Large-Community Attribute
A BGP community attribute is a set of destination addresses that have the same characteristics. The attribute is expressed as a list of one or more 4-byte values. Generally, the community attribute on the NetEngine 8000 F is in the format of aa:nn, where aa indicates an Autonomous System Number (ASN) and nn indicates an attribute ID defined by the network administrator. Due to the use of 4-byte ASNs, the BGP community attribute can no longer accommodate both an ASN and attribute ID. In addition, the community attribute offers limited flexibility because only one attribute ID is available. To address these shortcomings, the Large-Community attribute is defined. This attribute extends and can be used together with the community attribute. The Large-Community attribute is a set of one or more 12-byte values, each of which is in the format of Global Administrator:LocalData1:LocalData2.
The Global Administrator field is intended to represent a complete 4-byte ASN, but it can also be set to a value other than an ASN. Global Administrator, LocalData1, and LocalData2 are unsigned integers each ranging from 0 to 4294967295. The administrator can set Global Administrator, LocalData1, and LocalData2 according to requirements.
The Large-Community attribute can represent a 2-byte or 4-byte ASN, and has two 4-byte LocalData IDs. This allows an administrator to apply route-policies more flexibly. For example, the Large-Community attribute can be set to ME:ACTION:YOU or ASN:Function:Parameter.
Networking Application
On the network shown in Figure 1, Device B establishes an EBGP peer relationship with each of Device A and Device C. By setting the Large-Community attribute to 20:4:30 on Device B, you can disable the routes on Device A from being advertised to AS30.
