finds a level one parent route for the classfull, logical destination address.
4 - Router examines it's level 2 child route located under 172.160.0 and does not find a match for the 172.16.3.0 network.
5 - Router drops packet..

Instead, the router is still using classless routing decisions. Is there something I am missing?

Code: Select all
R1#show runn | s classless
no ip classless
R1#


Code: Select all
R1#show version |  s IOS Software
Cisco IOS Software, 3600 Software (C3640-IO3-M), Version 12.4(25d), RELEASE SOFTWARE (fc1)
R1#


Code: Select all
Gateway of last resort is 0.0.0.0 to network 0.0.0.0

     172.16.0.0/30 is subnetted, 1 subnets
C       172.16.1.0 is directly connected, Serial0/0
S*   0.0.0.0/0 is directly connected, Serial0/0
R1(config)#


Code: Select all
R1#ping 172.16.3.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.3.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 28/44/80 ms
R1#


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Re: Classfull routing table.

Sat Jan 22, 2011 3:33 pm

scottsee wrote:2 - Router examines its routing table to find a level 1 ultimate route - none found
3 - Router examines its routing table, finds a level one parent route for the classfull, logical destination address.
4 - Router examines it's level 2 child route located under 172.160.0 and does not find a match for the 172.16.3.0 network.


Level 1 ultimate, level 2, child? You have me confused :shock:

What's the subnet mask of the s0/0 interface?

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Re: Classfull routing table.

Sat Jan 22, 2011 4:28 pm

mellowd; the information that scottsee is working from is part of the CCNA Exploration curriculum, cribbed from a book called Cisco IP Routing by Alex Zinin.
The tl;dr is pretty much this: There are level 1 routes in the routing table; default routes, summary routes, and network routes. These level 1 routes may have child routes. Scottsee has summarised how he sees the routing table lookup happening after implementing the ip classless command.
At this point, I don't know the packet is getting forwarded out of the interface, it ought to be getting dropped. How about throwing a few more routes into the table so you've got more information to make a forwarding decision from?

[EDIT]
http://www.cisco.com/en/US/tech/tk365/t ... 4823.shtml
Towards the bottom is Cisco's explanation on how no ip classless is supposed to work

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Re: Classfull routing table.

Sat Jan 22, 2011 4:39 pm

Classfull routing is the devil. The world is moving to IPv6 and classfull routing is still a CCNA topic. So confusing.

Code: Select all
0.0.0.0/0 is directly connected, Serial0/0


Quad four route for the win? You've got a default route in there.
- Pete

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Re: Classfull routing table.

Sat Jan 22, 2011 5:24 pm

mellowd wrote:Level 1 ultimate, level 2, child? You have me confused :shock:


Glad I wasn't the only one... was about to hit the CCNA books again. :thinking:

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Re: Classfull routing table.

Sat Jan 22, 2011 5:44 pm

Project2501 wrote:Classfull routing is the devil. The world is moving to IPv6 and classfull routing is still a CCNA topic. So confusing.

Code: Select all
0.0.0.0/0 is directly connected, Serial0/0


Quad four route for the win? You've got a default route in there.


This

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Re: Classfull routing table.

Sat Jan 22, 2011 6:49 pm

Halo wrote:At this point, I don't know the packet is getting forwarded out of the interface, it ought to be getting dropped. How about throwing a few more routes into the table so you've got more information to make a forwarding decision from?



I added more loopbacks and made the network converge using RIPv2 but no Cigar. I'm still seeing the classfull routing table use the quad-zero default route..

Code: Select all
Gateway of last resort is 0.0.0.0 to network 0.0.0.0

     172.16.0.0/16 is variably subnetted, 7 subnets, 2 masks
C       172.16.5.1/32 is directly connected, Loopback0
C       172.16.7.1/32 is directly connected, Loopback2
C       172.16.6.1/32 is directly connected, Loopback1
C       172.16.1.0/30 is directly connected, Serial0/0
R       172.16.2.0/30 [120/1] via 172.16.1.2, 00:00:17, Serial0/0
R       172.16.100.1/32 [120/1] via 172.16.1.2, 00:00:17, Serial0/0
S*   0.0.0.0/0 is directly connected, Serial0/0
R1#ping 172.16.200.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.200.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 24/51/88 ms
R1#


Project2501 wrote:Quad four route for the win? You've got a default route in there.


Yes, but it shouldn't be seen by a classfull routing table in this situation.. I'll explain using the new routing table in this post..
- R1 is sending an ICMP echo to the address 172.16.200.1 /32 2 hops away.
- R1 has a classfull routing table enabled using no ip classless
- R1 examines the destination address for a classfull match (172.16.0.0 - Class B)
- R1 finds a parent classfull route for the destination network
- R1 examines each of the child routes within the parent route
- R1 finds no match for the destination 172.16.200.1 packet.
** With a classless routing table a router will re-examine level 1 routes for a less specific route the what is matched by the 172.16.0.0 classfull parent route. Allowing it to find any summary routes or quad-zero routes.
** With a classfull routing table, once the child routes are examined and no match is found, it will not re-examine the routing table to find a summary or default route..

That's the jist..

I'm getting sick so forgive me..

I just don't understand why these are being routed..
Last edited by scottsee on Sat Jan 22, 2011 7:12 pm, edited 1 time in total.

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Re: Classfull routing table.

Sat Jan 22, 2011 7:10 pm

try issuing "no ip cef" and see if that makes a difference.

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Re: Classfull routing table.

Sat Jan 22, 2011 7:23 pm

scottsee wrote:** With a classless routing table a router will re-examine level 1 routes for a less specific route the what is matched by the 172.16.0.0 classfull parent route. Allowing it to find any summary routes or quad-zero routes.


Wouldn't that explain why the quad zero is being used?

I can't see any routes that 172.16.3.1 would match otherwise.
- Pete

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Re: Classfull routing table.

Sat Jan 22, 2011 7:41 pm

Project2501 wrote:
scottsee wrote:** With a classless routing table a router will re-examine level 1 routes for a less specific route the what is matched by the 172.16.0.0 classfull parent route. Allowing it to find any summary routes or quad-zero routes.


Wouldn't that explain why the quad zero is being used?

I can't see any routes that 172.16.3.1 would match otherwise.


When you use the R1(config)no ip classless command, it turn the routing table from classless to classfull. So, it's not suppose to search for a quad zero route (or any other supernet route) when a parent route exists in the routing table for the network destination.

Understand?

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Re: Classfull routing table.

Sat Jan 22, 2011 7:45 pm

Jeff wrote:try issuing "no ip cef" and see if that makes a difference.


Still able to reach the destination address 172.16.200.1 from the following routing table.. I changed over to EIGRP, at this point I'm up for suggestions..


EDIT: I just noticed EIGRP added the default route from the HUB router because of a bad network statement. I didn't know EIGRP would add static routes that match a network statement in to the routing table.

Code: Select all
D       172.16.200.0/24 [90/2681856] via 172.16.1.2, 00:07:08, Serial0/0


After adding a correct network summary excludeing the static route from being populated in to the routing table, Classfull routing tables for EIGRP works as expected..


Code: Select all
Gateway of last resort is 0.0.0.0 to network 0.0.0.0

     172.16.0.0/16 is variably subnetted, 7 subnets, 3 masks
D       172.16.200.0/24 [90/2681856] via 172.16.1.2, 00:07:08, Serial0/0
C       172.16.5.1/32 is directly connected, Loopback0
C       172.16.7.1/32 is directly connected, Loopback2
C       172.16.6.1/32 is directly connected, Loopback1
C       172.16.1.0/30 is directly connected, Serial0/0
D       172.16.2.0/30 [90/2681856] via 172.16.1.2, 00:08:49, Serial0/0
D       172.16.100.1/32 [90/2809856] via 172.16.1.2, 00:08:49, Serial0/0
S*   0.0.0.0/0 is directly connected, Serial0/0
R1(config)#
Last edited by scottsee on Sat Jan 22, 2011 8:29 pm, edited 3 times in total.

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Re: Classfull routing table.

Sat Jan 22, 2011 7:56 pm

Huhh, I converged over to OSPF and the Classfull routing table works as expected.. Well at least I have a place to start. For some reason static routing & distance-vector routing aren't supporting classfull routing tables as expected... Time to figure out why..

Code: Select all
R1(config)#do show ip route
Codes: 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

Gateway of last resort is 0.0.0.0 to network 0.0.0.0

     172.16.0.0/16 is variably subnetted, 6 subnets, 2 masks
C       172.16.5.1/32 is directly connected, Loopback0
C       172.16.7.1/32 is directly connected, Loopback2
C       172.16.6.1/32 is directly connected, Loopback1
C       172.16.1.0/30 is directly connected, Serial0/0
O       172.16.2.0/30 [110/128] via 172.16.1.2, 00:02:18, Serial0/0
O       172.16.100.1/32 [110/129] via 172.16.1.2, 00:02:18, Serial0/0
S*   0.0.0.0/0 is directly connected, Serial0/0
R1(config)#ip classless
R1(config)#do ping 172.16.200.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.200.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 76/101/120 ms
R1(config)#no ip classless
R1(config)#do ping 172.16.200.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.200.1, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
R1(config)#ip classless
R1(config)#do ping 172.16.200.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.200.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 40/94/176 ms
R1(config)#

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Re: Classfull routing table.

Sat Jan 22, 2011 8:14 pm

scottsee wrote:
Jeff wrote:try issuing "no ip cef" and see if that makes a difference.


Still able to reach the destination address 172.16.200.1 from the following routing table.. I changed over to EIGRP, at this point I'm up for suggestions..

Code: Select all
Gateway of last resort is 0.0.0.0 to network 0.0.0.0

     172.16.0.0/16 is variably subnetted, 7 subnets, 3 masks
D       172.16.200.0/24 [90/2681856] via 172.16.1.2, 00:07:08, Serial0/0
C       172.16.5.1/32 is directly connected, Loopback0
C       172.16.7.1/32 is directly connected, Loopback2
C       172.16.6.1/32 is directly connected, Loopback1
C       172.16.1.0/30 is directly connected, Serial0/0
D       172.16.2.0/30 [90/2681856] via 172.16.1.2, 00:08:49, Serial0/0
D       172.16.100.1/32 [90/2809856] via 172.16.1.2, 00:08:49, Serial0/0
S*   0.0.0.0/0 is directly connected, Serial0/0
R1(config)#


In this table you do have an entry for the 200.1 so a ping should work.

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Re: Classfull routing table.

Sat Jan 22, 2011 8:18 pm

scottsee wrote:Huhh, I converged over to OSPF and the Classfull routing table works as expected.. Well at least I have a place to start. For some reason static routing & distance-vector routing aren't supporting classfull routing tables as expected... Time to figure out why..

Code: Select all
R1(config)#do show ip route
Codes: 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

Gateway of last resort is 0.0.0.0 to network 0.0.0.0

     172.16.0.0/16 is variably subnetted, 6 subnets, 2 masks
C       172.16.5.1/32 is directly connected, Loopback0
C       172.16.7.1/32 is directly connected, Loopback2
C       172.16.6.1/32 is directly connected, Loopback1
C       172.16.1.0/30 is directly connected, Serial0/0
O       172.16.2.0/30 [110/128] via 172.16.1.2, 00:02:18, Serial0/0
O       172.16.100.1/32 [110/129] via 172.16.1.2, 00:02:18, Serial0/0
S*   0.0.0.0/0 is directly connected, Serial0/0
R1(config)#ip classless
R1(config)#do ping 172.16.200.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.200.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 76/101/120 ms
R1(config)#no ip classless
R1(config)#do ping 172.16.200.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.200.1, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
R1(config)#ip classless
R1(config)#do ping 172.16.200.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.200.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 40/94/176 ms
R1(config)#


Something of note in regards to this route table vs the original one. In this table you have the 172.16.0.0 /16 (top of route table) and in the original it was a /30 which of course is not a classful mask.


EDIT: Nevermind just noticed your next table after that one did also have the /16 and had the ping work.

FWIW this link discusses the same question pretty much https://learningnetwork.cisco.com/message/89321

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Re: Classfull routing table.

Sat Jan 22, 2011 8:23 pm

A comedy interjection here:
If the default route is installed by IS-IS or OSPF, it will get used even in a no ip classless configured routing table.

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Re: Classfull routing table.

Sat Jan 22, 2011 8:27 pm

Jeff wrote:
scottsee wrote:
Jeff wrote:try issuing "no ip cef" and see if that makes a difference.


Still able to reach the destination address 172.16.200.1 from the following routing table.. I changed over to EIGRP, at this point I'm up for suggestions..

Code: Select all
Gateway of last resort is 0.0.0.0 to network 0.0.0.0

     172.16.0.0/16 is variably subnetted, 7 subnets, 3 masks
D       172.16.200.0/24 [90/2681856] via 172.16.1.2, 00:07:08, Serial0/0
C       172.16.5.1/32 is directly connected, Loopback0
C       172.16.7.1/32 is directly connected, Loopback2
C       172.16.6.1/32 is directly connected, Loopback1
C       172.16.1.0/30 is directly connected, Serial0/0
D       172.16.2.0/30 [90/2681856] via 172.16.1.2, 00:08:49, Serial0/0
D       172.16.100.1/32 [90/2809856] via 172.16.1.2, 00:08:49, Serial0/0
S*   0.0.0.0/0 is directly connected, Serial0/0
R1(config)#


In this table you do have an entry for the 200.1 so a ping should work.


Good eyes, I just corrected that and now EIGRP classfull routing works as expected, just like OSPF. Now it's just down to figuring out why it's not working with static routes and Rip..

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Re: Classfull routing table.

Sat Jan 22, 2011 11:41 pm

Jeff wrote:try issuing "no ip cef" and see if that makes a difference.


I must have had something configured incorrectly when I turned off Cisco express forwarding the first time. After I started the lab over and using the no ip cef command the classfullness behavior I was expecting to see using static routing occurred. I spent some time reading over the inherent differences between CEF and process switching. Like most material taught though the ICND series, the books leave out real world application. I guess it's nice to know that if you want classfull routing table behavior with static routing or RIP you'll need to disable CEF. Interestingly enough, OSPF and EIGRP both work fine CEF enabled.

Who would have known..

Thanks Guys..

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Re: Classfull routing table.

Sun Jan 23, 2011 8:49 am

Sorry, didn't I mention this in a email? Something about how Ch 8 is way beyond CCNA requirements and how the real world doesn't reflect this type of routing process?

The biggest traps for D-V labs using post-1995 gear, ip cef and long holddown timers - frequently catches out many Networking Academy instructors and students in the lab.

Like most material taught though the ICND series, the books leave out real world application.
Indicative of how the CCNA cert is caught between trying to test basics in a 21st century world. CEF is a CCNP area, though it could be mentioned in the Academy courses in my view. In CCNA Exploration 4 above-CCNA level ACLs are explained, but not assessed, to try and give more of a real world approach.

Aubrey
The illiterate of the 21st century will not be those who cannot read and write, but those who cannot learn, unlearn, and relearn. Alvin Toffler, "Future Shock" 1970

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Re: Classfull routing table.

Sun Jan 23, 2011 11:31 am

eaadams wrote:Sorry, didn't I mention this in a email? Something about how Ch 8 is way beyond CCNA requirements and how the real world doesn't reflect this type of routing process?

Aubrey


Yeah, now that you mention it. It's right there in your Email... :oops:

Email wrote:all good stuff but way beyond CCNA level, and IMO really not that relevant anymore in the real world with
CEF taking over after the first rotuing table look up.

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Re: Classfull routing table.

Mon Jan 24, 2011 1:22 am

scottsee wrote:
eaadams wrote:Sorry, didn't I mention this in a email? Something about how Ch 8 is way beyond CCNA requirements and how the real world doesn't reflect this type of routing process?

Aubrey


Yeah, now that you mention it. It's right there in your Email... :oops:

Email wrote:all good stuff but way beyond CCNA level, and IMO really not that relevant anymore in the real world with CEF taking over after the first routing table look up.
Hey, but look what you learned, and you'll never forget it now.

Aubrey
The illiterate of the 21st century will not be those who cannot read and write, but those who cannot learn, unlearn, and relearn. Alvin Toffler, "Future Shock" 1970

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