BGP Extended Access-List Filtering

Hello Elias

My apologies, I found it. This is the issue:

image

That should read “The prefix length has to be exactly /24 so we use subnet mask 255.255.255.0 with…”

Thanks for that, I’ll let Rene know.

Laz

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Hi Elias, Thanks for letting us know. I just fixed this.

Rene

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Hi
i advertised this network but
i didnt see them
20.0.0.0
30.0.0.0
it can be that you didnt use mask for them

Hello Bahri

In order for BGP to advertise a route, that route must exist in the routing table of the device. The route must match exactly, that is both the network address and the subnet mask. If there is no subnet mask in the routes that you advertised, as you mention above, then the classfull mask will be used. In this case it would be 255.0.0.0. So if the 20.0.0.0 255.0.0.0 and 30.0.0.0 255.0.0.0 networks are in the routing table, then they will be advertised.

If you are following the configs in the lesson, then these should indeed be advertised as the subnet mask is in fact 255.0.0.0, the classful mask,.

I hope this has been helpful!

Laz

Hi,
I have a question about “Filter anything with a /26 to /32 prefix length”.
Here is the access list:
R1(config)#access-list 106 permit ip 0.0.0.0 255.255.255.255 255.255.255.192 0.0.0.63
In your explanation, you are saying
“We want to match all prefixes from /26 to /32, by using this wildcard we tell the router that the last four bits have to match, we don’t care about the first four bits.”

If the wildcard mask for prefixes is 0.0.0.63, why is it responsible for 4 bits? Doesn’t it cover last 6 bits? Please explain.

Thanks in advance.

Hi Dmitriy,

You are correct, this is a type. A wildcard of 0.0.0.63 means the last six bits match and we ignore the first two bits.

Thanks! Just fixed this.

Rene

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Thank you for replying! And thank you for these materials, they are awesome.

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