BGP Allow AS In

I have one small problem with this lab.

"Ensure router Woody and Andy can still reach each others loopback interfaces when router Rex fails."

As we are using iBGP in AS1 we have two problems,

1. Woody's loopback will not reach Andy with out the use of a route reflector and vice versa.

2. iBGP's AD is 200 where as eBGP's is 20,

So by default all traffic will always take the path through Buzz and never through Rex as without Buzz Woddy and Andy would never know of each others loopbacks.

But anyway thanks for the lab.

Hello Andrew,

Thanks for your comment.

Within the autonomous system you need to configure IBGP and it should be a full-mesh. If you want you can use a route reflector if you don't want to configure all IBGP peerings.

You are right about the administrative distance but what is the loop-prevention mechanism for EBGP? If you see your own AS number in the AS Path for an update you will reject it because it's a loop. That's the default for BGP.

You can get around this by disabling the loop prevention mechanism for BGP, the title of this lab is the answer to that

Good luck!

Rene

Oh I got the purpose of the lab, disabling eBGP's metric, just wasn't overly keen on the wording of the last objective.

But I think that works well for this lab as it does make you think more the adding the allowas-in command.

Thanks again for the great resource.

Where's the lab?

If you register and login, you'll see the "attachments" at the bottom of the article.

Please share the configuration of this lab

Hi Sehgal,

I will soon...once I have time I'll be recording all the videos.

What part are you stuck on? If you drop questions here on the forum I'll be answering them.

Rene

After configuring the IBGP in As-1.I am not able to up the EBGP between Andy and BUZZ,But after configuring peer-group between andy and buzz,all nei. are comes up .is it right way for this lab???

Hello Vik,

You should only use a peer group when you have neighbors with the exact same configuration. Imagine having 4 EBGP peers with the same prefix-filter, that would be a good example of using a peer group.

In this example router Andy only has one IBGP and one EBGP router. You could configure a peer group on router Buzz because it has two EBGP neighbors in AS 1. To keep things simple I would just forget about the peer group and use the per neighbor commands. There has to be an error in your BGP neighbor configuration somewhere.

The lab is about the loop prevention mechanism of BGP. If it sees it's own AS number it will drop the traffic by default. Using BGP allow-AS in can change this behavior.

Thanks Rene for reply....
Yes,my all bgp nei. are up now,route also in routing table of every router but not able to ping.Allows in is confugured on both router l.e. Woddy and andy for nei. With buzz

Hi Rene

I am wondering have u ever used this allow as in feature in a real world?

The lab was good and everything worked fine a/c to the plan.

Thanks

I never used this in real life but if someone else did...please reply I think you have to be careful with this feature because we are telling BGP to ignore it's "loop-prevention" mechanism.