BGP: CONCEPT: STATES, ATTRIBUTES, ROUTE MAPS


TOPICS

What is BGP

BGP states

BGP attributes

Route Maps

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

BGP STATES

Output Check:

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 ATTRIBUTE

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

Reference Link

http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html

 

It tells in which sequence attributes are compared to select the best path to a destination

 

ROUTE-MAP

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 syntax

route−map map−tag [permit | deny]  [sequence−number]

Example: route-map  routetest  permit 10

Here, routetest is simply the name of the route-map

Case:

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.

 

Exception:

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.

 

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