BGP Question

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Mar 11th, 2008
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Hi Guys,


I was going through Doyles TCP/IP volume 2 for BGP,may be i am confused and may not able to understand.Below is my question.


He says that BGP doesnt keep the topology info of the internal As where as IGP does.And BGP works on top of IGP.

Can anyone please explain the above pharses.


Thanks

Mahmood

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guruprasadr Wed, 03/12/2008 - 00:10
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HI Mahmood, [Pls Rate if HELPS]


If i am not wrong on understanding the phrase:


BGP will not send / receive the periodic updates as how other IGP protocol does. The network updates are exchanged only when there are some Network Changes / Neighbor Changes / others Occur.


Note: For BGP the AD is lower than other IGP Protocols. So always BGP runs on top of IGP's.


Hope i am Informative.


Pls Rate if HELPS


Best Regards,


Guru Prasad R

royalblues Wed, 03/12/2008 - 01:13
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BGP does not form neighbors via any discovery process and hence requires an IGP to talk to the peer and form an adjacency.


The IGP can either be a dynamic routing protocol or static etc. Also it does not keep the topology of the network as a link state protocol would do..


HTH

Narayan

mohammedmahmoud Wed, 03/12/2008 - 03:22
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Hi Mahmood,


Agree with Narayan, BGP is extremely different than IGPs in the terms of operation, BGP requires the IGP for packet forwarding, think of BGP as an application that advertise prefixes but needs the IGP to forward the packets to the responsible next-hop.


BR,

Mohammed Mahmoud.

mahmoodmkl Wed, 03/12/2008 - 04:19
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HI


Mahmoud,


First i would like to say mubrook on getting u r number.


Can u explain me with a example how does BGP uses IGP for packet forwarding.really i am confused about the operation of the BGP.i know i am missing something which is minor just to get the concept of it.


here by prefixes u mean networks.

As to my understanding IGP operation is like this.


When u enable any IGP it is going to send and recieve updates from the interfaces which fall into the network command range and it is going to send the updates about the networks connected to the router.am i right in thinking this.


Mahmood

Richard Burts Wed, 03/12/2008 - 04:20
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I believe that there is a slightly different way to look at the question about differences in maintaining topology. If you think about our common IGPs (EIGRP, OSPF, and even RIP) their metric and selection criteria are based on information about physical path (bandwidth and delay, bandwidth, and hop count). So these protocols do maintain and use information about the physical topology of the network. Then think about BGP and its path selection. It does not use any topology information (no bandwidth, no delay, no hop count) but uses things such as weight, local preference, or length of the AS path.


As far as BGP running over an IGP, I believe that this is true. Since BGP does not maintain information about the local links, then BGP usually needs an IGP to understand the topology and to make sure that its exit points and next hop addresses are reachable.


HTH


Rick

mahmoodmkl Wed, 03/12/2008 - 04:28
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Hi Rick


thanks for the reply.


Sorry,i think i sounding stupid here.

I did not understand u r pharse that IGP's maintain and use information about physical topology of the network.Here by this u mean that IGP's have the routes information about the internal network.Please explain.


And BGP needs IGP to understand the topology of the network...Please explain with a example.


Thanks

Mahmood

mheusing Wed, 03/12/2008 - 04:26
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Hi Mahmood,


First, BGP is not aware of the internal topology, as iBGP is setup based on TCP connections to not directly connected neighbors. Usually a Loopback IP is used for iBGP sessions. To learn the Loopback IP of a configured iBGP neighbor you need an IGP announcing it. BGP itself will not contain any information on how to reach the neighbor IP, hence the statement, that it does not keep the internal topology.

Second, the BGP next hop for a prefix learned through an iBGP neighbor could be the iBGP neighbors Loopback IP. for a routing table entry a recursive lookup is required. Basically any router needs an outgoing interface or IP address of a directly connected network as next hop for the routing table. As the Loopback IP is not directly connected, a BGP router needs to lookup the routing table how to get to the Loopback IP (hence recursive) used as BGP next hop. This information, again, will be provided by an IGP.

Therefore an IGP is needed for iBGP to work and thus the statement "BGP works on top of IGP".


Hope this helps! Please use the rating system.


Regards, Martin

mohammedmahmoud Wed, 03/12/2008 - 05:42
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Mahmood,


Allah yebrek feek :). I believe that Rick and Martin have replied on you in details, please never hesitate to go further in quires, always baring in mind that BGP is a lot different than IGPs in operation.


BR,

Mohammed Mahmoud.

mahmoodmkl Fri, 03/14/2008 - 23:02
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Hi Guys,


Thanks for replying.


Martin IBGP is setup based on TCP connections so for IBGP neighbors need not be directly connected.


So u mean to say that for routing BGP needs IGP for its routing.


Thanks

Mahmood


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