BGP

BGP Weight Attribute Configure

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BGP Weight Attribute Configure

BGP Weight Attribute is a Cisco Proprietary attribute that influences a router how to reach a certain prefix. The difference between Local Preference and Weight is that the former is propagated within an AS and the latter is router locally significant. Weight can be used if there is one router connected to two or more AS’s or just to just one with two or more eBGP peers. Now, lets configure weight and later use a route-map for more complex use of weight. [boxads]

  • Weight is a Cisco Proprietary attribute for BGP that is “locally significant.”
  • Local Preference is a Well-known Discretionary attribute, Weight doesn’t belong to any category.
  • Default Weight for locally originated routes is 32768. Zero is the default for other routes.
  • Weight is not propagated to other routers within the AS.

In this lab, BGP Weight Attribute will be configured and will be using route map for more flexibility. Check the diagram below for details.

BGP Weight Attribute Cisco Proprietary attribute
BGP Weight Attribute Cisco Proprietary attribute

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BGP Attributes Categories

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BGP Attributes Categories

BGP Attributes Categories are 1) WELL-KNOWN, MANDATORY , 2) WELL-KNOWN, DISCRETIONARY, 3) OPTIONAL, TRANSITIVE, 4) OPTIONAL, NON-TRANSITIVE. details are: [boxads]

BGP Attributes Categories
BGP Attributes Categories

WELL-KNOWN, MANDATORY

AS-path: A list of the Autonomous Systems (AS) numbers that a route passes through to reach the destination. As the update passes through an AS the AS number is inserted at the beginning of the list. The AS-path attribute has a reverse-order list of AS passed through to get to the destination.

Next-hop: The next-hop address that is used to reach the destination.

Origin: Indicates how BGP learned a particular route. There are three possible types — IGP (route is internal to the AS), EGP (learned via EBGP), or Incomplete (origin unknown or learned in a different way).

WELL-KNOWN, DISCRETIONARY

Local Preference: Defines the preferred exit point from the local AS for a specific route.

Atomic Aggregate: Set if a router advertises an aggregate causes path attribute information to be lost.

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BGP Routing Protocol Overview

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BGP Routing Protocol Overview

Hi ! Today i will discuss about BGP Routing Protocol Overview. BGP is an Border gateway protocol, BGP mainly use for connect between different networks. It is the protocol used between Internet service providers (ISPs) and also can be used between an Enterprise and an ISP. BGP was built for reliability, scalability, and control. [boxads]

  • BGP stands for Border Gateway Protocol. Routers running BGP are termed BGP speakers.
  • BGP uses the concept of autonomous systems (AS). An autonomous system is a group of networks under a common administration. The Internet Assigned Numbers Authority (IANA) assigns AS numbers: 1 to 64511 are public AS numbers and 64512 to 65535 are private AS numbers.
  • Autonomous systems run Interior Gateway Protocols (IGP) within the system. They run an Exterior Gateway Protocol (EGP) between them. BGP version 4 is the only EGP currently in use.
  • Routing between autonomous systems is called interdomain routing.
  • The administrative distance for EBGP routes is 20. The administrative distance for IBGP routes is 200.
  • BGP neighbors are called peers and must be statically configured.
  • BGP uses TCP port 179. BGP peers exchange incremental, triggered route updates and periodic keepalives.
  • Routers can run only one instance of BGP at a time.
  • BGP is a path-vector protocol. Its route to a network consists of a list of autonomous systems on the path to that network.
  • BGP’s loop prevention mechanism is an autonomous system number. When an update about a network leaves an autonomous system, that autonomous system’s number is prepended to the list of autonomous systems that have handled that update. When an autonomous system receives an update, it examines the autonomous system list. If it finds its own autonomous system number in that list, the update is discarded.

BGP Databases

BGP uses three databases. The first two listed are BGP-specific; the third is shared by all routing processes on the router:

  • Neighbor database: A list of all configured BGP neighbors. To view it, use the show ip bgp summary command.
  • BGP database, or RIB (Routing Information Base): A list of networks known by BGP, along with their paths and attributes. To view it, use the show ip bgp command.
  • Routing table: A list of the paths to each network used by the router, and the next hop for each network. To view it, use the show ip route command.

BGP Message Types

BGP has four types of messages:

  • 1) Open: After a neighbor is configured, BGP sends an open message to try to establish peering with that neighbor. Includes information such as autonomous system number, router ID, and hold time.
  • 2) Update: Message used to transfer routing information between peers. Includes new routes, withdrawn routes, and path attributes.
  • 3) Notification: When a problem occurs that causes a router to end the BGP peering session, a notification message is sent to the BGP neighbor and the connection is closed.
  • 4) Keepalive: BGP peers exchange keepalive messages every 60 seconds by default. These keep the peering session active.
  • ROUTE-REFRESH: An optional message (negotiated during capability advertisement) that is sent to request dynamic BGP route updates from the Adj-RIB-Out table of a remote BGP speaker

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