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Re: BGP "no sync" posted 02/16/2002
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At 10:40 AM -0800 2/16/02, jonatale@xxxxxxxxxxxxx wrote:
The original intent was that each router have a unique ID, this is how other
vendors do it. Cisco allows the router to have a BGP ID and an OSPF ID, I
consider THIS a bug. But instead of fixing the bug they created another
[undocumented] "rule" (so they can make more money on service/training or maybe
because the fix would have required too much work/risk or [God-knows]).

-$0.02

Well, there are a couple of reasons why Cisco does it the way it does, and these are mostly historical interactions with some actually beneficial features.

Cisco is the only implementation I know of (MAYBE Zebra) that lets you have more than one OSPF process per physical router. I can't think of a reason in the protocol why you couldn't have the same ID in different OSPF domains, but I can picture some designer making that assumption.

The whole synchronization issue came into being at a time when consensus had not formed that it's a bad idea to redistribute full external routes into any IGP. IGP design of the time (John Moy discusses this in his book) was trying to achieve things we now do with logically full mesh iBGP.

The BGP-OSPF interaction RFC (1403) has been designated obsolete, but it's always hard to take something out of your code -- some customer may be using it. No sync did speed up convergence in some non-transit AS, but probably doesn't make all that much difference today.


j killion 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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