How does BGP use the AS path attribute?

A path attribute is a list of AS numbers that a route has traversed to reach a destination. BGP uses path attributes to provide more information about each route and to help prevent routing loops in an arbitrary topology.

What is filter list in BGP?

A filter list is a form of route policy that restricts the routes that will be advertised or accepted based on the AS-Path of the route. To configure a filter list, you must first create an AS-path access list based on the known paths you wish to permit.

How do I filter BGP routes?

You can combine route maps with prefix lists to filter the routes advertised to or received from a BGP peer, to control routes redistributed into BGP, and to set BGP attributes for specific routes. Prefix lists alone can be applied to a neighbor to filter route updates.

What is AS path filter?

Using the AS path filter we can permit or deny prefixes from certain autonomous systems. You can use this for things like: Accept only prefixes from directly connected autonomous systems. Accept only prefixes from directly connected autonomous systems AND one autonomous system behind the first one.

What is AS Path attribute?

AS Path Attribute is one of the mosth used BGP Path Attribute by Service Providers. In AS Path Attribute mechanism, whenever a route passes an AS (Autonomous System), it adds the number of AS it passed. So, AS Path Attribute is a list of AS numbers, that the router traverse.

What is an AS path?

The AS path allows us to see through which autonomous systems we have to travel to get to a certain destination and is also used in BGP for loop prevention. When a router sees its own AS number in the AS path, it will not accept the prefix.

What are BGP route maps?

Border Gateway Protocol (BGP) is one of the most important routing protocols used to exchange data between different autonomous systems (AS) on the Internet. Each of these individual systems are usually owned by an Internet Service Providers (ISPs) or other, major organization.

What is the difference between prefix list and distribution list?

The most notable and important difference is that a prefix-list allows you to filter networks also based on their subnet mask. ACLs used in distribute list filter networks only by network addresses but they do not perform matching on subnet mask (in other words, for an ACL used in distribute list, the networks 192.0.

What is AS path used for?

So the AS path lists all the ASes that need to be traversed to reach the location where the prefix that the path is attached to is advertised from. As such, a traceroute should encounter those same ASes. The main purpose of the AS path is to avoid loops.

What is BGP AS path prepend?

In the Border Gateway Protocol (BGP), prepending is a technique used to deprioritize a route by artificially increasing the length of the AS-PATH attribute by repeating an autonomous system number (ASN). Route selection in BGP prefers the shorter AS path length, assuming all other criteria are equal.

What is the BGP path selection criteria?

BGP Path Selection Criteria To do so, it examines the path attributes of any loop-free, synchronized (if synchronization is enabled) routes with a reachable next-hop in the following order: Choose the route with the highest weight. If weight is not set, choose the route with the highest local preference.

What is community list in BGP?

A BGP community list is used to create groups of communities to use in a match clause of a route map. A community list can be used to control which routes are accepted, preferred, distributed, or advertised, for example. You can also use a community list to set, append, or modify the communities of a route.

What is as PATH attribute in BGP?

AS Path Attribute is one of the mosth used BGP Path Attribute by Service Providers. In AS Path Attribute mechanism, whenever a route passes an AS (Autonomous System), it adds the number of AS it passed. So, AS Path Attribute is a list of AS numbers, that the router traverse.With this mechanism AS Path Attribute is also used…

How do I choose a BGP route for my router?

A BGP router will prefer routes that it installed into BGP itself over a route that another router installed in BGP. Prefer the path with the shortest AS path length. For example, AS path 1 2 3 is preferred over AS path 1 2 3 4 5.

How does as-path access-list work in BGP?

The as-path access-list works like the normal access-lists, there is a hidden “deny any” at the bottom. First we create the as-path access-list and then attach it to a route-map. In the BGP configuration you can attach the route-map to one of your BGP neighbors.

What is the default value of the BGP weight attribute?

The default value is 0 for all routes that are not originated by the local router. You can learn how it works in the BGP weight attribute lesson. The local preference is used within an autonomous system and exchanged between iBGP routers. We prefer the path with the highest local preference. The default value is 100.