ip prefix advertisement by bgp

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Feb 25th, 2009
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HI every body!

I have few questions about bgp.


Suppose an imaginary company ABC needs internet access.

Case #1

ABC contacts an isp and was given an ip address 11.11.11.1/24 . ABC uses nat(pat) to allow internet connectivity for its employees to internet. In this case , 11.11.11.1/24 is global internat address which is advertised by the isp.


case2# ABC contacts the IANA or local internet registry and gets the block 196.196.196.0/24.

Since abc has internet ip block but no internet connectivity , it contacts an isp and advertises its block to isp which in turn advertises it to internet.


===========================================


My questions are:

1) in the first case we don't need bgp to run on abc's edge router. The address allocated to abc, is already adverertised by isp. Correct or wrong?


2)In the 2nd case, we have to use bgp on our edge router because the address given by iana/ local internet registry is not being advertised by isps. So abc runs bgp on edge router to advertise its address to internet.Is it correct or wrong?

If it is correct,instead of using bgp, why don't we simply use some routing protocol between edge router and isp and advertises these networks to isp ?


3) how does the edge router running bgp connects to isp? Just to make my point,let say I have home-based business, i bought a cisco router to run bgp . How should this routerbe connected to isp? for example i have cable, i simply plug in the cable rj45 to router fast ethernet port?


thanks a lot!


Correct Answer by adamclarkuk_2 about 8 years 4 weeks ago

It is better to use BGP when you have multiple connections to providers, preferably different ones, for inbound and outbound redundancy and route selection as provider A might have better access to net x and you can use BGP to select the best route.


For inbound redundancy, you would advertise your prefix out of A and B but prepend the AS by say 5 times towards B. As BGP uses AS path lengths as it's selector in general ( it's 4 on the list in reality), then to the outside world, provider A will look closer and they will use that unless that sessions drops then they will use the connection via B.


This is the tip of the ice berg but you can see that with multiple connections, BGP is the best choice.



Correct Answer by Edison Ortiz about 8 years 4 weeks ago

My question is how to decide when to use bgp or not to use bgp at all when it comes to advertising the prefix to the internet(world)?


Sarah,


It's all about having a dynamic routing protocol between the ISP and the customer site.


If the customer site has more than one link towards the ISP, then it's ideal to run some kind of routing protocol (on this case BGP) between devices.


If the customer site has only one link, then there isn't much benefit to run BGP between devices. A static default route towards the ISP from the customer site will take care of traffic leaving the customer *AND* a static route from the ISP to the customer for traffic returning to the customer site will accomplish the design.


HTH,


__


Edison.

Correct Answer by adamclarkuk_2 about 8 years 4 weeks ago

Single homed just means one BGP connection upstream.


If you don't want to run BGP, your provider my advertise it for you and just route back to you through their network. Like you said you just have the static default on the edge pointing to them which, you redistribute back into your IGP, (I guessing you dont have a AS number). This will need some collaboration with you and your provider either way.

Correct Answer by adamclarkuk_2 about 8 years 4 weeks ago

For option 2 you would also need to purchase and AS number or (if you have a nice ISP) Peer with the ISP on private AS number and then they would pass that on into their BGP cloud(Not best practise though).

You wouldn't use an IGP to talk to your provider simply because BGP was designed for this purpose. BGP is designed to handle HUGE routing tables (which you do not want in your IGP), where as the IGP's are designed to converge as fast as possible, so you need a protocol demarc.


You can use any media to connect to the provider that they support. When I was learning BGP I had an ADSL line and my ISP let me peer with them over that, not recommended by the way. Normally you would have a dedicated link to the provider, say CAT5 from your kit in your COLO space to there switch infrastructure , they give you an IP address, a /30 say and your peer to the other end or what ever IP address they give you to peer to.

Correct Answer by Mohamad Qayoom about 8 years 4 weeks ago

1)Correct.

2)Correct.

3) You can simply plug in the cable rj45 to router fast ethernet port?

Regards,

Mohamad

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Correct Answer
Mohamad Qayoom Wed, 02/25/2009 - 07:10
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1)Correct.

2)Correct.

3) You can simply plug in the cable rj45 to router fast ethernet port?

Regards,

Mohamad

Correct Answer
adamclarkuk_2 Wed, 02/25/2009 - 07:18
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For option 2 you would also need to purchase and AS number or (if you have a nice ISP) Peer with the ISP on private AS number and then they would pass that on into their BGP cloud(Not best practise though).

You wouldn't use an IGP to talk to your provider simply because BGP was designed for this purpose. BGP is designed to handle HUGE routing tables (which you do not want in your IGP), where as the IGP's are designed to converge as fast as possible, so you need a protocol demarc.


You can use any media to connect to the provider that they support. When I was learning BGP I had an ADSL line and my ISP let me peer with them over that, not recommended by the way. Normally you would have a dedicated link to the provider, say CAT5 from your kit in your COLO space to there switch infrastructure , they give you an IP address, a /30 say and your peer to the other end or what ever IP address they give you to peer to.

sarahr202 Wed, 02/25/2009 - 07:43
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Thanks for your reply Adamclarkuk!


1) i understand that bgp is designed to handle huge routes. But in our case there is only one network to be advertised. You might argue while advertising this network, we end up huge routes from isp into our igp in our imaginary firm ABC. But we can prevent these routes on isp side from being advertised by using acl either on our edge router( which gives the benefit of being in control of ACL manipulation) or at isp router( which in turn requires isp cooperation).


My understanding is though we can use the above approach but it requires a lot of help, cooperation from isp. By using bgp, we just need to to peer with isp and let isp advertise our network. Is it correct or wrong concept?


Another question is about how to connect to isp router . You mention that various media can be used. I think the load we are expecting on our side from the internet dictates what link should be used to connect abc company to isp. For example if abc is online auction company and expects heavy traffic , we might want to use T1 or T3 line to connect our edge router running bgp to isp's router. Is it correct or wrong concept?



thanks a lot!

adamclarkuk_2 Wed, 02/25/2009 - 07:48
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If you are single homed, which it sounds like you are then it will be a very simple setup, send them your prefix and accept default.


I would recommend that you ask your provider for default only to keep it easy.


Choices are normally :-

Default Only

Partial table

Full routing table ( 270,000 + prefixes )

sarahr202 Wed, 02/25/2009 - 07:55
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Thanks for your reply Adamclarkuk!


"If you are single homed, which it sounds like you are then it will be a very simple setup, send them your prefix and accept default".


Does it mean i don,t have to run bgp on edge router to advertise my prefix to isp?


If yes, how do we do it? We just tell isp to advertise the prefix and use default route pointed to isp on our edge router?


Thanks a lot!

Correct Answer
adamclarkuk_2 Wed, 02/25/2009 - 08:03
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Single homed just means one BGP connection upstream.


If you don't want to run BGP, your provider my advertise it for you and just route back to you through their network. Like you said you just have the static default on the edge pointing to them which, you redistribute back into your IGP, (I guessing you dont have a AS number). This will need some collaboration with you and your provider either way.

sarahr202 Wed, 02/25/2009 - 08:20
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Thanks for your reply adamclarkuk!

So alternative is use default route pointing to isp and redistribute this route into our igp and have isp advertise our prefix .

By the way , I am just a student not running any company or working on some project. I am preparing for ccnp routing exam.


My question is how to decide when to use bgp or not to use bgp at all when it comes to advertising the prefix to the internet(world)?


Thanks a lot!

Correct Answer
Edison Ortiz Wed, 02/25/2009 - 08:28
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My question is how to decide when to use bgp or not to use bgp at all when it comes to advertising the prefix to the internet(world)?


Sarah,


It's all about having a dynamic routing protocol between the ISP and the customer site.


If the customer site has more than one link towards the ISP, then it's ideal to run some kind of routing protocol (on this case BGP) between devices.


If the customer site has only one link, then there isn't much benefit to run BGP between devices. A static default route towards the ISP from the customer site will take care of traffic leaving the customer *AND* a static route from the ISP to the customer for traffic returning to the customer site will accomplish the design.


HTH,


__


Edison.

Correct Answer
adamclarkuk_2 Wed, 02/25/2009 - 08:30
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It is better to use BGP when you have multiple connections to providers, preferably different ones, for inbound and outbound redundancy and route selection as provider A might have better access to net x and you can use BGP to select the best route.


For inbound redundancy, you would advertise your prefix out of A and B but prepend the AS by say 5 times towards B. As BGP uses AS path lengths as it's selector in general ( it's 4 on the list in reality), then to the outside world, provider A will look closer and they will use that unless that sessions drops then they will use the connection via B.


This is the tip of the ice berg but you can see that with multiple connections, BGP is the best choice.



sarahr202 Wed, 02/25/2009 - 08:48
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Thanks Edison and Adamclarkuk!

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