IBGP as IGP (Without any OSPF or ISIS or RIP etc)

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Jul 20th, 2010
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1) IGBP as IGP (Without any OSPF or ISIS or RIP etc)

I understand that IBGP can operate as the IGP without any other IGP configured, by creating full mesh connectivity between all the routers.

My Question is : How does the IBGP Router chooses the best path to a Specific Destination especially when multiple paths are available. In other IGPs there are metrics available, like hop count, cost etc to choose the optimal path, but in IBGP is there any specific attribute available to carry the metric which helps in dynamic path selection.

Second Question: What is the mechanism in IBGP to avoid the routing loops ( AS_PATH would work only in the inter AS routing )

Both the questions above are specific to the IBGP as IGP scenario

I am wondering how IBGP ( alone ) can operate as IGP, if the above mechanisms are not available.

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Mohamed Sobair Tue, 07/20/2010 - 01:45
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Hi,


You wil still need an IGP for IBGP so the IGP is not ommited.


-- IBGP uses AS - Path attributes to determine which path it takes for particular destination, those are BGP best path selection criteria:


1- prefer the path with the Highest wieght

2- prefer the path with the highest Higest local preference

3- prefer locally originated routes

4- prefer the shortest as path

5- prefer the path with the lowest origin code.

6- prefer the path with the lowest MED value.

7- prefer the Ebgp path over the IBGP path

8- prefer the path with lowest bgp router - id


Second Question:


BGP uses split-horizon rule to prevent routing loops, a BGP speaker wont accept any updates that contains its own AS in the AS path list by default.


However, In certain circumistances, a BGP route-reflector and BGP confederation disables BGP Spilit Horizon rule and reduces the need of a full mesh IBGP connectivity within an AS.


a route reflector will advertise any routes learned from its clients to all IBGP clients and its non client.

a route reflector will advertis any routes learned from non clients to its IBGP clients only.



Hope this addressed your question,


Mohamed

Jon Marshall Tue, 07/20/2010 - 02:01
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Hi Mohamed


You wil still need an IGP for IBGP so the IGP is not ommited.


Can you clarify what you mean by this as you don't need an IGP if you are running a full mesh IBGP setup.


Jon

Mohamed Sobair Tue, 07/20/2010 - 02:11
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Hi Jon,


Cisco recommends peering IBGP uses loopack interfaces instead of the physical interface, So based on this, He will still needs an IGP on each bgp speaker for the peering loopback.


If he uses the physical interface for peering, then he wouldnt need it of course, but i have included my answer based on the above.



HTH
Mohamed

swilsons Tue, 07/20/2010 - 05:09
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Hi Mohammed,

The Attributes that you have mentioned will be used for the best Route Selection in the Inter AS routing scenario . My Question is about enabling the I-BGP ( IBGP alone) as
the IGP in a Network . Let us assume there is no External Routes in this network. Also let us assume there is no Loop back configured and all the IBGP full mesh connectivity is to the interface IP.
Also let us assume it is I-BGP full mesh (Logical) connectivity in the NW and hence no Confideration or Reflector configuration is required

The question again: What is the mechanism in I-BGP which helps in optimal path selection . In RIP we have hop count which is carried along with the Routing Update , In OSPF there is Cost which is carried in the LSA, But in BGP , i could not locate any attribute that could help in the best path selection in I-BGP alone scenario

Regards,

Stella Florence W

Mohamed Sobair Tue, 07/20/2010 - 05:14
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The BGP best path selection criteria is applied for both IBGP and EBGP speakers not Only Inter AS, its also applied for Intra AS.



HTH

Mohamed

swilsons Tue, 07/20/2010 - 23:05
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In my view . these Attributes here does not seem to help the best route selection in the IBGP scenario ( Assuming , IBGP is the only IGP)


<2- prefer the path with the highest Higest local preference>

This is useful in selecting the exit path out of the AS , but this will not be useful in the intra AS routing

<3- prefer locally originated routes>

In this Scenario, all the routes are locally originated, so this can not help

<4- prefer the shortest as path>

In this scenarios , all the Routes are internal and will have the same AS PATH

<5- prefer the path with the lowest origin code.>

All the Routes will be having the lowest origin code

<6- prefer the path with the lowest MED value.>
MED is specific to inter AS routing and not applicable to the scenario in question

<7- prefer the Ebgp path over the IBGP path>
All are IBGP, so this will not help in choosing the best path among the IGMP routes

<8- prefer the path with lowest bgp router - id>
This will not always work for selecting the best path among the IGMP routes. The lowest BGP ID can not be the only criteria and if that is the case it would result in non-optimal path

milan.kulik Wed, 07/21/2010 - 01:01
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Hi,


IMHO, Mohamed is right, the BGP best path selection  algorithm is applied to all prefixes, no matter if received by eBGP or iBGP.

It's even more complex, see http://www.cisco.com/en/US/customer/tech/tk365/technologies_tech_note09186a0080094431.shtml

(there's a step 8. Prefer the path with the lowest IGP metric to the BGP next hop., e.g.).


But many attributes are equal for all prefixes belonging to your local AS.

Which means the best path selection algorithm might chose a path which is not the best one from an IGP metric point of view (number of hops, bandwidth available, etc.).


You still could use weight and local-preference to manipulate the best path selection.

But doing so, there's no loop detection available within your AS in a case you made that inconsistently.


Generally, it's not a good idea to run iBGP as an IGP if your topology is not a really primitive one (like 3 routers in a triangle, e.g.).

BGP was NOT designed to run as an IGP and that's it!


BR,

Milan

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