Advanced BGP - Conditional Matching

6 important questions on Advanced BGP - Conditional Matching

What two categories of Access Control Lists (ACLs) are there?

Standard ACL: Define packets solely on the source network.
Extended ACL: Define packets on Source, Destination, Protocol, Port or other packet attributes.

Both Standard and Extended ACLs can be Named or Numbered. What numbers can be given to the ACLs?

1-99 and 1300-1999 for standard ACLs
100-199 and 2000-2699 for extended ACLs

When using ACLs for IGP Network Selection, which type of ACL is used and how?

Extended ACL is used, where the Source host is the network and the Destination host is the subnet mask.
So permit ip host host means the network is selected.
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How do Extended ACLs work when used for matching BGP Routes?

When using Extended ACLs for matching BGP routes the Source fields specify a network and the Destination field specifies which Subnets are allowed to be selected within that network.

permit ip

This means look in the network for network matches that have a subnet mask of /25 to /32.

Instead of ACLs to identify networks in a routing protocol, a simpler option would be to use Prefix Matching. How does Prefix Matching work?

It is done by using a network with a prefix to specify the network and a le for Equal or Lesser or ge for Equal or Greater to specify the specific subnets to match. ge 22 le 26
Means any subnet in the range with a mask equal or greater then 22 and equal or lesser than 26

How are Regular Expressions used in BGP when identifying routes?

When matching Networks is to complex, another option would be to match all networks from a specific ASN.
This matching is done user Regular Expressions against the AS Number.

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