Cisco 3845 Software Guide - Page 619

Chapter 29 Configuring IP Unicast Routing Configuring BGP • When multiple entries of a prefix list match a given prefix, the sequence number of a prefix list entry identifies the entry with the lowest sequence number. By default, sequence numbers are generated automatically and incremented in units of five. If you disable the automatic generation of sequence numbers, you must specify the sequence number for each entry. You can specify sequence values in any increment. If you specify increments of one, you cannot insert additional entries into the list; if you choose very large increments, you might run out of values. You do not need to specify a sequence number when removing a configuration entry. Show commands include the sequence numbers in their output. Before using a prefix list in a command, you must set up the prefix list. Beginning in privileged EXEC mode, follow these steps to create a prefix list or to add an entry to a prefix list: Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Command Purpose configure terminal Enter global configuration mode. ip prefix-list list-name [seq seq-value] deny | Create a prefix list with an optional sequence number to deny or permit network/len [ge ge-value] [le le-value] permit access for matching conditions. You must enter at least one permit or deny clause. • network/len is the network number and length (in bits) of the network mask. • (Optional) ge and le values specify the range of the prefix length to be matched.The specified ge-value and le-value must satisfy this condition: len < ge-value < le-value < 32 ip prefix-list list-name seq seq-value deny | (Optional) Add an entry to a prefix list, and assign a sequence permit network/len [ge ge-value] [le le-value] number to the entry. end Return to privileged EXEC mode. show ip prefix list [detail | summary] name Verify the configuration by displaying information about a prefix list [network/len] [seq seq-num] [longer] or prefix list entries. [first-match] copy running-config startup-config (Optional) Save your entries in the configuration file. To delete a prefix list and all of its entries, use the no ip prefix-list list-name global configuration command. To delete an entry from a prefix list, use the no ip prefix-list seq seq-value global configuration command. To disable automatic generation of sequence numbers, use the no ip prefix-list sequence number command; to reenable automatic generation, use the ip prefix-list sequence number command. To clear the hit-count table of prefix list entries, use the clear ip prefix-list privileged EXEC command. Configuring BGP Community Filtering One way that BGP controls the distribution of routing information based on the value of the COMMUNITIES attribute. A community is a group of destinations that share some common attribute. Each destination can belong to multiple communities. AS administrators can define to which communities a destination belongs. By default, all destinations belong to the general Internet OL-23400-01 Cisco ME 3800X and 3600X Switch Software Configuration Guide 29-53