BGP4

In this section:

  1. BGP4 Overview
  2. BGP4 Peering
  3. BGP4 Message Types
  4. BGP4 attributes
  5. BGP4 Best Path Selection Algorithm
    The BGP decision process is applied to the routes contained in the Routing Information Base, Incoming (RIB-In) which contains routes learned from inbound update messages. The output of the decision process is the set of routes that will be advertised to BGP speakers in local or remote autonomous systems and are stored in the Adjacency RIB, Outgoing (RIB-Out).
  6. Implementation of BGP4
  7. Device ID
  8. BGP global mode
  9. Configuring a local AS number
    The local AS number (ASN) identifies the AS in which the BGP device resides. The following task configures the local ASN in which the device resides.
  10. Neighbor configuration
  11. Peer Groups
  12. Advertising the default BGP4 route
    By default, a BGP device does not originate and advertise a default route using BGP4. A BGP4 default route is the IP address 0.0.0.0 and the route prefix 0 or network mask 0.0.0.0. For example, 0.0.0.0/0 is a default route. A BGP device can be configured to advertise the default IPv4 route to all BGP4 neighbors and to install that route in the local BGP4 route table.
  13. Four-byte AS numbers
  14. Cooperative BGP4 route filtering
  15. BGP4 Parameters
  16. Route Redistribution
  17. Advertised Networks
  18. Importing routes into BGP4
    Routes can be explicitly specified for advertisement by BGP.
  19. Route reflection
  20. Route flap dampening
    A route flap is a change in the state of a route, from up to down or down to up. A route state change causes changes in the route tables of the devices that support the route.
  21. Aggregating routes advertised to BGP neighbors
    A device can be configured to aggregate routes in a range of networks into a single IP prefix.
  22. Advertising the default BGP4 route
    By default, a BGP device does not originate and advertise a default route using BGP4. A BGP4 default route is the IP address 0.0.0.0 and the route prefix 0 or network mask 0.0.0.0. For example, 0.0.0.0/0 is a default route. A BGP device can be configured to advertise the default IPv4 route to all BGP4 neighbors and to install that route in the local BGP4 route table.
  23. Advertising the default BGP4 route to a specific neighbor
    A BGP device can be configured to advertise the default IPv4 route to a specific neighbor.
  24. Multipath load sharing
    Unlike IGP, BGP does not perform multipath load sharing by default. Therefore, the maximum number of paths across which BGP can balance the traffic is set to 1 by default. You can change this value by using the maximum-paths command.
  25. Specifying the Weight Added to Received Routes
    The weight that the device adds to received routes can be specified. The following task changes the weight from the default for routes that are received from a specified BGP neighbor.
  26. Using the IPv4 Default Route as a Valid Next Hop for a BGP4 Route
    In certain cases, such as when a device is acting as an edge device, it can be configured to use the default route as a valid next hop.
  27. Adjusting defaults to improve routing performance
    The following examples illustrate a variety of options for enabling and fine-tuning route flap dampening.
  28. Next-hop recursion
    For each BGP4 route learned, the device performs a route lookup to obtain the IPv4 address of the next hop for the route. A BGP4 route is eligible for addition in the IPv4 route table only if the following conditions are true:
  29. Route Filtering
  30. BGP Regular Expression Pattern-Matching Characters
    The following table illustrates the functions of BGP regular expression pattern-matching characters and illustrates their use.
  31. Timers
  32. BGP4 outbound route filtering
    The BGP4 Outbound Route Filtering Capability (ORF) feature is used to minimize the number of BGP updates sent between BGP peers.
  33. BGP4 Confederations
    A large autonomous system (AS) can be divided into multiple subautonomous systems and grouped into a single BGP4 confederation.
  34. BGP community and extended community
    A BGP community is a group of destinations that share a common property. Community information identifying community members is included as a path attribute in BGP UPDATE messages. You can perform actions on a group using community and extended community attributes to trigger routing decisions.
  35. BGP4 graceful restart
    BGP4 graceful restart (GR) allows for restarts where BGP neighboring devices participate in the restart, helping to ensure that no route and topology changes occur in the network for the duration of the restart.
  36. Generalized TTL Security Mechanism support
    Generalized TTL Security Mechanism (GTSM) is a lightweight security mechanism that protects external Border Gateway Protocol (eBGP) peering sessions from CPU utilization-based attacks using forged IP packets. GTSM prevents attempts to hijack the eBGP peering session by a host on a network segment that is not part of either BGP network, or by a host on a network segment that is not between the eBGP peers.
  37. Disabling the BGP AS_PATH Check Function
    A device can be configured so that the AS_PATH check function for routes learned from a specific location is disabled, and routes that contain the recipient BGP speaker's AS number are not rejected.
  38. Matching on a destination network
    A route map that matches on a destination network can be configured.
  39. Matching on a Next-Hop Device
    A route map that matches on a next-hop device can be configured.
  40. Route Map Continue Statement for BGP4 Routes
    A continue statement in a route map directs program flow to skip over route map instances to another, user-specified instance. If a matched instance contains a continue statement, the system looks for the instance that is identified in the statement.
  41. Clearing diagnostic buffers
    The device stores the following BGP4 diagnostic information in buffers:
  42. Displaying BGP4 Statistics
    Various show ip bgp commands verify information about BGP4 configurations.
  43. Displaying BGP4 Neighbor Statistics
    Various show ip bgp neighbors commands verify information about BGP4 neighbor configurations.