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9.8.1 BGP Community Lists

BGP community list is a user defined BGP communites attribute list. BGP community list can be used for matching or manipulating BGP communities attribute in updates.

There are two types of community list. One is standard community list and another is expanded community list. Standard community list defines communities attribute. Expanded community list defines communities attribute string with regular expression. Standard community list is compiled into binary format when user define it. Standard community list will be directly compared to BGP communities attribute in BGP updates. Therefore the comparison is faster than expanded community list.

— Command: ip community-list standard name {permit|deny} community

This command defines a new standard community list. community is communities value. The community is compiled into community structure. We can define multiple community list under same name. In that case match will happen user defined order. Once the community list matches to communities attribute in BGP updates it return permit or deny by the community list definition. When there is no matched entry, deny will be returned. When community is empty it matches to any routes.

— Command: ip community-list expanded name {permit|deny} line

This command defines a new expanded community list. line is a string expression of communities attribute. line can include regular expression to match communities attribute in BGP updates.

— Command: no ip community-list name
— Command: no ip community-list standard name
— Command: no ip community-list expanded name

These commands delete community lists specified by name. All of community lists shares a single name space. So community lists can be removed simpley specifying community lists name.

— Command: show ip community-list
— Command: show ip community-list name

This command display current community list information. When name is specified the specified community list's information is shown. 

          # show ip community-list
          Named Community standard list CLIST
              permit 7675:80 7675:100 no-export
              deny internet
          Named Community expanded list EXPAND
              permit :
          
          # show ip community-list CLIST
          Named Community standard list CLIST
              permit 7675:80 7675:100 no-export
              deny internet