No relevant resource is found in the selected language.

This site uses cookies. By continuing to browse the site you are agreeing to our use of cookies. Read our privacy policy>Search

Reminder

To have a better experience, please upgrade your IE browser.

upgrade

ME60 V800R010C10SPC500 Feature Description - WAN Access 01

This is ME60 V800R010C10SPC500 Feature Description - WAN Access
Rate and give feedback:
Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
BGP Fundamentals

BGP Fundamentals

BGP Operating Modes

BGP is called Internal BGP (IBGP) when it runs within an AS; it is called External BGP (EBGP) when it runs between ASs, as shown in Figure 9-2.

Figure 9-2 BGP operating modes

Roles in Transmitting BGP Messages

  • Speaker: Any ME device that sends BGP messages is called a BGP speaker. The speaker receives or generates new routing information and then advertises the routing information to other BGP speakers. After receiving a route from another AS, a BGP speaker compares the route with its local routes. If the route is better than its local routes, or the route is new, the speaker advertises this route to all other BGP speakers.

  • Peer: BGP speakers that exchange messages with each other are called peers.

BGP Messages

BGP runs by sending five types of messages: Open, Update, Notification, Keepalive, and Route-refresh.

  • Open: The first message sent after a TCP connection is set up is an Open message, which is used to set up BGP peer relationships. After a peer receives an Open message and the peer negotiation is successful, the peer sends a Keepalive message to confirm and maintain the peer relationship. Then, peers can exchange Update, Notification, Keepalive, and Route-refresh messages.

  • Update: This type of message is used to exchange routes between BGP peers.

    • An Update message can advertise multiple reachable routes with the same attributes. These route attributes are applicable to all destination addresses (expressed by IP prefixes) in the Network Layer Reachability Information (NLRI) field of the Update message.

    • An Update message can be used to delete multiple unreachable routes. Each route is identified by its destination address (using the IP prefix), which identifies the routes previously advertised between BGP speakers.

    • An Update message can be used only to delete routes. In this case, it does not need to carry the route attributes or NLRI. In addition, an Update message can be used only to advertise reachable routes. In this case, it does not need to carry information about the deleted routes.

  • Notification: When BGP detects an error, it sends a Notification message to its peer. The BGP connection is then torn down immediately.

  • Keepalive: BGP periodically sends Keepalive messages to peers to maintain peer relationships.

  • Route-refresh: This type of message is used to request that the peer resend all reachable routes.

    If all BGP routers are enabled with the Route-refresh capability and the import policy of BGP changes, the local BGP router sends a Route-refresh message to its peers. After receiving the Route-refresh message, the peers resend their routing information to the local BGP router. In this manner, BGP routing tables are dynamically refreshed and new routing policies are used without tearing down BGP connections.

BGP Finite State Machine

The BGP Finite State Machine (FSM) has six states: Idle, Connect, Active, OpenSent, OpenConfirm, and Established.

Three common states during the establishment of BGP peer relationships are Idle, Active, and Established.

  • In the Idle state, BGP denies all connection requests. This is the initial status of BGP.

  • In the Connect state, BGP decides subsequent operations after a TCP connection is established.

  • In the Active state, BGP attempts to establish a TCP connection.

  • In the OpenSent state, BGP is waiting for an Open message from the peer.

  • In the OpenConfirm state, BGP is waiting for a Notification or Keepalive message.

  • In the Established state, BGP peers can exchange Update, Route-refresh, Keepalive, and Notification messages.

The BGP peer relationship can be established only when both BGP peers are in the Established state. Both peers send Update messages to exchange routes.

BGP Processing

  • BGP adopts TCP as its transport layer protocol. Therefore, a TCP connection must be available between the peers. BGP peers negotiate parameters by exchanging Open messages to establish a BGP peer relationship.

  • After the peer relationship is established, BGP peers exchange BGP routing tables. BGP does not periodically update a routing table. When BGP routes change, BGP updates the changed BGP routes in the BGP routing table by sending Update messages.

  • BGP sends Keepalive messages to maintain the BGP connection between peers.

  • After detecting an error on a network, BGP sends a Notification message to report the error and the BGP connection is torn down.

BGP Attributes

BGP route attributes are a set of parameters that describe specific BGP routes. With BGP route attributes, BGP can filter and select routes. BGP route attributes are classified into the following types:

  • Well-known mandatory: This type of attribute can be identified by all BGP routers and must be carried in Update messages. Without this attribute, errors occur in the routing information.

  • Well-known discretionary: This type of attribute can be identified by all BGP routers. This type of attribute is optional and, therefore, is not necessarily carried in Update messages.

  • Optional transitive: This indicates the transitive attribute between ASs. A BGP router may not recognize this attribute, but the router still receives it and advertises it to other peers.

  • Optional non-transitive: If a BGP router does not recognize this type of attribute, the router does not advertise it to other peers.

The most common BGP route attributes are as follows:

  • Origin

    The Origin attribute defines the origin of a route. The Origin attribute is classified into the following types:

    • Interior Gateway Protocol (IGP): This attribute type has the highest priority. IGP is the Origin attribute for routes obtained through an IGP in the AS from which the routes originate. For example, the Origin attribute of the routes imported to the BGP routing table using the network command is IGP.

    • Exterior Gateway Protocol (EGP): This attribute type has the second highest priority. The Origin attribute of the routes obtained through EGP is EGP.

    • Incomplete: This attribute type has the lowest priority. Incomplete is the Origin attribute type of all routes that do not have the IGP or EGP Origin attribute. For example, the Origin attribute of the routes imported using the import-route command is Incomplete.

  • AS_Path

    The AS-Path attribute records all ASs through which a route passes from the local end to the destination in distance-vector (DV) order.

    When a BGP speaker advertises a local route:

    • When advertising the route beyond the local AS, the BGP speaker adds the local AS number to the AS_Path list and then advertises it to the neighboring ME devices through Update messages.

    • When advertising the route within the local AS, the BGP speaker creates an empty AS_Path list in an Update message.

    When a BGP speaker advertises a route learned from the Update messages of another BGP speaker:

    • When advertising the route beyond the local AS, the BGP speaker adds the local AS number to the left of the AS_Path list. From the AS_Path attribute, the BGP router that receives the route learns the ASs through which the route passes to the destination. The number of the AS that is nearest to the local AS is placed on the left of the list, whereas other AS numbers are listed in sequence.

    • When advertising the route within the local AS, the BGP speaker does not change the AS_Path attribute.

    The AS_Path attribute has four types:
    • AS_Sequence: records in reverse order all the ASs through which a route passes from the local device to the destination.
    • AS_Set: records without an order all the ASs through which a route passes through from the local device to the destination. The AS_Set attribute is used in route summarization scenarios. After route summarization, the device records the unsequenced AS numbers because it cannot sequence the numbers of ASs through which specific routes pass. No matter how many AS numbers an AS_Set contains, BGP regards the AS_Set as one AS number when calculating routes.
    • AS_Confed_Sequence: records in reverse order all the sub-ASs within a BGP confederation through which a route passes from the local device to the destination.
    • AS_Confed_Set: records without an order all the sub-ASs within a BGP confederation through which a route passes from the local device to the destination. The AS_Confed_Set attribute is used in route summarization scenarios in a confederation.

    The AS_Confed_Sequence and AS_Confed_Set attributes are used to prevent routing loops and to select routes among the various sub-ASs in a confederation.

  • Next_Hop

    Different from the Next_Hop attribute in an IGP, the Next_Hop attribute in BGP is not necessarily the IP address of a neighboring ME device. In most cases, the Next_Hop attribute in BGP complies with the following rules:

    • When advertising a route to an EBGP peer, a BGP speaker sets the Next_Hop of the route to the address of the local interface through which the BGP peer relationship is established.

    • When advertising a locally generated route to an IBGP peer, a BGP speaker sets the Next_Hop of the route to the address of the local interface through which the BGP peer relationship is established.

    • When advertising a route learned from an EBGP peer to an IBGP peer, the BGP speaker does not change the Next_Hop of the route.

  • MED

    The Multi-Exit-Discriminator (MED) is transmitted only between two neighboring ASs. The AS that receives the MED does not advertise it to a third AS.

    Similar to the cost used by an IGP, the MED is used to determine the optimal route when traffic enters an AS. When a BGP peer learns multiple routes that have the same destination address but different next hops from EBGP peers, the route with the smallest MED value is selected as the optimal route if all other attributes are the same.

  • Local_Pref

    The Local_Pref attribute indicates the BGP priority of a route. It is available only to IBGP peers and is not advertised to other ASs.

    The Local_Pref attribute is used to determine the optimal route when traffic leaves an AS. When a BGP ME device obtains multiple routes to the same destination address but with different next hops through IBGP peers, the route with the largest Local_Pref value is selected.

Translation
Download
Updated: 2019-01-04

Document ID: EDOC1100059473

Views: 15642

Downloads: 10

Average rating:
This Document Applies to these Products
Related Version
Related Documents
Share
Previous Next