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BGP "no synchronization"

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.

Reply to
Bod43
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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

Reply to
Merv

, FastEthernet2/0

LocPrf Weight Path

=A0100 =A0 =A0 =A00 i

=A0 0 =A0 =A0100 =A0 =A0 =A00 i

=A0 =A0100 =A0 =A0 =A00 2 i

=A0 =A00 =A0 =A0100 =A0 =A0 =A00 2 i

=A0 =A0200 =A0 =A0 =A00 2 i

=A0 =A00 =A0 =A0100 =A0 =A0 =A00 2 i

LocPrf Weight Path

=A0100 =A0 =A0 =A00 i

=A0 0 =A0 =A0100 =A0 =A0 =A00 i

=A0 =A0100 =A0 =A0 =A00 2 i

=A0 =A00 =A0 =A0100 =A0 =A0 =A00 2 i

=A0 =A0100 =A0 =A0 =A00 2 i

=A0 =A00 =A0 =A0200 =A0 =A0 =A00 2 i

, FastEthernet2/0

Thanks, not digested it all yet but looks just the business. Not yet got Doyle Vol 2 - That will be the problem.

Reply to
Bod43

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