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New Member

BGP issues

Multihomed router located at HQ has multiple 3 paths to the public internet(3 ISP).

1.Please do clarify if the service providers advertise the publicly routed IP block to the public network concurrently or only one does while others occupy redundancy state ? taking loop effect into consideration.

2.Does AS-Prepend superceeds,Weight attribute to influence path to the internet ?

3.What happens when looking glass tool shows a blank result for an enterprise AS no agreed to be advertised by the 3 service providers.

4.What is the best way to troubleshoot,narrow-down and detect where issue lies when BGP internet link ?

5.A service provider claims a problem to be from NIXP.Can you shed more light on this sort of incident?

I can confidently handle layer-1 and few of layer-2 issues.

I really need useful tips on route advertisement on the public internet.

Thank You for your usual apt response..

Hall of Fame Super Silver

Re: BGP issues

Hello Femi,

real BGP multihoming requires that the enterprise has its own public IP address block and its own BGP AS number; both can be asked to the regional internet registry:

ARIN for North America

RIPE for Europe

APNIC for Asia pacific and so on.

Without your own AS number and your own public IP address block a combination of BGP and NAT can be used to implement redundancy.

BGP builds loop free paths by examining the AS path attribute (loop avoidance rule says discard routes whose AS path already contains my AS number).


Weight can be used on local node (it is a Cisco feature) to give preference to outbound path to the internet.

AS path prepending is a way to try to influence the return path from the internet to your IP prefixes.


Before getting your own AS number and IP address block it is likely the enterprise is using an IP address block from one provider.


traceroute made from different internet points can help

you can find a list of looking glasses allowing traceroutes in

BGP commands that can help are:

sh ip bgp summary

sh ip bgp neighbors

sh ip bgp neigh x.x.x.x received-routes

sh ip bgp neigh x.x.x.x advertised-routes


The service provider is likely connecting to the NIXP. However, there can be different policies in BGP exchange of routes at the NIXP, and of course also a NIXP can have technical troubles like a lan switch fault for example or a misbeahiving device that corrupts other devices ARP table (just to make two examples)

Hope to help


Re: BGP issues

Just to make an addition to Question 2:

BGP path selection based on weight has precedence over selecting paths based on AS-Path length.

To be specific, here is the exact sequence of decision making (only the beginning of the whole process):

1. Choose the prefix with the highest weight.

2. If weights are equal, choose the highest local preference.

3. If local preferences are equal, select the prefix originated by the local router.

4. Select the prefix with the shortest AS-path length.

5. If AS-path lengths are equal, select the prefix with the lowest origin code

6. If the origin codes are the same, select the prefix with the lowest MED value.


As you can see, decision making using weights is earlier in the sequence.

But, as Giuseppe says, be aware, that weight is Cisco specific, and you won't find it in other vendors' BGP implementations.



New Member

Re: BGP issues

Can I conclude that the 3 service providers advertise the publicly routed IP blocks at the same time and I am left with an option to use the Bgp attributes (weight & As prepend path,etc)to influence enterprise internet outbound path and the return path from the internet.

For example:ISP A,can route me to the internet using weight attribute while ISP B,handles return path adopting

AS-Path prepend feature.

I stand corrected on above phrase,

Thanks to the DUO for your apt response.I appreciate immensely.


Hall of Fame Super Silver

Re: BGP issues

Hello Femi,

your understanding is correct you can choice ISPA as your preferred outgoing path using weight until you have a single border router multihomed (weight is not sent to anyone it is local on device).

If in the future you will have multiple BGP routers you will need to use local preference attribute.

You can try to influence what is your preferred inbound path by prepending the advertisements sent to the other paths/ISP.

If you want to have ISPB as preferred you need to make worse= prepend your AS number towards ISPA and ISPC

It is in both cases you that perform actions not your providers unless there are specific agreements.

Hope to help


New Member

Re: BGP issues

It is most preffered to have 3 ISP from maximum uptime point of view...But what do you mean by the caption below.Please make it more explicit......

''If you want to have ISPB as preferred you need to make worse= prepend your AS number towards ISPA and ISPC''


Hall of Fame Super Silver

Re: BGP issues

Hello Femi,

you use prepending on your side to make the AS path attribute to looks like longer then real.

So AS path prepending is to be used on the links you want to be the less preferred return paths.

You send not prepended BGP advertisements towards the ISP you would like to use as preferred return path

Hope to help