Rip summarization

Auto-summary will override a manual summary except in the case that both the following conditions are true.

1) Split Horizon is not enabled
2) If the manual summary shares the same major network as the interface its configured on.

This doesnt make sense to me!
If the manual summary shares the same address as the interface its configured on,auto-summary will not be used as the major network boundardy has not been crossed, so of course the manual summary will be used.
Also what has Split Horizon to do with summarization?
Can anyone enlighten me to what cisco mean by this?

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Comments

mikej412

ed_the_lad wrote:

This doesnt make sense to me!

I can't wrap my head around it either.... I see the sample, but it really isn't an example of anything. I can't see the "why."

It feels more like a "programming feature" where it was easier to change the doc than the code.

Some poor overworked programmer probably used the wrong bit somewhere as a flag -- and that's how split horizon got in there. You disable split horizon... which can cause problems, but the other router then uses poison reverse..... So yeah -- use split horizon as a flag, but make sure the doc gets changed.

EdTheLad

mikej412 wrote:

ed_the_lad wrote:

This doesnt make sense to me!

Some poor overworked programmer probably used the wrong bit somewhere as a flag -- and that's how split horizon got in there. You disable split horizon... which can cause problems, but the other router then uses poison reverse..... So yeah -- use split horizon as a flag, but make sure the doc gets changed.

But i cant even see where split horizon changes behaviour, i tried to sim something up but still i cant see where this comes into play.Under what circumstances does split horizon effect summarization?
Below is an example from cisco.

In the following example configuration, the major network is 10.0.0.0. The 10 in the address defines a Class A address space, allowing space for 0.x.x.x unique hosts where x defines unique bit positions in the addresses for these hosts. The summary of the major net defines the prefix as implied by the class (A, B, or C) of the address, without any network mask. The summary address 10.2.0.0 overrides the automatic summary address of 10.0.0.0, 10.2.0.0 is advertised out interface E1, and 10.0.0.0 is not advertised.

Router(config)# interface Ethernet1
Router(config-if)# ip address 10.1.1.1 255.255.255.0
Router(config-if)# ip summary-address rip 10.2.0.0 255.255.0.0
Router(config-if)# no ip split-horizon
Router(config-if)# exit
Router(config)# router rip
Router(config)# network 10.0.0.0

It says the summary address 10.2.0.0 overrides the automatic summary address 10.0.0.0,10.2.0.0 is advertised but 10.0.0.0 is not advertised.What has this to do with split-horizon? This behaviour will be the same with split-horizon enabled or disabled.The auto-summary 10.0.0.0/8 will not be sent as the interface shares the same major network address.Auto-summary only occurs when the major net changes which is not the case here.Has the author completely screwed up or am i missing something?[/i]

Ching01

Ed, RIP v1 does not support manual summarization (Refer to page 10 of Sybex 5th edition). If you disable the no split horizon rule then it cause a routing loop.

EdTheLad

Ching01 wrote:

Ed, RIP v1 does not support manual summarization (Refer to page 10 of Sybex 5th edition). If you disable the no split horizon rule then it cause a routing loop.

Its a typo in the cisco example, i'm referring to ripv2, i know what split-horizon is for, my issue is with cisco's example of manual summarization
overriding auto-summarization.They say this will occur if 2 conditions are true

1) Split-horizon is not enabled
2) If the manual summary shares the same major net as the interface its configured on.

How can one simulate this? How can an auto-summary occur and override
a manual summary if the update shares the same major net as the interface.
Auto-summary will not occur.

A

B
10.0.0.1/30 10.0.0.2/30

If A has a manual summary " ip summary-address 10.0.0.0 255.255.0.0"
Auto-summary will never be used for a 10.x.x.x network as auto-summary only occurs if the major net changes.In this case with or without split-horizon being enabled the manual summary will occur.

Cisco says if split-horizon is enabled an auto-summary 10.0.0.0/8 will
take precedence and override the manual summary.
Cisco says if split horizon is diabled the manual summary will take precedence.

I have tryed both and as expected only the manual summary is sent in both cases.

Ching01

Yeah, your right I simulate your example and it's giving me the same output. I disable and enable split horizon but nothing change, it still showing me the advertise summary address.

Here's my config: router A
interface Loopback0
ip address 10.2.0.1 255.255.255.252
no ip directed-broadcast
!
interface FastEthernet0/0
ip address 10.0.0.1 255.255.255.252
no ip directed-broadcast
ip summary-address rip 10.0.0.0 255.255.0.0
duplex auto
speed auto
!
router rip
version 2
network 10.0.0.0

>>>>>>>>>>>>>>>for router B

!
interface FastEthernet0/0
ip address 10.0.0.2 255.255.255.252
no ip directed-broadcast
duplex auto
speed auto
!
router rip
version 2
network 10.0.0.0
!
ip classless
no ip http server
!
!

Herer the routing table for B
Gateway of last resort is not set

10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
R 10.2.0.0/30 [120/1] via 10.0.0.1, 00:00:19, FastEthernet0/0
R 10.0.0.0/16 [120/1] via 10.0.0.1, 00:00:19, FastEthernet0/0
C 10.0.0.0/30 is directly connected, FastEthernet0/0
B#

show ip int f0/0
FastEthernet0/0 is up, line protocol is up
Internet address is 10.0.0.1/30
Broadcast address is 255.255.255.255
Address determined by setup command
MTU is 1500 bytes
Helper address is not set
Directed broadcast forwarding is disabled
Multicast reserved groups joined: 224.0.0.9
Outgoing access list is not set
Inbound access list is not set
Proxy ARP is enabled
Security level is default
Split horizon is enabled <<<<<<I also did disabling this but still same

Ching01

I think I got it what cisco is saying, I tried to add a loopback address of 10.0.0.0 if split horizon is disable. On rotuer B the table is like this :
Gateway of last resort is not set

10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
R 10.0.0.0/16 [120/1] via 10.0.0.1, 00:00:01, FastEthernet0/0
C 10.0.0.0/30 is directly connected, FastEthernet0/0
B#

However, if splithorizon is enable the routing table look like this:

Gateway of last resort is not set

10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
R 10.0.0.0/16 [120/1] via 10.0.0.1, 00:00:25, FastEthernet0/0
C 10.0.0.0/30 is directly connected, FastEthernet0/0
R 10.0.0.4/30 [120/1] via 10.0.0.1, 00:00:38, FastEthernet0/0

I am just wondering, when I disable and then enable the split horizon
tjh table is showing like this and again not the same output as split horizon is enable:

Gateway of last resort is not set

10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
R 10.0.0.0/16 [120/1] via 10.0.0.1, 00:00:11, FastEthernet0/0
C 10.0.0.0/30 is directly connected, FastEthernet0/0
B#
00:41:33: RIP: sending v2 flash update to 224.0.0.9 via FastEthernet0/0 (10.0.0.2)
00:41:33: RIP: Update contains 1 routes
00:41:33: RIP: Update queued
00:41:33: RIP: Update sent via FastEthernet0/0

EdTheLad

I dont think your output is valid, the 10.0.0.4 update has not been refreshed, the time should be less than 30 sec, your output has 38sec,do a clear ip route * after each test to ensure you are seeing current updates.

Ching01

I did try issuing clear ip ro * , still showing the same output.

EdTheLad

I tryed a setup exactly the same as you and changing the split-horizon has no effect.Since the time on your update for 10.0.0.4 is 38sec it means you have not received an update for that route.The default hello time is 30 sec, after 180 sec that route will enter an invalid state as the summary is surpressing the route.

Ching01

Ed, I tried clearing the forwarding table and as I told you still the same.

mikej412

So what IOS version(s) are/have you tried/trying?

Ching01 -- are you real router or Dynamips?

Ching01

So what IOS version(s) are/have you tried/trying?

Actually,I am using both dynamips and real cisco routers.