Routing

When a BGP filtering method has been changed, the BGP session may need to be cleared for the new filtering method to take effect. Typically when a BGP policy changes, the BGP table must be processed by the router for the filtering changes to take effect. There are two methods of clearing a BGP session,…

Route maps can filter routes in or out to neighbours. Route maps can also provide attribute modification properties to routes advertised outbound or inbound too. Route maps are applied on a per neighbour basis, and different route maps can be activated for inbound and outbound directions. ip prefix-list lan-addresses 192.168.0.0/16 ge 16 ip prefix-list public-addresses…

Selecting routes from a BGP neighbour by using the AS path requires an AS Path Access Control List. An AS Patch Access Control List is very different from a standard or extended access control list. An AS Path ACL utilises regular expressions as part of their filtering of routes based on the neighbours AS Path…

Prefix list filtering allows filtering of routes on a specific neighbour basis by using a prefix list. To create a prefix list, use the command ip prefix-list then apply it to a neighbour using the neighbour x.x.x.x prefix-list y followed by in or out. In will filter inbound, and out will filter outbound. ! Allow…

Distribute List filtering allows filtering of network prefixes on BGP using access control lists on a neighbour specific basis. The distribution list can be activated on a neighbour with the command neighbour x.x.x.x distribute-list y followed by in for inbound filtering or out for outbound filtering ip access-list extended restrict-routes ! Allows 192.168.0.0 – 192.168.255.255…

BGP route filtering is a method of selectively identifying and selecting routes that are advertised from BGP neighbours. By filtering routes, traffic flows can be manipulated. A common scenario of route filtering is from ISPs to customers. An ISP will implement route filtering to ensure only the customers prefixes are allowed over the BGP link…

Route maps can filter networks much in the same way as access control lists or prefix lists, and much more. Route maps have additional capabilities such as allowing the modification or addition of network attributes in BGP. To modify or add network attributes in BGP, a route-map must be referenced by the routing protocol. This…

There can be instances where identifying all the networks from a certain provider may be too complicated or numerous, and a regular expression may be more suitable to filter all networks from a certain AS. Regular expression can be used for this. Quick guide to regular expression Modifier Purpose _ Matches a space ^ Indicates…

Prefix lists provide an alternative method to matching networks with a routing protocol. The prefix list can identify a specific IP address, network, or an entire network range. It is flexible enough to allow of selection of multiple network ranges too. Prefix matching in it’s simplest form can be defined with a 192.168.1.0/24, where it…