On 30 Mar, 19:41, Merv <merv.hr...@rogers.com> wrote:
> 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 * NO SYNCHRONIZATION
> > The internal router has
>
> > router bgp 1
> > *no synchronization
> > *bgp log-neighbor-changes
> > *neighbor 1.1.1.17 remote-as 1
> > *neighbor 1.1.1.21 remote-as 1
> > *no auto-summary
>
> > RH#sh ip rou
> > * * *1.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
> > D * * * 1.1.1.0/30 [90/2816] via 1.1.1.17, 00:53:16, FastEthernet2/0
> > D * * * 1.1.1.0/24 [90/2560] via 1.1.1.21, 00:53:18, FastEthernet3/0
> > D * * * 1.1.1.4/30 [90/2816] via 1.1.1.21, 00:53:15, FastEthernet3/0
> > D * * * 1.1.1.12/30 [90/5120] via 1.1.1.21, 00:53:18, FastEthernet3/0
> > * * * * * * * * * * [90/5120] via 1.1.1.17, 00:53:18, FastEthernet2/0
> > C * * * 1.1.1.16/30 is directly connected, FastEthernet2/0
> > C * * * 1.1.1.20/30 is directly connected, FastEthernet3/0
> > * * *2.0.0.0/24 is subnetted, 2 subnets
>
> > !! *IBGP ROUTES *ideal
> > B * * * 2.2.2.0 [200/0] via 1.1.1.1, 00:03:59
> > B * * * 2.2.3.0 [200/0] via 1.1.1.5, 00:02:31
>
> > RH#sh ip bgp
>
> > * *Network * * * * *Next Hop * * * * * *Metric LocPrf Weight Path
> > r i1.1.1.0/24 * * * 1.1.1.21 * * * * * * * * 0 * *100 * * *0 i
> > r>i * * * * * * * * 1.1.1.17 * * * * * * * * 0 * *100 * * *0 i
> > * i2.2.2.0/24 * * * 1.1.1.5 * * * * * * * * *0 * *100 * * *0 2 i
> > *>i * * * * * * * * 1.1.1.1 * * * * * * * * *0 * *100 * * *0 2 i
> > *>i2.2.3.0/24 * * * 1.1.1.5 * * * * * * * * *0 * *200 * * *0 2 i
> > * i * * * * * * * * 1.1.1.1 * * * * * * * * *0 * *100 * * *0 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
> > *synchronization
>
> > RH#sh ip bgp
>
> > * *Network * * * * *Next Hop * * * * * *Metric LocPrf Weight Path
> > r>i1.1.1.0/24 * * * 1.1.1.17 * * * * * * * * 0 * *100 * * *0 i
> > r i * * * * * * * * 1.1.1.21 * * * * * * * * 0 * *100 * * *0 i
> > * i2.2.2.0/24 * * * 1.1.1.1 * * * * * * * * *0 * *100 * * *0 2 i
> > * i * * * * * * * * 1.1.1.5 * * * * * * * * *0 * *100 * * *0 2 i
> > * i2.2.3.0/24 * * * 1.1.1.1 * * * * * * * * *0 * *100 * * *0 2 i
> > * i * * * * * * * * 1.1.1.5 * * * * * * * * *0 * *200 * * *0 2 i
> > RH#
>
> > RH#sh ip route
>
> > * * *1.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
> > D * * * 1.1.1.0/30 [90/2816] via 1.1.1.17, 00:59:39, FastEthernet2/0
> > D * * * 1.1.1.0/24 [90/2560] via 1.1.1.21, 00:59:41, FastEthernet3/0
> > D * * * 1.1.1.4/30 [90/2816] via 1.1.1.21, 00:59:38, FastEthernet3/0
> > D * * * 1.1.1.12/30 [90/5120] via 1.1.1.21, 00:59:41, FastEthernet3/0
> > * * * * * * * * * * [90/5120] via 1.1.1.17, 00:59:41, FastEthernet2/0
> > C * * * 1.1.1.16/30 is directly connected, FastEthernet2/0
> > C * * * 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 * - * 1.1.1.0/30 [90/2816] via 1.1.1.17
> > * * *1.1.1.5 * - * 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 *the 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 *and 2.2.3.0/24 * *would 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 *there 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.
|
Similar Threads
