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Re: BGP "no sync" posted 02/16/2002
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yup, there it is:
"
   The BGP identifier MUST be the same as the OSPF router id at all
   times that the router is up."

I'd say it is a Cisco non-compiance; but then again, I'd say the king was
naked...

:-)

Brian Dennis wrote:

> John,
> Are you referring to RFC 1403?
>
> http://www.faqs.org/rfcs/rfc1403.html
>
> Brian Dennis, CCIE #2210 (R&S)(ISP/Dial) CCSI #98640
> 5G Networks, Inc.
> brian@xxxxxx
>
> >From RFC 1403
>
> 3.  BGP Identifier and OSPF router ID
>
>    The BGP identifier MUST be the same as the OSPF router id at all
>    times that the router is up.
>
>    This characteristic is required for two reasons.
>
>      i    Synchronisation between OSPF and BGP
>
>           Consider the scenario in which 3 ASBRs, RT1, RT2, and RT3,
>           belong to the same autonomous system.
>
>                                      +-----+
>                                      | RT3 |
>                                      +-----+
>                                         |
>
>                           Autonomous System running OSPF
>
>                                  /               \
>                              +-----+          +-----+
>                              | RT1 |          | RT2 |
>                              +-----+          +-----+
>
>           Both RT1 and RT2 have routes to an external network X and
>           import it into the OSPF routing domain.  RT3 is advertising
>           the route to network X to other external BGP speakers.  RT3
>
>           must use the OSPF router ID to determine whether it is using
>           RT1 or RT2 to forward packets to network X and hence build the
>           correct AS_PATH to advertise to other external speakers.
>
>           More precisely, RT3 must determine which ASBR it is using to
>           reach network X by matching the OSPF router ID for its route
>           to network X with the BGP Identifier of one of the ASBRs, and
>           use the corresponding route for further advertisement to
>           external BGP peers.
>
>      ii   It will be convenient for the network administrator looking at
>           an ASBR to correlate different BGP and OSPF routes based on
>           the identifier.
>
> -----Original Message-----
> From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx]On Behalf Of
> John Neiberger
> Sent: Tuesday, January 15, 2002 7:44 AM
> To: j killion; ccielab@xxxxxxxxxxxxxx
> Subject: Re: RE: BGP "no sync"
>
> It's documented in an RFC, but I can't remember which one.
> This requirement has been obsoleted by a new RFC but Cisco
> hasn't changed the behavior.
>
> Take the following example:
>
> [A]---(ospf)-----[B]----(ospf)----[C]
>  \--------------(ibgp)-------------/
>
> A and C are iBGP peers.  C has an external BGP connection and
> it's redistributing eBGP into OSPF.  It then advertises those
> same routes via iBGP to A.
>
> A learns these prefixes via OSPF from B and via iBGP from C.
> There is your mismatch.  If you were to do a 'show ip bgp
> a.b.c.d' on one of those routes it would show as not
> synchronized even though you'd think it would be.
>
> You can tweak either the OSPF router ID on B or the BGP router
> ID on C.  You have to be careful when you do this but it's one
> way to fix the problem, depending on your topology.
>
> HTH,
> John
>
> ________________________________________________
> Get your own "800" number
> Voicemail, fax, email, and a lot more
> http://www.ureach.com/reg/tag
>
> ---- On Tue, 15 Jan 2002, j killion (jkillion1977@xxxxxxxxx)
> wrote:
>
> > I've heard someone else mention the OSPF/BGP router ID
> > rule.  What does this rule state and is it documented
> > somewhere?  Must the ID's be the same for it to work
> > w/ "no sync"?
> > The same formula is used to calculate the OSPF and BGP
> > ID, so wouldn't they be the same (unless you started
> > one process, added a loopback, then started the
> > other)?
> >
> >
> >
> >
> > --- "Yadav, Arvind K (CAP, GECIS)"
> > <Arvind.Yadav@xxxxxxxxxxxx> wrote:
> > > Pls check , OSPF router ID must match the BGP router
> > > ID
> > >
> > > Arvind
> > >
> > >
> > >  -----Original Message-----
> > > From:       Tony Hanks
> > > [mailto:jhconsulting2001@xxxxxxxxx]
> > > Sent:       Tuesday, January 15, 2002 11:57 AM
> > > To: Bob Sinclair; j killion
> > > Cc: ccielab@xxxxxxxxxxxxxx
> > > Subject:    RE: BGP "no sync"
> > >
> > > Bob,
> > >
> > > I have to disagree.  The route in BGP table is not
> > > flagged as a "best" route
> > > (>).  Hence, BGP will not under any circumstances
> > > inject it into the routing
> > > table.
> > >
> > > J,
> > >
> > > The link might explain why the path isn't marked as
> > > "not sync".
> > >
> > > http://www.cisco.com/warp/public/459/25.shtml
> > >
> > > Tony Hanks
> > > MCSE+I, CCNP
> > > Network Infrastructure Engineer
> > > J & H Consulting Inc.
> > >
> > >
> > >
> > >
> > > -----Original Message-----
> > > From: nobody@xxxxxxxxxxxxxx
> > > [mailto:nobody@xxxxxxxxxxxxxx]On Behalf Of
> > > Bob Sinclair
> > > Sent: Monday, January 14, 2002 6:59 PM
> > > To: j killion
> > > Cc: ccielab@xxxxxxxxxxxxxx
> > > Subject: Re: BGP "no sync"
> > >
> > >
> > > It seems to me you have no problem.  The BGP table
> > > shows 100.1.1.0 to be an
> > > ibgp route  (admin distance 200).  The OSPF route is
> > > in the IP table (FIB)
> > > because it has a lower admin distance (110).  I
> > > think you will find that if
> > > you remove OSPF, the BGP route will take its place,
> > > assuming next hop is
> > > reachable.  Try:  sh ip b 100.1.1.0, and see if all
> > > is well with the route.
> > >
> > > -Bob
> > >
> > > ----- Original Message -----
> > > From: "j killion" <jkillion1977@xxxxxxxxx>
> > > To: <ccielab@xxxxxxxxxxxxxx>
> > > Sent: Monday, January 14, 2002 8:43 PM
> > > Subject: BGP "no sync"
> > >
> > >
> > > > I thought I understood the operation of IBGP and
> > > the
> > > > "no sync" cmd, but now I'm not so sure.  Correct
> > > me if
> > > > I'm wrong, but the two IBGP rules that must be met
> > > in
> > > > order for an IBGP route to become active is 1) The
> > > > router must be able to reach the next hop IP, and
> > > 2)
> > > > There must be a match for the subnet in the IGP
> > > table.
> > > >  The second rule can be circumvented w/ the use of
> > > "no
> > > > sync"....Sound good so far?  How is this
> > > explained?
> > > >
> > > > bart#sh ip bg
> > > > BGP table version is 1, local router ID is
> > > 152.1.11.1
> > > > Status codes: s suppressed, d damped, h history, *
> > > > valid, > best, i - internal
> > > > Origin codes: i - IGP, e - EGP, ? - incomplete
> > > >
> > > >    Network          Next Hop            Metric
> > > LocPrf
> > > > Weight Path
> > > > * i100.1.1.0/24     152.1.1.2                0
> > > 100
> > > >     0 i
> > > > bart#ping 152.1.1.2
> > > > !!!!!
> > > > bart#sh ip rou
> > > >      100.0.0.0/24 is subnetted, 1 subnets
> > > > O 100.1.1.0 [110/65] via 152.1.1.2, 00:05:53,
> > > Serial0
> > > >
> > > > As you can see, the BGP route isn't active yet I
> > > can
> > > > ping the next hop IP *and* I have an IGP route for
> > > > 100.1.1.0/24.  If I add "no sync" the route
> > > becomes
> > > > active.  What am I missing?
> > > >
> > > > Thanks
> > > >
> > > >
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