Re: BGP weight

Learned lessons and confirmed what I already knew :)

BGP router will advertise to EBGP router only preferred routes.
Weight command changes preferred route, therefore the other less important
route is not advertised.
Preferred route is sent back to a router who advertised it and it s dropped.
(AS-PATH contains our own AS)

Thenks to all for your input.

On 19 November 2010 15:35, garry baker <baker.garry_at_gmail.com> wrote:

> not next-hop ip address but the AS number of 200 being sent back to R2, see
> output and debug below:
>
>
> R1#sh ip bgp neighbors 10.1.12.2 advertised-routes
> BGP table version is 5, local router ID is 10.1.14.1
> 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
> *> 4.0.0.0 10.1.12.2 50000 200 300 400 i
>
> Total number of prefixes 1
>
>
> R2#debug ip bgp updates
> BGP updates debugging is on for address family: IPv4 Unicast
> R2#
> *Mar 1 00:38:27.791: %BGP-5-ADJCHANGE: neighbor 10.1.12.1 Down Peer closed
> the session
> *Mar 1 00:38:38.199: %BGP-5-ADJCHANGE: neighbor 10.1.12.1 Up
> *Mar 1 00:38:38.203: BGP(0): 10.1.12.1 send UPDATE (format) 4.0.0.0/8,
> next 10.1.12.2, metric 0, path 300 400
> *Mar 1 00:38:38.307: BGP(0): updgrp 1 - 10.1.12.1 updates replicated for
> neighbors:
> *Mar 1 00:38:59.443: BGP(0): 10.1.12.1 rcv UPDATE w/ attr: nexthop
> 10.1.12.1, origin i, originator 0.0.0.0, path 100 200 300 400, community ,
> extended community
> *Mar 1 00:38:59.447: BGP(0): 10.1.12.1 rcv UPDATE about 4.0.0.0/8 --
> DENIED due to: AS-PATH contains our own AS;
>
>
>
> --
> Garry L. Baker
>
> "There is no 'patch' for stupidity." - www.sqlsecurity.com
>
>
> On Fri, Nov 19, 2010 at 2:24 PM, garry baker <baker.garry_at_gmail.com>wrote:
>
>> what does the command 'sh ip bgp neighbors 10.1.12.2 advertised-routes'
>> show?
>>
>> wouldnt it be showing that the R2 address as the next-hop now from R1,
>> which would not be a legit route for R2 to put into its bgp table right?
>>
>> i will throw this up in a lab when i get a minute...
>>
>>
>> --
>> Garry L. Baker
>>
>> "There is no 'patch' for stupidity." - www.sqlsecurity.com
>>
>>
>>
>> On Fri, Nov 19, 2010 at 1:30 PM, Jack Router <pan.router_at_gmail.com>wrote:
>>
>>> I have a question about BGP weight and how it affects routing on other
>>> routers. Here is an example:
>>>
>>> AS100 AS200
>>> R1-------------R2
>>> | |
>>> | |
>>> | |
>>> R4-------------R3
>>> AS400 AS300
>>>
>>> Router R4 advertises network 4.0.0.0
>>> Rip runs on all routers and advertises links between routers.
>>>
>>> R2 sees two paths to 4.0.0.0 :
>>>
>>> Network Next Hop Metric LocPrf Weight Path
>>> * 4.0.0.0 10.1.12.1 0 100 400 i
>>> *> 10.1.23.3 0 300 400 i
>>>
>>> Now, on router R1 I define weight of 50000 for all routes coming from
>>> AS200:
>>> # neighbor 10.1.12.2 weight 50000
>>>
>>> R2 sees now only one route to 4.0.0.0:
>>> Network Next Hop Metric LocPrf Weight Path
>>> *> 4.0.0.0 10.1.23.3 0 300 400 i
>>>
>>> R2 removed path via R1 which is good thing because this would create a
>>> loop.
>>> My question is:
>>> How R2 knows not to use R1 anymore ?
>>>
>>>
>>> Blogs and organic groups at http://www.ccie.net
>>>
>>> _______________________________________________________________________
>>> Subscription information may be found at:
>>> http://www.groupstudy.com/list/CCIELab.html

Blogs and organic groups at http://www.ccie.net
Received on Fri Nov 19 2010 - 18:18:09 ART