Dell Brocade DCX, Configuration Manual

Page: 832 / 1436

760 BigIron RX Series Configuration Guide
53-1001810-01
Configuring confederations
26
When router ID comparison is enabled, the path comparison algorithm compares the router IDs of
the neighbors that sent the otherwise equal paths.
•
If BGP4 load sharing is disabled (maximum-paths 1), the device selects the path that came
from the neighbor with the lower router ID.
•
If BGP4 load sharing is enabled, the device load shares among the remaining paths. In this
case, the router ID is not used to select a path.
NOTE
Router ID comparison is disabled by default.
To enable router ID comparison, enter the following command at the BGP configuration level of the
CLI.
BigIron RX(config-bgp)# compare-routerid
Syntax: [no] compare-routerid
For more information, refer to “How BGP4 selects a path for a route” on page 740.
Configuring confederations
A 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-related
traffic, thus reducing the complexity of the Interior Border Gateway Protocol (IBGP) mesh among
the 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 BGP
sessions to all the other BGP routers within the AS. This is feasible in smaller ASs but becomes
unmanageable in ASs containing many BGP routers.
When you configure BGP routers into a confederation, all the routers within a sub-AS (a subdivision
of the AS) use IBGP and must be fully meshed. However, routers use EBGP to communicate
between different sub-ASs.
NOTE
Another method for reducing the complexity of an IBGP mesh is to use route reflection. However, if
you 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 an
AS. The term “sub-AS” distinguishes ASs within a confederation from ASs that are not in a
confederation. For the viewpoint of remote ASs, the confederation ID is the AS ID. Remote ASs do
not know that the AS represents multiple sub-ASs with unique AS IDs.
NOTE
You can use any valid AS numbers for the sub-ASs. If your AS is connected to the Internet, Brocade
recommends that you use numbers from within the private AS range (64512 – 65535). These are
private ASs numbers and BGP4 routers do not propagate these AS numbers to the Internet.