From: fwells12 (fwells12@xxxxxxxxxxx)
Date: Wed Nov 28 2001 - 20:21:00 GMT-3
Thanks for the reply. This was well worth investigating. Unfortunately it
was not my problem.
----- Original Message -----
From: <dwhitley@dynis.com>
To: <fwells12@hotmail.com>; <ccielab@groupstudy.com>
Sent: Wednesday, November 28, 2001 7:39 AM
Subject: RE: Has anyone come across this problem before?
> BGP is not supposed to enter a route into the BGP table unless it has a
> route in it's IGP table first right?
> Yes, and No
> If you are trying to use the network command under router bgp, then yes.
> Now if a BGP route is recieved from a neighbor and you have no valid route
> to it, then No. You will have a BGP table entry with no corresponding
> routing table entry. That's why turning off sync fixes your problem.
>
> Here is a document link that might help, and an excerpt from the same.
You
> listed OSPF as one of your IGP's and this is a little gotcha that has
gotten
> me before.
>
> Paths marked as "not synchronized" in the show ip bgp <longer-prefixes>
> output. If BGP synchronization is enabled, which it is by default in Cisco
> IOS. Software, there must be a match for the prefix in the IP routing
table
> in order for an internal (iBGP) path to be considered a valid path. If the
> matching route is learned from an OSPF neighbor, its OSPF router ID must
> match the BGP router ID of the iBGP neighbor. Most users prefer to disable
> synchronization using the no synchronization BGP subcommand.
>
> http://www.cisco.com/warp/public/459/25.shtml
>
> Good Luck
> Dean Whitley
>
>
>
>
> -----Original Message-----
> From: fwells12 [mailto:fwells12@hotmail.com]
> Sent: Wednesday, November 28, 2001 2:28 AM
> To: Nigel Taylor; ccielab@groupstudy.com
> Subject: Re: Has anyone come across this problem before?
>
>
> Thanks for your reply Nigel. Unfortunately my topology is too complicated
> to pass along.
>
> In regards to you comments I want to point out that the router in question
> has the loopback network in it's igp route-table prior to the network
> statement being added to BGP on the router that has the loopback. Once
that
> is done, the other router loses it's igp route-table route. Now, that is
> all well and good because it now learns the route via BGP which has a
lower
> AD -but the ip forwarding table does not get a replacement BGP route for
> that network! The odd part for me is however that the BGP route IS
injected
> into the BGP table!!
>
> 'No sync' does remedy the problem but I want to understand why it is
> behaving like it is...
>
>
> ----- Original Message -----
> From: "Nigel Taylor" <nigel_taylor@hotmail.com>
> To: <ccielab@groupstudy.com>; "fwells12" <fwells12@hotmail.com>
> Sent: Tuesday, November 27, 2001 10:16 PM
> Subject: Re: Has anyone come across this problem before?
>
>
> > fwells12,
> > I'm a bit lost as to the layout of your sample topology?
> > Remember that if the route is being advertised through the IGP(eigrp,
> ospf,
> > etc..) then let's say you advertise that route into bgp(using the
network
> > command) then it would depend on how the router in question is
> > learning/receiving the BGP routes for the network(loopback) in question.
> > If the router is receiving a eBGP route then what you see is correct in
> that
> > eBGP AD is 20, which would be better that the IGP metric from EIGRP or
> OSPF.
> > This eBGP route would replace the existing route.
> >
> > Also, that eBGP route using most-likely the directly connected interface
> > would then have a valid route to that network and in effect add the
route
> to
> > both the BGP table and the RIB.
> >
> > >BGP is not supposed to enter a route into the BGP table unless it has a
> > route
> > > in it's IGP table first right?
> >
> > This is correct.
> >
> > The important thing to remember here is there are inbound and outbound
> rules
> > that apply here as it pertains to eBGP routes versus iBGP routes,
entering
> > an ext AS's or moving within an existing AS.
> >
> > But remember once you disable "synchronization" you are allowing bgp to
> add
> > a route(s) from the BGP table to the RIB, by telling BGP to overlook the
> > requirement of having a route/path in the RIB.
> >
> > Post the config and route table in question..
> >
> > - Nigel
> >
> > ----- Original Message -----
> > From: "fwells12" <fwells12@hotmail.com>
> > To: <ccielab@groupstudy.com>
> > Sent: Wednesday, November 28, 2001 12:02 AM
> > Subject: Has anyone come across this problem before?
> >
> >
> > > I have a route in my igp that disappears when I advertise that route
> from
> > BGP.
> > > The network is on a loopback interface being redistributed from EIGRP
to
> > OSPF.
> > > Before I add it to BGP, all my other routers (regardless of IGP) can
see
> > it
> > > just fine. Once I add it to BGP via a network statement all my BGP
> > speakers
> > > can see the route in their BGP tables (including the router in
question)
> > and
> > > in all their IP forwarding tables as BGP routes -except the router in
> > > question. The odd thing is, the router that does not have it in its
IP
> > > forwarding table any more, does have it in its BGP table! -how does
that
> > work?
> > > BGP is not supposed to enter a route into the BGP table unless it has
a
> > route
> > > in it's IGP table first right?
> > >
> > > Sync is enabled
> > > IOS is 11.3 IP/IPX/AT/DEC and has been used extensively without
previous
> > > problems.
> > >
> > > Thoughts please...
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