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EBGP and point-to-point link with two routers

Cisco CCNP1 V3.0 curriculum (9.3.2 EBGP and IBGP )says:

"EBGP session – Occurs between routers in two different autonomous systems. These routers are usually adjacent to one another, sharing the same medium and a subnet ".

And in another place Cisco CCNP1 V3.0 curriculum (9.3.3 EBGP and IBGP configuration ) :

"Because EBGP sessions are typically point-to-point, there is no need to use update-source command with EBGP".

1- Isn't this logical point-to-point ?

2- When cisco says "adjacent" how will these two routers be connected (let us say customer and ISP)?,,,,it is obvious it will be connected through Telecom cloud ,,,Am I right ? Doesn't that mean only these two routers will occupy the link (point-to-point),,,

3- What does the word "adjacent" mean ? does it mean for instance that a customer router far away form ISP router couple of kilometers ? or it can be meant one router in HongKong and other in Japan ?

4- This question depend on the answer of question 3. Could you please look at the link below (connection between RTA and RTC) it is in a red arrow which cisco says it refers to logical link:

http://img52.imageshack.us/img52/1042/ebgpandlogical9jz.jpg

4 REPLIES
Silver

Re: EBGP and point-to-point link with two routers

1) It is point-to-Point (logical/physical). You can bring up BGP over GRE tunnels as well.

2) It means point-to-point at Network Layer not at DataLink or Physical layer so my switches and mux will pass through the frames.

3) Adjacent is over a LAN/Point-to-Point location. YOu can have a BGP session between routers in SJ and Tokyo (EBGP/IBGP).

4) Sorry cudnt open the pic :-(

HTH

New Member

Re: EBGP and point-to-point link with two routers

(((It is point-to-Point (logical/physical). )))

I did not get this very well, could you explain more.

(((It means point-to-point at Network Layer not at DataLink or Physical layer so my switches and mux will pass through the frames.)))

It is very easy to understand this concept,,,but if it is at networl layer,,,what does that mean ? does it mean the the two routers (customer and ISP or SJ and Tokyou)reside within same subnet ? or it does mean we can have only two layer 3 devices (SJ router and Tokyo router) and other devices between these two routers are switches belong to Telecom cloud?

(((Adjacent is over a LAN/Point-to-Point location)))

I did not get this .

(((Sorry cudnt open the pic :-( )))

Just give another try.

New Member

Re: EBGP and point-to-point link with two routers

Hi,

To answer your questions

1. May be logical (ethernet) or physical (serial line) as long as routers are sharing the same medium and subnet. You will notice they use the term "usually", this is because in some instances it might be necessary to create a BGP neighbor relationship which is more than one hop from each other...

2. Yes, ajacent in this context means connected, usually directly or through a cloud. It might be that the cloud is Frame Relay, in which case it might be point-to-point or point-to-multipoint...

3. Adjacent means on the same Layer 3 network, in other words, on the same subnet. You may have a point to point link from Hong Kong to Mars that is on the same subnet...

4.On the diagram it looks like RTA and RTC is directly connected via serial link.

Hope this helps...

Regards

JC

New Member

Re: EBGP and point-to-point link with two routers

(((1. May be logical (ethernet) or physical (serial line) as long as routers are sharing the same medium and subnet. )))

Why did you say logical with ethernet ? it is obvious for me with serial link,,,,but again my question was about real live not Lab (connected two routers through serial cable) ?

(((use the term "usually", this is because in some instances it might be necessary to create a BGP neighbor relationship which is more than one hop from each other...)))

When they used word "usually", does that mean there is possibility of creating a EBGP not as point-to-point ?,,,anothe issue raised ,If there are more than one hops (within WAN cloud)in the way between two the routers (customer and ISP or between HongKong And Japan) how are we going to create EBGP realation ?

(((Yes, ajacent in this context means connected, usually directly or through a cloud)))

In the real life can we have directly connected ?

(((Adjacent means on the same Layer 3 network, in other words, on the same subnet)))

Yes I can imagine that .

(((.On the diagram it looks like RTA and RTC is directly connected via serial link. )))

This link cisco says , it logical link it does not say it is physical link,,,could you kindly check the notation for read color.

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