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BGP Question, Multiple ISPs.

Posted on 2009-04-27
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Last Modified: 2012-05-06
I currently have a single Cisco 3825 doing BGP between 2 vendors.   One 10MB Circuit and a 4.5MB bonded T1's.

I am going to be adding a 15MB circuit and dropping the 4.5MB one and adding a second 3825 for the new ISP, and using the existing 3825 for the 10MB.

I think I pretty much have the BGP to the ISP's figured out by using the example I have in the existing 3825, but here is my question.

I am going to be running 2 ASA5510's in Active/Standby, and was going to run OSPF on both 3825's, both announcing the default route with the same cost.   That should load-balance my outbound traffic.  Inbound traffic would be load-shared from BGP.

I have seen comments around talking about running BGP between the 3825's.  Is this needed?  What would I gain by doing this?  Would I need a 3rd interface in the 3825's?

Thoughts?

Thanks!

-Mark
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Question by:MarkJenks
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by:cstosgale
ID: 24244886
If the routers are both in the same AS, then I would recommend peering them. However, your configuration above will probably work fine either way as it is a very simple setup.

from version 8 you can now use EIGRP with the ASAs which may be a better choice of IGP (but this is mostly personal preference). This should allow you to do unequal cost load balancing though, which may be beneficial in this scenario. you will need to redistribute your routes with different metrics however.
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by:MarkJenks
ID: 24245764
Are you referring to Fast Peering or something like that?

http://www.cisco.com/en/US/docs/ios/12_3t/12_3t14/feature/guide/gt_bsfda.html

I've been using OSPF for years.  Does EIGRP work better for these things?
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by:cstosgale
ID: 24246165
No, I simply meant setting them up as iBGP neighbours so they were aware of their respective routes via BGP. this will be particularly beneficial if you are getting more than a default route from your ISP as this will allow the edge routers to route the traffic using the best path even if your ASA has chosen the other router if that makes sense. i.e., your ASA can just have two default routes, and the routers can dictate which link the traffic goes down based on the shortest path.

If you are more comfortable with OSPF, stick with that. The only real advantage of EIGRP in this case is unequal cost load balancing. This allows you to send 70% of the traffic down one link and 30% down the other for example. given the difference in bandwidth between your links it probably isn't worth it.
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by:MarkJenks
ID: 24246889
Okay, I went out looking at running iBGP between the 3825's.  From what I can tell, I should run a loopback interface on both of them to make sure that I don't have a problem in case one of the the ISP's goes down and the interface goes down.

I see examples of this, and they make sense.   Does the loopback interface need to be in a different subnet as my current one?  I've never used one before, but I see them all listed as a 255.255.255.255 subnet.  I can just assign it a .253 and .254 from my /24 subnet?

The next question in the Remote-AS #'s.   I'm guessing those are assigned from the ISP, but can I just use any one that I want between the 3825's?

I guess I really only know how to connect BGP to the ISP, but not create my own addition to it.

-Mark  
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by:MarkJenks
ID: 24247108
Instead of running OSPF of the 3825's, I just saw somewhere that they were using GLBP to share an IP.

I really don't see a difference in it at all.  

In one you are doing default-routes in OSPF, it seems to me as the same as doing GLBP.

You thoughts?
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by:cstosgale
ID: 24253903
I would recommend using OSPF as it is more flexible and can react more quickly to changes in the BGP routing. Also, I amm pretty sure you GLBP is only supported on cat 6500s so you would have to use HSRP or VRRP neither of which do load balancing.
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by:cstosgale
ID: 24253997
With regard to your previou comment:-

You can use any unused IP addresses for your loopback addresses, but they must be in a range that is unused on the rest of your network. This is because a loopback interface is still an interface.

e.g. Your router has 192.168.1.1/24 on its fa0/0 interface. If you try to configure 192.168.1.10/32 on your loopback interface, IOS will give you an error because it overlaps with fa0/0.

there is no point in using a subnet mask less than 255.255.255.255 as the loopback interface doesn't connect to anything so there can't be any other IPs on that link.

With regards to AS numbers, your ISP will configure their router with your AS number on their side, so it has to match or they won't establish a neighbourship. If you have been given different AS numbers for yourself by your two ISPs, you won't be able to set up an iBGP neighbourship between your two routers as they will be in different ASes. For this to work, you would have to have a public AS number that both your ISPs are connecting to.

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by:MarkJenks
ID: 24256702
Okay, it just hit me like a ton of bricks.  Been looking at BGP and didn't understand the iBGP part of it, but now I do.  I use my own ASN and configure a next-hop-self between each other.  That part all makes sense now.  I was looking at the config of my existing 3825 and the AS's are one number off and thought they were "created", but they are actual AS's.  DUH!

As for the loopback interfaces.  If I put them in the upper range of my /24 and keep it away from my /26 that I am going to be using for my ASA's, I can just just .253 and .254 for my loopbacks.

Now the next step, should I just look at using the loopback for my iBGP, or look at using it for my eBGP also?
I'm looking at this page now:

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a0080093f25.shtml#ebgploopconfigloop

What are your thoughts on using loopbacks for these?  I'm looking at trying to create the most redundant connection that I can with this configuration.
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by:cstosgale
ID: 24265429
I would recomend using loopbacks for your ibgp neighbourships but real addresses for your ebgp neighbourships. As there is only one interface you can connect to each ebgp neighbour through, using loopbacks for this won't increase redundancy but will increase configuration complexity. It also means your ISPs would need to know how to get to your loopback addresses.
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by:MarkJenks
ID: 24265972
So, what would I gain for adding loopbacks on the iBGP?

Is it that if the interface goes down (3750 stack, vlan across them, one 3825 to each), wouldn't they go out their own eBGPs?  

Or is it because traffic could be routed out ISPA from ISPB's router, and the failover would be faster?

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cstosgale earned 500 total points
ID: 24289210
It's not really going to make much difference to be honest. Either way, if the ebgp route is lost, ospf will send an update and the 3750 will use the other route.

It's more about for what is best practice, and what will support you better as / if your network grows.
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