Mailing List Archive

Hi ... I think the cisco-nas list is nearly defunct (it is/was focused
on our now end-of-sale AS5xxx digital modem products.)

If you don't get an answer to this query, you might try the cisco-nsp
list (http://puck.nether.net/mailman/listinfo/cisco-nsp).

Cheers,

Aaron

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On 2/1/2012 5:50 PM, annkwok80@gmail.com (annkwok80) wrote:
> Hello
>
> I read this prefix list.
>
> Can I know why there is "le 24" after network block in /22 and /21
>
> Why don't have "le 24" after /24?
>
> I also saw another prefix list before. They use "le 32" instead of
> "le 24"
>
> What are their different?
>
> ip prefix-list prefix-filter-as100 seq 10 permit 202,168.136.0/22 le 24
> ip prefix-list prefix-filter-as100 seq 20 permit 202,22.92.0/22 le 24
> ip prefix-list prefix-filter-as100 seq 30 permit 202,21.148.0/22 le 24
> ip prefix-list prefix-filter-as100 seq 40 permit 203,178.88.0/21 le 24
> ip prefix-list prefix-filter-as100 seq 50 permit 178.88.74.0/24
> <http://178.88.74.0/24>
>
> Thank you so much
>
>
> _______________________________________________
> cisco-nas mailing list
> cisco-nas@puck.nether.net
> https://puck.nether.net/mailman/listinfo/cisco-nas
_______________________________________________
cisco-nas mailing list
cisco-nas@puck.nether.net
https://puck.nether.net/mailman/listinfo/cisco-nas

On Thursday, February 02, 2012 08:50:18 AM annkwok80 wrote:

> Can I know why there is "le 24" after network block in
> /22 and /21

As Aaron mentioned, this is probably more appropriate on the
'c-nsp' mailing list, but I'll try and answer your questions
anyway.

The reason you have 'le xx' or 'ge xx' after the prefix is
to define additional bit boundaries that warrant further
examination of that particular prefix, without resorting to
manually defining the additional prefix itself.

So it's like saying, "of this /22, also consider a /23 from
the same block, a /24 from the same block, e.t.c.", without
manually typing the /23 or /24 prefix as well. It saves time
and effort.

> Why don't have "le 24" after /24?

Well, in this particular case, 'le xx' means "anything
longer than or equal to xx'. Above, 'x.x.x.x/24 le 24' is
the same as saying 'x.x.x.x/24', because you're not asking
the router to consider any prefix longer than a /24, while
the original prefix is a /24 anyway.

But, if you did something like 'x.x.x.x/24 le 29', that
means from the /24 prefix, the router can also consider /25,
/26, /27, /28 and /29.

Hope I didn't confuse you :-).

> I also saw another prefix list before. They use "le 32"
> instead of "le 24"

'le 32' means the router will consider prefixes that are up
to 32 bits long, i.e., /24, /25, 26, /27, /28, /29, /30, /31
and /32.

> ip prefix-list prefix-filter-as100 seq 10 permit
> 202,168.136.0/22 le 24

> ip prefix-list
> prefix-filter-as100 seq 20 permit 202,22.92.0/22 le 24

> ip prefix-list prefix-filter-as100 seq 30 permit
> 202,21.148.0/22 le 24

> ip prefix-list prefix-filter-as100
> seq 40 permit 203,178.88.0/21 le 24

All the above prefix lists basically say, "consider the
prefix and all longer subnets of it up to a maximum length
of /24", e.g., 202.168.136.0/22, and any other subnet from
this prefix up to a maximum prefix length of /24.

> ip prefix-list
> prefix-filter-as100 seq 50 permit 178.88.74.0/24

This one basically matches only that particular /24. Any
subnets that are generated from this /24 will not be
considered by the router.

Hope this helps.

Cheers,

Mark.