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Posted by on March 31, 2008, 1:51 am
If you were Registered and logged in, you could reply and use other advanced thread options > On Mar 30, 11:18 am, Bo...@hotmail.co.uk wrote:
>
>
>
>
>
> > Hi,
>
> > I wonder if anyone can assist with a BGP
> > understanding issue?
>
> > I have a lab setup with BGP and it seems that
> > I do not understand "bgp synchronisation".
>
> > I have 2 AS border routers and a single internal
> > router with IBGP fully meshed across the three.
> > There is also another AS clearly but that does
> > seem to be related to the issue.
>
> > The internal router has routes to the bgp next hop
> > however the routes do not get put in the table
> > unless I turn off synchronisation.
>
> > WITH =A0 NO SYNCHRONIZATION
> > The internal router has
>
> > router bgp 1
> > =A0no synchronization
> > =A0bgp log-neighbor-changes
> > =A0neighbor 1.1.1.17 remote-as 1
> > =A0neighbor 1.1.1.21 remote-as 1
> > =A0no auto-summary
>
> > RH#sh ip rou
> > =A0 =A0 =A01.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
> > D =A0 =A0 =A0 1.1.1.0/30 [90/2816] via 1.1.1.17, 00:53:16, FastEthernet2=
/0
> > D =A0 =A0 =A0 1.1.1.0/24 [90/2560] via 1.1.1.21, 00:53:18, FastEthernet3=
/0
> > D =A0 =A0 =A0 1.1.1.4/30 [90/2816] via 1.1.1.21, 00:53:15, FastEthernet3=
/0
> > D =A0 =A0 =A0 1.1.1.12/30 [90/5120] via 1.1.1.21, 00:53:18, FastEthernet=
3/0
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 [90/5120] via 1.1.1.17, 00:53:18=
, FastEthernet2/0
> > C =A0 =A0 =A0 1.1.1.16/30 is directly connected, FastEthernet2/0
> > C =A0 =A0 =A0 1.1.1.20/30 is directly connected, FastEthernet3/0
> > =A0 =A0 =A02.0.0.0/24 is subnetted, 2 subnets
>
> > !! =A0IBGP ROUTES =A0ideal
> > B =A0 =A0 =A0 2.2.2.0 [200/0] via 1.1.1.1, 00:03:59
> > B =A0 =A0 =A0 2.2.3.0 [200/0] via 1.1.1.5, 00:02:31
>
> > RH#sh ip bgp
>
> > =A0 =A0Network =A0 =A0 =A0 =A0 =A0Next Hop =A0 =A0 =A0 =A0 =A0 =A0Metric=
LocPrf Weight Path
> > r i1.1.1.0/24 =A0 =A0 =A0 1.1.1.21 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 0 =A0=
=A0100 =A0 =A0 =A00 i
> > r>i =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 1.1.1.17 =A0 =A0 =A0 =A0 =A0 =A0 =A0=
=A0 0 =A0 =A0100 =A0 =A0 =A00 i
> > * i2.2.2.0/24 =A0 =A0 =A0 1.1.1.5 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A00 =
=A0 =A0100 =A0 =A0 =A00 2 i
> > *>i =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 1.1.1.1 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =
=A0 =A00 =A0 =A0100 =A0 =A0 =A00 2 i
> > *>i2.2.3.0/24 =A0 =A0 =A0 1.1.1.5 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A00 =
=A0 =A0200 =A0 =A0 =A00 2 i
> > * i =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 1.1.1.1 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =
=A0 =A00 =A0 =A0100 =A0 =A0 =A00 2 i
> > RH#
>
> > All OK.
>
> > Now turn on synchronisation.
>
> > RH(config)#router bgp 1
> > RH(config-router)#synch
>
> > RH#clear ip bgp *
>
> > We really have:
>
> > router bgp 1
> > =A0synchronization
>
> > RH#sh ip bgp
>
> > =A0 =A0Network =A0 =A0 =A0 =A0 =A0Next Hop =A0 =A0 =A0 =A0 =A0 =A0Metric=
LocPrf Weight Path
> > r>i1.1.1.0/24 =A0 =A0 =A0 1.1.1.17 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 0 =A0=
=A0100 =A0 =A0 =A00 i
> > r i =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 1.1.1.21 =A0 =A0 =A0 =A0 =A0 =A0 =A0=
=A0 0 =A0 =A0100 =A0 =A0 =A00 i
> > * i2.2.2.0/24 =A0 =A0 =A0 1.1.1.1 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A00 =
=A0 =A0100 =A0 =A0 =A00 2 i
> > * i =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 1.1.1.5 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =
=A0 =A00 =A0 =A0100 =A0 =A0 =A00 2 i
> > * i2.2.3.0/24 =A0 =A0 =A0 1.1.1.1 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A00 =
=A0 =A0100 =A0 =A0 =A00 2 i
> > * i =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 1.1.1.5 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =
=A0 =A00 =A0 =A0200 =A0 =A0 =A00 2 i
> > RH#
>
> > RH#sh ip route
>
> > =A0 =A0 =A01.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
> > D =A0 =A0 =A0 1.1.1.0/30 [90/2816] via 1.1.1.17, 00:59:39, FastEthernet2=
/0
> > D =A0 =A0 =A0 1.1.1.0/24 [90/2560] via 1.1.1.21, 00:59:41, FastEthernet3=
/0
> > D =A0 =A0 =A0 1.1.1.4/30 [90/2816] via 1.1.1.21, 00:59:38, FastEthernet3=
/0
> > D =A0 =A0 =A0 1.1.1.12/30 [90/5120] via 1.1.1.21, 00:59:41, FastEthernet=
3/0
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 [90/5120] via 1.1.1.17, 00:59:41=
, FastEthernet2/0
> > C =A0 =A0 =A0 1.1.1.16/30 is directly connected, FastEthernet2/0
> > C =A0 =A0 =A0 1.1.1.20/30 is directly connected, FastEthernet3/0
> > RH#
>
> > Since I have EIGRP routes to the BGP next hop
> > i.e. 1.1.1.1 =A0 - =A0 1.1.1.0/30 [90/2816] via 1.1.1.17
> > =A0 =A0 =A01.1.1.5 =A0 - =A0 1.1.1.4/30 [90/2816] via 1.1.1.21
>
> > I would have though that I was Synchronised and would not
> > need NO SYCHRONIZATION in the BGP config??
>
> > Clearly I am mistaken.
>
> > Anyone care to explian?
>
> > BSCI tomorrow - don't suppose I will need this but
> > I have gone this far and would like to understand it
> > while the iron is hot as it were.
>
> It is NOT =A0the next hop that has to be learned via the IGP, it is the
> BGP route/prefix ITSELF that must be learned via the IGP.
>
> So in your example 2.2.2.0/24 =A0and 2.2.3.0/24 =A0 =A0would have to be
> learned via EIGRP before an IBGP router should install the route
>
> And to test BGP should not be runninging on the Internal router just
> the IGP.
>
> So remove BGP from you internal route advertise 2.2.2.0/24 on one of
> the external routers and see if your other external router is then
> willing to install with
>
> The BGP synronization feature was meant to be used for the scenario
> where you are providing transit but not all routers in the path are
> running BGP. in other words the traffic would be dropped at the
> internal router not running BGP because obviously =A0there is no BGP
> route and there is no IGP route EITHER.
>
> This feature is never used in an ISP environment as far as I know. On
> IOS XR for CRS, no sync is the default unlike IOS where sync is still
> the default- Hide quoted text -
>
> - Show quoted text -
Thanks, not digested it all yet but looks just the business.
Not yet got Doyle Vol 2 - That will be the problem.
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