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Community Member

BGP With 3 Links

Hi Experts

Please have a look to the attached topology

i have two Cisco 3845 routers connected to 3 different ISPs ,

ISP 1 with link bandwidth of 24 Mbps

ISP 2 with link bandwidth of 16 Mbps

ISP 3 with link bandwidth of 8 Mbps

i have a public AS from a ripe along with 2 Class Address (Public independent)

questions

----------------

1) what is the best design and configuration to utilize the 3 Links ,outbound and inbound (since we have our public address along with AS) my boss told me,all These  3 links  must be active

2)what is the recommended design and configuration for the whole topology , pls share the best gotchas

3)what is the need of iBGP?why we need it when we run bgp?

thanks

Ibrahim

10 REPLIES

Re: BGP With 3 Links

You list 3 x ISPs so I am assuming that they each have their own AS numbers.

If so one thing to mindful of is to avoid becoming a transit AS for one ISP to the other. This wastes your bandwidth and router CPU cycles.

See below a link explaining how to avoid this. It also provides a guide for a multi-homing configuration.

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a008009456d.shtml

Community Member

BGP With 3 Links

Hi Sean

i only receive a default route from them, ofcourse they have their own AS

anyway , y didn"t answer my post

jamil

Super Bronze

Re: BGP With 3 Links

Disclaimer

The    Author of this posting offers the information contained within this    posting without consideration and with the reader's understanding that    there's no implied or expressed suitability or fitness for any  purpose.   Information provided is for informational purposes only and  should not   be construed as rendering professional advice of any kind.  Usage of  this  posting's information is solely at reader's own risk.

Liability Disclaimer

In    no event shall Author be liable for any damages whatsoever  (including,   without limitation, damages for loss of use, data or  profit) arising  out  of the use or inability to use the posting's  information even if  Author  has been advised of the possibility of such  damage.

Posting

#1 Best way, if supported on your IOS, is OER/PfR for outbound.

Inbound is a problem.  With your two public class Cs, you could advertise one as "better" on the 8 & 16 Mbps links, and the other as "better" on the 24 Mbps link.  You could also talk to you providers about what they are willing to do, such as passing the preferences in community strings.  All inbound, though, is going to be impacted by what's further downstream so it's not 100% unless instead of advertising one class C subnet as "better" then the other as described, you only advertise the particular subnet and only provide the other only if its link fails.  Even doing all this assumes your two public class C subnets will draw equal traffic.

#2 The only major item I notice, you have 8 Mbps one router and the 16 & 24 Mbps on the other.  If the latter fails it takes most of your bandwidth.  Better would be 8 & 16 Mbps on one router with the 24 Mbps on the other.  Doing this guarantees you won't lose more than half your bandwidth and also likely better distributes the load across the two routers.

#3 Normally really required if your taking more than a default route from your providers so all your internal routers know the external routes.  Taking just an external you don't really need to run iBGP, but you can still pass the default given to you from your providers to the other iBGP router.  For example, right now if your 8 Mbps link failed, but your router doesn't, how does that router know to use your other router?  Probably via iBGP.

PS:

One issue, which is difficult to deal with without OER/PfR, how do you direct traffic to your two internal BGP routers?  By default, equal AS paths will use the egress router you're on, so, for instance, if traffic starts on the 8 Mbps router it won't use the 16/24 Mbps (this might be why you're configured with 8 Mbps on the one router, as a backup, you might direct all traffic to the 16/24 - and even then, it will prefer one link, and it might not be the 24).

Community Member

BGP With 3 Links

Hi Joseph

i read in the link attached by sean type of default route towards ISPs,does it solve the issue outbound?

thanks

Super Bronze

BGP With 3 Links

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

Perhaps, depends on how optimal you want your link usage to be.  First you have two separate routers, and so you have the issue of dividing the traffic between them.  Second, your links are all different capacities.  Default routes could get you to both routers and get you to use two links on the same router, which would solve your "3 links must be active" but it wouldn't optimally load balance across them.

Community Member

Re: BGP With 3 Links

HI Joseh

can i use default-route only, on R2 so that both links passes the traffic, now on R2 only 24 Mbps passing traffic while the 16 Mpbs sitting idle no traffic

is the default route solve the issue?

on R2 do i have to add two default route each one point to the next hope of the 16Mbps and 24 Mbps?

did the default route  affect the BGP Session?

i have a routing questions?why the default route trigger the 16Mbps ,while BGP not?

thanks

ibrahim

Super Bronze

Re: BGP With 3 Links

Disclaimer

The   Author of this posting offers the information contained within this   posting without consideration and with the reader's understanding that   there's no implied or expressed suitability or fitness for any purpose.   Information provided is for informational purposes only and should not   be construed as rendering professional advice of any kind. Usage of  this  posting's information is solely at reader's own risk.

Liability Disclaimer

In   no event shall Author be liable for any damages whatsoever (including,   without limitation, damages for loss of use, data or profit) arising  out  of the use or inability to use the posting's information even if  Author  has been advised of the possibility of such damage.

Posting

The default route you have now is coming from your BGP peers, correct?  If so, it might be following the default routing rules for BGP and only using one path.  There's a hidden command that should overcome this (bgp bestpath as-path multipath-relax) with max-paths equal two.

If you use a static default route, it should use both links, but then you need to deal with loss connectivity with your BGP peers.

Community Member

Re: BGP With 3 Links

Hi Joseph

Please,Provide me a sample BGP Configuration according the topology,please take into consideration default route  from ISPs,also a public as along with 3 class C Subnet from ripe

thanks in advance

ibrahim

Super Bronze

Re: BGP With 3 Links

Disclaimer

The   Author of this posting offers the information contained within this   posting without consideration and with the reader's understanding that   there's no implied or expressed suitability or fitness for any purpose.   Information provided is for informational purposes only and should not   be construed as rendering professional advice of any kind. Usage of  this  posting's information is solely at reader's own risk.

Liability Disclaimer

In   no event shall Author be liable for any damages whatsoever (including,   without limitation, damages for loss of use, data or profit) arising  out  of the use or inability to use the posting's information even if  Author  has been advised of the possibility of such damage.

Posting

Unfortnately, I don't work that much in BGP, and I couldn't "simply" provide you a good example configuration off the top of my head.  However, another poster might contribute such.

Community Member

Re: BGP With 3 Links

Hi Jo

pls make for me as favor to refer back when i configure this topology

thanks

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