I posted the following to the CCIE list and received no comment whatsoever.
apparently if a topic isn't directly related to any of the practice labs, or
is not perceived as being necessary to pass the Lab, no one is interested.
since I know the folks on this list like to learn things, I thought I'd post
here
-----Original Message-----
From: Chuck Larrieu [mailto:[EMAIL PROTECTED]]
Sent: Monday, July 23, 2001 10:10 PM
To: CCIE_Lab Groupstudy List
Subject: IP Summary-address RIP
ran into this one while digging around CCO
IP summary-address rip network mask
this command is new to IOS 12.1
it is an interface command, and ip split-horizon must be disabled for it to
work.
http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121cgcr/ip_c
/ipcprt2/1cdrip.htm#xtocid1773719
watch the word wrap
>From CCO:
Example 1: Correct Configuration
The following example shows how the ip summary-address rip command works
with autosummary addressing in RIP. In the example, the major network is
10.0.0.0. The summary address 10.2.0.0 overrides the autosummary address of
10.0.0.0, so that 10.2.0.0 is advertised out interface e1 and 10.0.0.0 is
not advertised.
----------------------------------------------------------------------------
----
Note If split horizon is enabled, neither autosummary nor interface
summary addresses (those configured with the ip summary-address rip command)
are advertised.
----------------------------------------------------------------------------
----
router rip
router int e1
ip address 10.1.1.1 255.255.255.0
(config-if)# ip summary-address rip 10.2.0.0 255.255.0.0
no ip split-horizon
router rip
network 10.0.0.0
[end of CCO quote]
finally got this one to work. the doc CD omits the requirement that the
sender must be a RIPv2 speaker. which makes sense, if you stop and think
about it. sample configs below my signature.
I got to wondering about the no ip split-horizon requirement on the
interface where the summary takes place. if the RIP process receives the
summarization information from an interface, then I guess it considers
advertising that summary back out that address as a violation of split
horizon. make sense? I can't think of any other reason for the requirement.
leads to some interesting possibilities. for example, suppose you have some
kind of RLAN private network, with lots of home user DSL lines aggregated
into an ATM DS3 at the core. you want to conserve address space by
allocating /29's to the home office users, but your RIP network requires a
mask of /24.
don't laugh, guys, I have a customer with exactly this situation :->
you can advertise a /24 summary into your rip network. Cisco routers by
default listen to RIPv2 advertisements, even if they are RIPv1 routers. So
the RIPv2 advertisements will be accepted and placed into the routing table.
I don't know if this is true with other vendor's products.
there are obviously some pitfalls. with split horizon disabled, one must be
aware of the implications. I have not yet tried various flavors of route
maps or distribute lists to limit what goes out. also, one must take care
that the RIPv1 routers are able to readvertise the summarized route.
Assuming, as in the case I have mentioned, the RIPv1 router has only /24's
on its attached interfaces, this should be OK. just something to be aware
of.
in terms of CCIE lab applicability, I suppose this can be one more tool in
your arsenal. but only if your pod features 12.1 or better.
Chuck
interface Loopback0
ip address 10.1.1.1 255.255.255.248
!
interface Loopback1
ip address 10.1.1.9 255.255.255.248
!
interface Loopback2
ip address 10.1.1.17 255.255.255.248
!
interface Loopback3
ip address 10.1.1.25 255.255.255.248
!
interface Loopback4
ip address 10.1.1.33 255.255.255.248
!
interface Ethernet0
ip address 190.190.99.3 255.255.255.0
no ip split-horizon
ip summary-address rip 10.1.1.0 255.255.255.0
interface Serial1
ip address 190.190.23.3 255.255.255.0
no ip split-horizon
ip summary-address rip 10.1.0.0 255.255.0.0
clockrate 2000000
!
router rip
version 2
passive-interface Loopback0
passive-interface Loopback1
passive-interface Loopback2
passive-interface Loopback3
passive-interface Loopback4
network 10.0.0.0
network 190.190.0.0
no auto-summary
another router:
I 171.171.0.0/16 [100/9076] via 190.190.23.3, 00:01:10, Serial1
S 100.0.0.0/8 is directly connected, Serial1
190.190.0.0/24 is subnetted, 2 subnets
C 190.190.23.0 is directly connected, Serial1
I 190.190.99.0 [100/8576] via 190.190.23.3, 00:01:10, Serial1
10.0.0.0/16 is subnetted, 1 subnets
R 10.1.0.0 [120/1] via 190.190.23.3, 00:00:03, Serial1
note the summarized route 10.1.0.0/16 ( from serial 1 above )
another router:
C 171.171.0.0/16 is directly connected, Loopback0
190.190.0.0/24 is subnetted, 2 subnets
I 190.190.23.0 [100/8576] via 190.190.99.3, 00:01:09, Ethernet0
C 190.190.99.0 is directly connected, Ethernet0
10.0.0.0/24 is subnetted, 1 subnets
R 10.1.1.0 [120/1] via 190.190.99.3, 00:00:22, Ethernet0
note the summarized route as a /24 ( from the ethernet interface above )
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=13699&t=13699
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