Re: Simple Doubt regarding BGP

Hi Ryan,

I suppose that summarizes the initial link?

===================================================================================================================================
Q. How does BGP behave differently with auto-summary enabled or disabled?

    A. Auto-summary behavior has changed across Cisco IOS software releases.
Initially, auto-summary was enabled by default. However, with Cisco bug ID
CSCdu81680 ( registered customers only) this behavior has changed. In the
latest Cisco IOS, auto-summary is disabled by default. When auto-summary is
enabled, it summarizes the locally originated BGP networks to their
classfull boundaries. Auto-summary is enabled by default in BGP. When
auto-summary is disabled, the routes introduced locally into the BGP table
are not summarized to their classfull boundaries. When a subnet exists in
the routing table and these three conditions are satisfied, then any subnet
of that classfull network in the local routing table will prompt BGP to
install the classfull network into the BGP table.

        * Classfull network statement for a network in the routing
table
        * Classfull mask on that network statement
        * Auto-summary enabled

    For example, if the subnet in the routing table is 75.75.75.0 mask
255.255.255.0, and you configure network 75.0.0.0 under the router bgp
command, and auto-summary is enabled, BGP introduces the classfull network
75.0.0.0 mask 255.0.0.0 in the BGP table.

    If these three conditions are not all met, then BGP does not install any
entry in the BGP table unless there is an exact match in the local routing
table.

    Note: If the AS that performs BGP does not own the complete classfull
network, Cisco recommends that you issue the no auto-summary command under
router bgp in order to disable auto-summary.
===================================================================================================================================

--------------------------------------------------
From: "Ryan West" <rwest_at_zyedge.com>
Sent: Tuesday, April 28, 2009 3:31 AM
To: "nAyYAR" <nyrhh_at_hotmail.co.uk>; "Raghav Bhargava"
<raghavbhargava12_at_gmail.com>
Cc: <ccielab_at_groupstudy.com>
Subject: RE: Simple Doubt regarding BGP

> Nayyar,
>
> The classful boundary with auto-summary enabled allows you to enter
> network 2.0.0.0 to pull in a /24 that falls within that class boundary.
> The same rule of having a matching existing route in your RIB applies here
> too:
>
> Rack1R1(config-router)#auto-summary
> Rack1R1(config-router)#net 2.2.2.2
> Rack1R1(config-router)#do s deb
> IP routing:
> BGP updates debugging is on for address family: IPv4 Unicast
>
>
>
> Rack1R1(config-router)#net
> *Apr 28 02:22:11.389: BGP: Import walker start version 0, end version 1
> *Apr 28 02:22:11.389: BGP: ... start import cfg version = 0
> Rack1R1(config-router)#net 3.3.3.3
> Rack1R1(config-router)#do s ip b
>
> Rack1R1(config-router)#net 2.0.0.0
> Rack1R1(config-router)#
> *Apr 28 02:22:59.701: BGP(0): route 2.0.0.0/8 up
> *Apr 28 02:22:59.701: BGP(0): nettable_walker 2.0.0.0/8 route sourced
> locally
> Rack1R1(config-router)#net 3.3.3.3 mask 255.255.255.255
> Rack1R1(config-router)#
> *Apr 28 02:23:30.009: BGP(0): route 3.3.3.3/8 up
> *Apr 28 02:23:30.009: BGP(0): nettable_walker 3.0.0.0/8 route sourced
> locally
> *Apr 28 02:23:30.009: BGP(0): nettable_walker 3.3.3.3/32 route sourced
> locally
>
> And below is the added /24 in a class a boundary:
>
> Rack1R1(config-router)#in lo 3
> Rack1R1(config-if)#ip add
> *Apr 28 02:26:33.789: %LINEPROTO-5-UPDOWN: Line protocol on Interface
> Loopback3, changed state to up
> Rack1R1(config-if)#ip add 4.4.4.4 255.255.255.0
> Rack1R1(config-if)#exit
> Rack1R1(config)#rb 100
> Rack1R1(config-router)#net 4.0.0.0
> Rack1R1(config-router)#
> *Apr 28 02:26:55.761: BGP(0): route 4.0.0.0/8 up
> *Apr 28 02:26:55.761: BGP(0): nettable_walker 4.0.0.0/8 route sourced
> locally
>
> Rack1R1(config-router)#do s ip b
> BGP table version is 5, local router ID is 150.1.1.1
> Status codes: s suppressed, d damped, h history, * valid, > best, i -
> internal,
> r RIB-failure, S Stale
> Origin codes: i - IGP, e - EGP, ? - incomplete
>
> Network Next Hop Metric LocPrf Weight Path
> *> 2.0.0.0 0.0.0.0 0 32768 i
> *> 3.0.0.0 0.0.0.0 0 32768 i
> *> 3.3.3.3/32 0.0.0.0 0 32768 i
> *> 4.0.0.0 0.0.0.0 0 32768 i
>
> -ryan
>
> -----Original Message-----
> From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of
> nAyYAR
> Sent: Monday, April 27, 2009 10:12 PM
> To: Raghav Bhargava
> Cc: ccielab_at_groupstudy.com
> Subject: Re: Simple Doubt regarding BGP
>
> Hi Raghav,
>
> I believe the behavior is related to BGP auto-summary ... have a look at
> this link it might help.
>
> http://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a00800949e8.shtml#five
>
>
> --------------------------------------------------
> From: "Raghav Bhargava" <raghavbhargava12_at_gmail.com>
> Sent: Tuesday, April 28, 2009 2:55 AM
> To: "Cisco certification" <ccielab_at_groupstudy.com>
> Subject: Simple Doubt regarding BGP
>
>> Hi Experts,
>>
>> Very basic question
>>
>> Suppose I have this ip address configured on my loopback Interface of R1
>>
>> 2.2.2.2/8
>>
>> Now under my BGP Process if i advertise this network as
>>
>>
>> network 2.2.2.2
>>
>> OR
>>
>> network 2.0.0.0
>>
>> Now in first case I don't see this network in the BGP table of the other
>> router but
>> if I follow the second case the network is seen.
>>
>> Can anyone please explain why is it doing like this??
>>
>> --
>> Warm Regards
>> Raghav
>>
>>
>> Blogs and organic groups at http://www.ccie.net
>>
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>
>
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Received on Tue Apr 28 2009 - 04:03:33 ART