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BGP Subnets

The BGP subnets list can be used to list prefixes, or subnetworks (whose router configuration files are unavailable) originated from a particular router or AS node. Various BGP attributes associated with the subnetwork can be defined in the subnet file.

Note:

If useliveBGPrtbl=1 is set in the dparam file, or in Design > Design Options > Path Placement > BGP, then the subnets information will be ignored.

  1. The subnet file can be viewed from the File Manager or from Network > Protocols > BGP > BGP Subnets... menu. To add, modify, or delete BGP subnets in the subnet file, navigate to Network > Protocols > BGP > BGP Subnets... to bring up the BGP Subnets window. The following figure shows a subnet entry for AS node, AS1301, being modified.
    Figure 1: Modifying a BGP Subnet Graphical user interface for configuring BGP subnets, showing a table of BGP subnets, a configuration panel for adding or modifying subnets, and a dialog for editing subnet details.

  2. Note the Protocol field, which defaults to bgp. Specifying “bgp” indicates that this is the prefix advertised from the router. In-policies still need to be applied to this route by the router receiving the route. Specifying “bgptbl” in this field indicates the route that is in the router’s routing table. It has already been accepted by the router’s in policy, but may or may not be the preferred route. This option is used for routes received from other Autonomous Systems, since their configuration files may not be available.

    To illustrate how to use the BGP subnet list (accessed via Network > Protocols > BGP > BGP Subnets...), a sample network and the corresponding BGP subnet list are shown in the following two figures. Note that within the BGP subnet list, ASnode AS1301 is declaring that it can reach subnet 10.10.1.0/24, which has an AS_PATH attribute that includes 65500. ASnode AS1301 is also declaring that it can reach subnet 10.10.2.0/24, which has an AS_PATH attribute that includes 65002.

    Figure 2: View BGP Subnets WindowGUI for managing BGP subnets showing node name, prefix, mask, protocol, MED, route-map, community, AS-path, VRF, with filter and edit options.
    Figure 3: Main View and BGP View of the networkTwo network topology maps showing BGP configuration. Left: Standard view with routers RTA, RTB, RTC, RTD, RTE, RTG, RTH in AS1301, AS65100, AS65200, AS65500, AS1401. Blue lines indicate internal AS connections; red lines indicate external AS connections. Right: BGP view with red for EBGP, green for IBGP, blue for non-BGP connections, highlighting BGP relationships.

  3. To see the BGP InPolicy defined at a router RTE, bring up the BGP Neighbors table and switch to the In Policy tab, as shown in the following figure. In this particular example, the InPolicy at router RTE is defined by a Cisco route-map and says that if an incoming route has 65001 included in its AS_PATH attribute, then set the LOCAL_PREF attribute to 123; otherwise, set the LOCAL_PREF attribute to 89. The InPolicy at router RTG is the same except that 65002 is matched for instead of 65001.

    Figure 4: BGP In Policy for RTEScreenshot of a network tool showing BGP neighbors with details like Node, AS, Interface, and applied routing policies.

  4. Continuing with our example, we bring up our BGP routing table to verify that the LOCAL_PREF attribute got set correctly to 123 for AS1301’s subnetwork 10.10.2.0/24, which has 65002 included in its AS_PATH attribute.

    Figure 5: BGP Routing table from RTH to AS1301 subnet 10.10.2.0/24Graphical interface for analyzing BGP routing tables with options to specify source and destination nodes, display routing info, and view route properties.
    Note:

    In Internet routing, community is another commonly-used attribute to tag a particular route. Each service provider can define its own policy based on this attribute of the incoming route. The subnet file helps the user to simulate routing behavior to various Internet destinations.

  5. Finally, we can do a path trace from a router, say RTH, in AS65500 (which includes routers RTH, RTE, RTG) to AS1301’s subnetwork 10.10.2.0 and verify that RTG is indeed the preferred exit point for AS65500, as indicated by the higher LOCAL_PREF value of 123. The following figure shows the path trace.

    Figure 6: Path trace illustrating the RTG being the preferred exit pointNetwork topology diagram showing routers RTA, RTB, RTC, RTD, RTE, RTF, RTH, and RTG in AS 65501, AS 65100, AS 65200, and AS 65500. Analyzing traffic demand between RTH and AS1301.