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RE: iBGP based network. [9:2300] posted 12/10/2002
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Hi Alok,

See inline...

[..snip..]
> 
> okie no multihop, i agreee........that doesnt matter to me 
> right..no IGP either...

Well, it matters in the sense that without multihop, you have no resilience
at all.

[..snip..]

> ...hmm so full mesh is a nice problem isnt it :o).......i 
> wont next-hop
> self, ill next-hop adjacent neighbour..but i still need a full mesh

Yep, you need a full mesh or a really messy RR setup with every RR setting
next-hop to the outbound interface to all neighbours.

> > Why would you want to do this, what benefit does it bring?
> 
> just.. :o) ....wondering wht "iBGP not bouncing to another 
> iBGP" ever gives
> me actually....was it made with the intention of allowing 
> "smaller routing
> tables"......or it has a reason...u still have a "source" 
> field in BGP to
> recognise the source of the update dont u? so that "RIP equivalent" of
> "split horizon" cud be knocked off with that....

The reason a route learned via iBGP isn't announced to other iBGP neighbours
is to prevent looping.  In eBGP, you have the AS path to tell you when
there's a loop (i.e. if you see your own AS number, there's a loop).  Since
every router in iBGP is in your AS, you can't use the AS path as a loop
detection mechanism.  Therefore, you have to prevent loops by mandating that
you can't announce routes learned via iBGP to another iBGP neighbour.  The
obvious exception to this rule is if you're a route reflector.  Then, you
have to introduce cluster ids and originators to solve the problem of
induced loops.

> > I'm not even going to bother with the question regarding duplicate
> addresses
> > in the network!  Suffice to say that you've got a single 
> subnet appearing
> on
> > two interfaces on router y.  Which one do you choose to send the BGP
> packet?
> > What have you been smoking? ;-)
> 
> there was more than that i missed... i had the same IP 
> appearing on one side
> of the x router  and the same was on its other interface....
> 
> that was the bad bit.......sorry abt that.
> 
> i realised that immediately after i sent the mail and the "pot" wore
> off....that ..its A-x-w-y-z-B ;o)
> 
> now can i have the same addresses on the interfaces between 
> x-w and y-z
> ...yup not with all those issues u mentioned.....
> :o)

Yes, you could have the same address on the two links you mention provided
you're setting next-hop self along the path and provided that w and y are
route reflectors and are setting next-hop for all routes announced to each
other to be the outbound interface towards each other.  If w and y are not
RRs, then they will not announce anything to each other.

> actually here its linux-1 , linux 2, linux 3 , linux 4 linux 5 linux 6
> ...that i use... and i just relaised i dropped one linux when 
> i sent that
> mail....
> 
> anything else that fails?

When trying to troubleshoot in a network like this, your sanity ;-)  The
complexity introduced here, for the sake of not running an IGP, is IMHO
unsustainable in real life.

Regards,

Guy

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