752 BigIron RX Series Configuration Guide53-1001986-01Configuring confederations26When router ID comparison is enabled, the path comparison algorithm compares the router IDs ofthe neighbors that sent the otherwise equal paths.• If BGP4 load sharing is disabled (maximum-paths 1), the device selects the path that camefrom the neighbor with the lower router ID.• If BGP4 load sharing is enabled, the device load shares among the remaining paths. In thiscase, the router ID is not used to select a path.NOTERouter ID comparison is disabled by default.To enable router ID comparison, enter the following command at the BGP configuration level of theCLI.BigIron RX(config-bgp)# compare-routeridSyntax: [no] compare-routeridFor more information, refer to “How BGP4 selects a path for a route” on page 732.Configuring confederationsA confederation is a BGP4 Autonomous System (AS) that has been subdivided into multiple,smaller ASs. Subdividing an AS into smaller ASs simplifies administration and reduces BGP-relatedtraffic, thus reducing the complexity of the Interior Border Gateway Protocol (IBGP) mesh amongthe BGP routers in the AS.The Brocade implementation of this feature is based on RFC 3065.Normally, all BGP routers within an AS must be fully meshed, so that each BGP router has BGPsessions to all the other BGP routers within the AS. This is feasible in smaller ASs but becomesunmanageable in ASs containing many BGP routers.When you configure BGP routers into a confederation, all the routers within a sub-AS (a subdivisionof the AS) use IBGP and must be fully meshed. However, routers use EBGP to communicatebetween different sub-ASs.NOTEAnother method for reducing the complexity of an IBGP mesh is to use route reflection. However, ifyou want to run different Interior Gateway Protocols (IGPs) within an AS, configure a confederation.You can run a separate IGP within each sub-AS.To configure a confederation, configure groups of BGP routers into sub-ASs. A sub-AS is simply anAS. The term “sub-AS” distinguishes ASs within a confederation from ASs that are not in aconfederation. For the viewpoint of remote ASs, the confederation ID is the AS ID. Remote ASs donot know that the AS represents multiple sub-ASs with unique AS IDs.NOTEYou can use any valid AS numbers for the sub-ASs. If your AS is connected to the Internet, Brocaderecommends that you use numbers from within the private AS range (64512 – 65535). These areprivate ASs numbers and BGP4 routers do not propagate these AS numbers to the Internet.Figure 114 shows an example of a BGP4 confederation.
