BGP

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Apr 12th, 2008
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guys just a quick one why we turn on or off summarization in BGP???? can someone please give an exmple.....what r the factors which push to turn off or turn on the sum.....by default its on if i am not wrong.....secondly do we have to use static routes always in BGP???? in which scenario we use static routes and in which we dont...thanks a million in advance

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mibrahim81 Sun, 04/13/2008 - 01:43
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Summarization with network command


1. When Auto Summary is enabled


When auto-summary is enabled, it summarizes the locally originated BGP networks (network x.x.x.x) to their classfull boundaries (auto-summary is enabled by default in BGP) then


1. Any Subnet (component route, specific route) of that classfull network in the local routing table

2. Or the classfull network in routing table


prompts BGP to install the classfull network into the BGP table


So you need at least specific route in the routing table to force router to install the classfull network in BGP table and advertises it to BGP neighbors.


Ex. When you announce the network 172.16.0.0 via network command


router bgp 1

network 172.16.0.0 mask 255.255.0.0

auto-summary


The router will not install this route in BGP Table until it find the exact route Or at least specific route like 172.16.10.0/24 in routing table



2. When Auto Summary is disabled


To install any route in BGP table the

router must has an exact match for this

route in the routing table.


Ex. When you announce the network 172.16.0.0 via network command


router bgp 1

network 172.16.0.0 mask 255.255.0.0

no auto-summary


the router will not install this route in BGP Table until it has the exact route in routing table

to solve this issue we can add static route pointing to null 0


ip route 172.16.0.0 255.255.0.0 null 0






mounir.mohamed Sun, 04/13/2008 - 01:49
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One Comment, Auto-summary not enabled by default.



Auto-summary behavior has changed across Cisco IOS releases. Initially, auto-summary was enabled by default. However, with Cisco bug ID CSCdu81680, this behavior has changed. In the latest Cisco IOS, auto-summary is disabled by default.


Check This.


Router(config-router)#do sh ver | inc imag

System image file is "sup-slot0:/c7200-p-mz.122-25.S14.bin"


Router(config)#router bgp 1

Router(config-router)#do sh run | be router bg

router bgp 1

no synchronization

bgp log-neighbor-changes

no auto-summary

mibrahim81 Sun, 04/13/2008 - 02:49
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Dear Mounir,


Auto-summary behavior has changed across Cisco IOS releases. Initially, auto-summary was enabled by default. In the latest Cisco IOS, auto-summary is disabled by default.


Thanks

mounir.mohamed Sun, 04/13/2008 - 02:57
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What is the benefits of repeating the same words Mentioned above?


You said it's enabled by default and i'm clarifying that it's disabled by default in New IOS version, so it's was small correction.


Check the Bug-ID above if you don't believe that, or even try to lab it up.






Richard Burts Sun, 04/13/2008 - 10:20
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Mounir


At first I thought that it was an unusual post. But then I noticed that it is not just a repetition of what you said. He repeated your correction and then he says "thanks". I take that as an acknowledgement that he understands and accepts your correction.


HTH


Rick

mounir.mohamed Sun, 04/13/2008 - 15:12
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I thought that also, so i confirm the Bug-ID and the above example to make it clear, but no problem at all.


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