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IBGP Peering

 
 
Sandy Manning
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      11-26-2004
I have 3-routers that are directly connected to each other. Here is
an example drawing of the connectivity. One ethernet from RTRA to
RTRB and one ethernet from RTRB to RTRC.

RTRA----RTRB----RTRC

RTRA runs EBGP and IBGP. RTRB IBGP only, and RTRC EBGP and IBGP. RTRA
connects to AS0. RTRA, RTRB, and RTRC are in AS2. RTRC connects to
AS3.

How can I configure the IBGP to run properly in this topology?

Sandy Manning
BGP Neophite
 
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Arnold Nipper
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      11-26-2004
On 26.11.2004 21:43 Sandy Manning wrote


> I have 3-routers that are directly connected to each other. Here is
> an example drawing of the connectivity. One ethernet from RTRA to
> RTRB and one ethernet from RTRB to RTRC.
>
> RTRA----RTRB----RTRC
>
> RTRA runs EBGP and IBGP. RTRB IBGP only, and RTRC EBGP and IBGP. RTRA
> connects to AS0. RTRA, RTRB, and RTRC are in AS2. RTRC connects to
> AS3.
>
> How can I configure the IBGP to run properly in this topology?
>


Given

RTRA----RTRB network: 192.168.0.0/24, A .1, B .2
RTRB----RTRC network: 192.168.1.0/24, B .2, C .1

RTRA:
=====
router bgp 2
neighbor 192.168.0.2 remote-as 2
neighbor 192.168.1.1 remote-as 2

RTRB:
=====
router bgp 2
neighbor 192.168.0.1 remote-as 2
neighbor 192.168.1.1 remote-as 2

RTRC:
=====
router bgp 2
neighbor 192.168.0.1 remote-as 2
neighbor 192.168.1.2 remote-as 2


Make sure by IGP that RTRA knows how to reach RTRC and that RTRC knows
how to reach RTRA.




Arnold
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Arnold Nipper, AN45
 
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Toby
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      11-26-2004
Hi

Not going to do all the work for you, but some things to consider

RTRB shouldnt need to run BGP as it is only in One AS, BGP is used to pass
routing updates between AS's
IBGP is used to pass routing info between border routers within one AS for
onward transport. i.e AS0 info needs passing to AS3 and is achieved by IBGP
between RTRA and RTRC so a IBGP neibourship needs to be established between
RTRA &C not B
EBGP passes routing updates between AS's
RTRA,B & C should have a Internal routing protocol running between them such
as OSPF, ISIS. Use passive interface commands to stop leakage of this
between AS's
Packets being routed need to have a destination network in the routing table
even when running through routers not running BGP so BGP is normally
redistributed into the OSPF, ISIS but you have to be careful not to create
routing loops so dont redistribute OSPF, ISIS into B
Your BGP neighbours need to be reachable to create the peering to start with
so you may have to include static routes to ensure this.
If you are using loopback addresses to peer to you will need to include
neighbor x.x.x.x update-source Loopback0
And for EBGP neighbours you may need neighbor x.x.x.x ebgp-multihop 3 where
3 is the number of hops from your loopback to the neighbours loopback.

Regards

Toby

"Sandy Manning" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) om...
>I have 3-routers that are directly connected to each other. Here is
> an example drawing of the connectivity. One ethernet from RTRA to
> RTRB and one ethernet from RTRB to RTRC.
>
> RTRA----RTRB----RTRC
>
> RTRA runs EBGP and IBGP. RTRB IBGP only, and RTRC EBGP and IBGP. RTRA
> connects to AS0. RTRA, RTRB, and RTRC are in AS2. RTRC connects to
> AS3.
>
> How can I configure the IBGP to run properly in this topology?
>
> Sandy Manning
> BGP Neophite



 
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Arnold Nipper
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      11-26-2004
On 26.11.2004 22:30 Toby wrote

> RTRB shouldnt need to run BGP as it is only in One AS, BGP is used to pass
> routing updates between AS's


Not commenting on all you wrote: but how will RTRB know where to forward
packets for prefix e.g. 212.250.0.0/16. Assume RTRB defaults to RTRC but
212.250.0.0/16 has to be routed via RTRA.



Arnold
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Toby
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      11-26-2004
Hi

I agree I have thought of this as a transit AS only and answered in a
rush.just trying to give food for thought and it is wrong with the
redistribution etc. I realise this myself after I posted it.

Toby

"Arnold Nipper" <(E-Mail Removed)> wrote in message
news:co8as0$b25$(E-Mail Removed)...
> On 26.11.2004 22:30 Toby wrote
>
>> RTRB shouldnt need to run BGP as it is only in One AS, BGP is used to
>> pass routing updates between AS's

>
> Not commenting on all you wrote: but how will RTRB know where to forward
> packets for prefix e.g. 212.250.0.0/16. Assume RTRB defaults to RTRC but
> 212.250.0.0/16 has to be routed via RTRA.
>
>
>
> Arnold
> --
> Arnold Nipper, AN45



 
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Ivan Ostreš
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      11-28-2004
In article <co866h$35c$(E-Mail Removed)>, http://www.velocityreviews.com/forums/(E-Mail Removed) says...
> RTRB:
> =====
> router bgp 2
> neighbor 192.168.0.1 remote-as 2
> neighbor 192.168.1.1 remote-as 2
>
> Make sure by IGP that RTRA knows how to reach RTRC and that RTRC knows
> how to reach RTRA.
>
>


I would expect here that router B will NOT pass prefixes learned from
peer A to peer C and vice versa since that is one of the IBGP rules.
(You need to have a full mesh of internal BGP routers or route
reflectors).

So, I would configure RTRB as route-reflector:

router bgp 2
no sync
neighbor 192.168.0.1 remote-as 2
neighbor 192.168.0.1 route-reflector-client
neighbor 192.168.1.1 remote-as 2
neighbor 192.168.1.1 route-reflector-client

Now it should work ok.

HTH,

--
-Ivan.

*** Use Rot13 to see my eMail address ***
 
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Arnold Nipper
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      11-28-2004
On 28.11.2004 14:48 Ivan Ostreš wrote


> In article <co866h$35c$(E-Mail Removed)>, (E-Mail Removed) says...
>> RTRB:
>> =====
>> router bgp 2
>> neighbor 192.168.0.1 remote-as 2
>> neighbor 192.168.1.1 remote-as 2
>>
>> Make sure by IGP that RTRA knows how to reach RTRC and that RTRC knows
>> how to reach RTRA.
>>
>>

>
> I would expect here that router B will NOT pass prefixes learned from
> peer A to peer C and vice versa since that is one of the IBGP rules.
> (You need to have a full mesh of internal BGP routers or route
> reflectors).
>


If you read carefully you see that RTRA, RTRB and RTRC *are* fully meshed.

> So, I would configure RTRB as route-reflector:
>


You don't need it.

> Now it should work ok.
>


It already will have done before ...




Arnold
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Sandy Manning
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      11-28-2004
Arnold, Ivan and, Toby,

I did think of using RR (route-reflectors) but, I read that in order
to setup RR's the router configured as the RR must be the hub and the
others must act as the spokes? If this is true, the problem with my
configuration is that I cannot change the physical connectivity
meaning that RTRA must stay the EBGP peer to the remote-AS AS0.

If the hub rule is true should I only run BGP on RTRA and RTRC and use
OSPF to route between them? IF this is hub rule is false, how can I
setup BGP under this rigid configuration to make IBGP work without a
full mesh or RR's?

RTRA----RTRB----RTRC

Sandy Manning



(E-Mail Removed) (Sandy Manning) wrote in message news:<(E-Mail Removed). com>...
> I have 3-routers that are directly connected to each other. Here is
> an example drawing of the connectivity. One ethernet from RTRA to
> RTRB and one ethernet from RTRB to RTRC.
>
> RTRA----RTRB----RTRC
>
> RTRA runs EBGP and IBGP. RTRB IBGP only, and RTRC EBGP and IBGP. RTRA
> connects to AS0. RTRA, RTRB, and RTRC are in AS2. RTRC connects to
> AS3.
>
> How can I configure the IBGP to run properly in this topology?
>
> Sandy Manning
> BGP Neophite

 
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Sandy Manning
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      11-28-2004
Guys,

I thought of using a RR (route-reflector) but from what I have read on
RR's the rule is, the router configured as the RR must be the
HUB-router in the configuration, act as the EBGP peer to the remote-as
and, the other routers must connected as and act as spokes? My
problem is for design reasons, I cannot change the physical
configuration of the network. So, RTRA must stay the EGBP connection
to the remote-as AS0.

RTRA(Bdr for AS0-remote and AS1)-----RTRB(AS1)-----RTRC(Brdr for AS1
and AS2)

IF the HUB RULE FOR RR'S is true? Should I, on RTRA run EBGP to the
remote-as AS0 and also run an OSPF process. RTRB and RTRC run an OSPF
process. RTRC run BGP Process for redistribution of the remote-as AS2
routes?

or

If the Hub rule is not true for RR's how can I configure a RR without
the hub and spoke?

Thanks in advance for your help.

Sandy Manning
BGP Neophite
 
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Arnold Nipper
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      11-28-2004
On 28.11.2004 22:55 Sandy Manning wrote


> Guys,
>
> I thought of using a RR (route-reflector) but from what I have read on
> RR's the rule is, the router configured as the RR must be the
> HUB-router in the configuration, act as the EBGP peer to the remote-as
> and, the other routers must connected as and act as spokes?


Check
http://www.cisco.com/en/US/products/...html#wp1001965
for a comprehensive tutorial on route reflecting.

btw: if RTRA, RTRB and RTRC is all you have, you don't need a route
reflector.



Arnold
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