CCNP Enterprise Core (350-401)

Local Preference is a well-known discretional path attribute. It is included with advertisements throughout an AS. It is the second preferred path attribute when deciding a best path, after weight. It’s default value is 100, and a higher value is a preferred value. Local preference is a 32-bit value, between 0 to 4,294,967,295 Local preference…

BGP Weight is a Cisco defined path attribute and the first step on selecting the best path in BGP. The path with the higher weight is preferred. Weight is a 16-bit value, between 0 to 65,535. A path that originates on the router has a default weight of 32,768. A path that originates from elsewhere…

BGP consists of two parts, Network Layer Reachability Information and Path Attributes. The path attributes are a huge factor in deciding the preferred path for traffic to take. On top of this only the best path is advertised to neighbours in BGP. BGP automatically installs the first route it receives for a new destination as…

Typically routers always select the path by examining the prefix length of as network entry, with the chosen path being based on prefix length. The longest prefix length always being preferred. The logic can be used to influence path selection in BG. Path attributes can be used to influence traffic too, but there may be…

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…