From: dwhitley@xxxxxxxxx
Date: Wed Nov 28 2001 - 12:39:08 GMT-3
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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