From: Andrew Bratchell (a.bratchell@caeuk.com)
Date: Sun Oct 27 2002 - 14:49:25 GMT-3
Liozos,
If you have network prefix 10.1.1.0/24 advertised by R1 to R2 via OSPF.
R1 has a loopback IP address of 100.100.100.1 and that is being used as the router-id for OSPF at R1. R2 has a loopback IP address of 200.200.200.1 that is also the router-id for OSPF on that router.
R2 is also an IBGP router that happens to be a route reflector client. Because we have not configured a BGP router-ID at R2, R2 will also use this IP address as the BGP router-id.
R2 redistributes the OSPF route 10.1.1.0/24 into BGP.
R3 is the route reflector and is also running OSPF.
When you look at the BGP table of R3, you will see the BGP route 10.1.1.0/24 but it will be out of sync. Therefore R3 will NOT advertise that route to other route reflector clients or outside its AS.
The problem is caused because the IGP routing protocol i.e. OSPF for network prefix 10.1.1.0/24 when you look at it using the command show ip route 10.1.1.0 is tagged with the OSPF router-id of R1.
BUT when R2 redistributed that OSPF route into BGP, it was marked with the BGP router-id of 200.200.200.1
It is because of this that they are not in sync.
You can confirm this in this example by executing the command show ip bgp 10.1.1.0 at R3.
To fix this, you can :-
1) Turn off sync (what most people do)
2) Use confederations
3) Use another IGP like EIGRP (probably not practical)
4) Tweak the router-id so that they make at the appropriate places.
Using solution 4, the answer could be :-
1st set the OSPF router-id to 66.66.66.1 at R1.
2nd set the BGP router-id to 66.66.66.1 at R2.
In this scenario, setting the OSPF and BGP router-ids to the same value at R2 will not work.
Hope this makes sense.
If you want my notes for this issue with diagrams, routing table output etc, let me know and I will email it to you (word 2k format)
Thanks
Andy
-----Original Message-----
From: LoizosCisco [mailto:david_steven2001@yahoo.com]
Sent: 27 October 2002 17:24
To: Andrew Bratchell; Richard Davidson; groupstudy
Subject: RE: BGP sync
Andrew,
What if I do not use router-id for ospf and bgp
router-id? Both should shoose the highest loopback ip
and both will have the same router ID and that should
be good right? But if I assign router-id for ospf and
not for bgp then that should be a problem right? Is it
a good idea to assign the same id for both ospf and
bgp rather than letting them use the highest IP ???
Thanks
Loizos
--- Andrew Bratchell <a.bratchell@caeuk.com> wrote:
> Richard,
> Without knowing your exact topology, it is difficult
> to give an accurate answer BUT -
> when using OSPF as the underlying IGP you can have
> BGP sync issues.
> The reason is that the OSPF Router-ID for the IGP
> route must be the same as the BGP router-ID.
> If they are different they are considered out of
> sync.
> This is a common problem when using Route reflectors
> for example.
>
> Look at the following doc
> http://www.cisco.com/warp/public/459/25.shtml
> If you do a search on the archieves, you will find
> this a common issue that many people have come
> across.
>
> You have the following options to fix this problem
> (based upon you using Route reflectors for example)
>
> 1) Turn off BGP sync
> 2) Use confederations
> 3) Change the OSPF router-ID and BGP router-id's on
> the appropriate routers so that they match.
> 4) Use another IGP instead, eg EIGRP
>
> I have reseached and documented this issue. If you
> would like an example pls email me directly and I
> will forward the word doc I have created.
>
> Thanks
> Andy
>
> -----Original Message-----
> From: Richard Davidson [mailto:rich@myhomemail.net]
> Sent: 27 October 2002 06:00
> To: groupstudy
> Subject: BGP sync
>
>
> This is the rull of bgp sync as I know it. If the
> IGP
> knows the network that BGP has than bgp will be in
> sync. In the show ip route and sho ip bgp I have a
> route to each of the bgp networks but they are not
> all
> in sync. Am I missing something. Shouldn't they
> (bgp
> routes) all be in sync.
>
> Rich
>
> r2#sho ip bgp
> BGP table version is 3162, local router ID is
> 223.255.2.1
> 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
> *>i137.20.10.0/24 137.20.64.5 20
> 100
> 0 ?
> *>i137.20.20.0/24 137.20.25.2 84
> 100
> 0 ?
> * i137.20.25.0/24 137.20.64.5 11
> 100
> 0 ?
> * i137.20.33.0/26 137.20.25.2 75
> 100
> 0 ?
> * i137.20.48.0/20 137.20.25.2 12
> 100
> 0 ?
> * i137.20.64.0/20 137.20.25.2 11
> 100
> 0 ?
> * i137.20.100.34/32 137.20.25.2 10
> 100
> 0 ?
> * i137.20.100.35/32 137.20.25.2 74
> 100
> 0 ?
> * i137.20.240.0/20 137.20.25.2 11
> 100
> 0 ?
> *>i172.168.70.0/24 137.20.10.70 170
> 100
> 0 3 i
> * i172.168.80.0/24 137.20.86.1 0
> 100
> 0 1 i
> *>i200.200.100.0 137.20.25.2 75
> 100
> 0 ?
> * i200.200.200.0 137.20.25.1 0
> 100
> 0 ?
>
> r2#sho ip route
> Codes: C - connected, S - static, I - IGRP, R - RIP,
> M
> - mobile, B - BGP
> D - EIGRP, EX - EIGRP external, O - OSPF, IA
> -
> OSPF inter area
> N1 - OSPF NSSA external type 1, N2 - OSPF
> NSSA
> external type 2
> E1 - OSPF external type 1, E2 - OSPF external
> type 2, E - EGP
> i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS
> level-2, ia - IS-IS inter area
> * - candidate default, U - per-user static
> route, o - ODR
> P - periodic downloaded static route
>
> Gateway of last resort is 0.0.0.0 to network 0.0.0.0
>
> C 223.255.2.0/24 is directly connected,
> Loopback223
> O IA 200.200.200.0/24 [110/66] via 137.20.100.34,
> 00:18:09, Serial0
> 137.20.0.0/16 is variably subnetted, 14
> subnets,
> 6 masks
> O E1 137.20.200.16/28 [110/164] via
> 137.20.100.34,
> 00:18:10, Serial0
> O 137.20.240.0/20 [110/65] via 137.20.100.34,
> 00:18:20, Serial0
> O 137.20.224.0/20 [110/1626] via
> 137.20.100.34,
> 00:18:20, Serial0
> O IA 137.20.25.0/24 [110/65] via 137.20.100.34,
> 00:18:10, Serial0
> C 137.20.20.0/24 is directly connected,
> Ethernet0
> O E2 137.20.10.0/24 [110/20] via 137.20.100.34,
> 00:18:11, Serial0
> O E1 137.20.40.16/28 [110/164] via 137.20.100.34,
> 00:18:11, Serial0
> O 137.20.48.0/20 [110/66] via 137.20.100.34,
> 00:18:21, Serial0
> O IA 137.20.33.0/26 [110/129] via 137.20.100.34,
> 00:18:11, Serial0
> O E2 137.20.86.0/24 [110/20] via 137.20.100.34,
> 00:18:12, Serial0
> C 137.20.100.32/27 is directly connected,
> Serial0
> O 137.20.100.35/32 [110/128] via
> 137.20.100.34,
> 1d02h, Serial0
> O 137.20.100.34/32 [110/64] via 137.20.100.34,
> 1d02h, Serial0
> O 137.20.64.0/20 [110/65] via 137.20.100.34,
> 00:18:22, Serial0
> C 200.200.100.0/24 is directly connected,
> Loopback0
> 172.168.0.0/24 is subnetted, 2 subnets
> O E2 172.168.70.0 [110/500] via 137.20.100.34,
> 00:18:12, Serial0
> O E2 172.168.80.0 [110/500] via 137.20.100.34,
> 00:18:12, Serial0
> S* 0.0.0.0/0 is directly connected, Ethernet0
> r2#
>
>
>
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