What is BGP
What is BGP?
BGP is a dynamic protocol that allows communication between two autonomous networks.
Example: ABC company wants to connect to Internet. Internet will be provided by the ISP.
For company ABC, ISP is a autonomous network(Unknown). So, To exchange routes between ABC company and ISP, BGP is required
If everything is fine, run the below mentioned command
Point to noted is, in the column State/PfxRcd, there should be a number, may be 0, doesn’t matter
It means BGP neighborship has gone through all the states and learned this much number of prefix or routes or subnets
BGP attributes is a metric used to describe the characteristics of a BGP paths. Attributes are contained in
update messages passed between BGP peers.
Mandatory attributes must be present in all update message passed between BGP peers.
Discretionary attributes may or may not be present in update message.
BGP BEST PATH SELECTION
It tells in which sequence attributes are compared to select the best path to a destination
Route-Map is a set of conditions(combination of match and set conditions), to control or modify routing information or traffic.
BGP route maps are of two types
those that support both match and set clauses,
and those that support only match clauses
Route-Map is more like a Function defined in programming Language, “C”
In C-Language, Function is defined using If and Then conditions. Whenever required during programming,
Function is simply called in that point
In BGP, Route-Map is defined using Match and set conditions. When required to modify traffic, Route-Map is called.
When Route-map is required?
BGP route maps can be applied to modify or control inbound routes, outbound routes, and redistributed routes.
route−map map−tag [permit | deny] [sequence−number]
Example: route-map routetest permit 10
Here, routetest is simply the name of the route-map
In route-map, we have option of permit and deny.
In route-map, Access list is used. In Access list, again we have the option of permit and deny
So, to avoid confusion, it is preferred to use permit in case of Route map
permit and deny in case of Access-List
How permit works?
If the match criteria are met and we have a permit then the routes will be controlled as specified by the set action and we break out of the list
How deny works?
If the match criteria are met and we have a deny then the route will not be controlled and we break out of the list.
If we finish the list without a match then the route we are looking at will not be accepted nor forwarded.
when route-map is used for filtering BGP updates rather than when redistributing between protocols, you can not filter on the inbound when using a “match” on the ip address.