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a question about distribute list

access-list 101 ip permit 172.0.0.0 0.255.255.255 255.0.0.0 0.0.0.0

I saw this command on the BSCN book --distribute list

but I do not know what it mean.

it says " a router running BGP get one route to 172.10.0.0/16 ,one route to 172.20.0.0/16 and a aggregate route to 172.0.0.0/8,if the access-list command be used with the neighbor distribute-list command ,then the router only advertise the 172.0.0.0/8,do not advertise 172.10.0.0/16 and 172.20.0.0/16

1 REPLY
Cisco Employee

Re: a question about distribute list

the source part of the access-list matches the network and the destination part matches the mask.

So, in this example

172.0.0.0 0.255.255.255 = 172.x.x.x everything which starts with 172.

Then we have

255.0.0.0 0.0.0.0 = host 255.0.0.0 which means the mask 255.0.0.0 or 8 bits mask (/8)

Together it means every routing entry like 172.x.x.x /8

Everything else will be denied.

Hope this will clarify.

Gilles.

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