BGP "no synchronization"

Discussion in 'Cisco' started by Bod43@hotmail.co.uk, Mar 30, 2008.

  1. Guest

    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.

    Thanks.
    , Mar 30, 2008
    #1
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  2. Merv Guest

    On Mar 30, 11:18 am, 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
    Merv, Mar 30, 2008
    #2
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  3. Guest

    On 30 Mar, 19:41, Merv <> wrote:
    > On Mar 30, 11:18 am, 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.
    , Mar 31, 2008
    #3
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