1. © 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-1 Optimizing BGP Scalability Limiting the Number of Prefixes Received from a BGP Neighbor

2. © 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-2 Outline Overview Limiting the Number of Routes Received from a Neighbor Configuring the BGP Maximum-Prefix Function Monitoring the BGP Maximum-Prefix Function Summary

3. © 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-3 Limiting the Number of Routes Received from a Neighbor Definition of problem: All other filtering mechanisms specify only what you are willing to accept but not how much. A misconfigured BGP neighbor can send a huge number of prefixes that can exhaust the memory of a router or overload the CPU (several Internet-wide incidents have already occurred). A new tool is needed to establish a hard limit on the number of prefixes received from a neighbor.

4. © 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-4 Configuring the BGP Maximum-Prefix Function neighbor ip-address maximum-prefix maximum [threshold] [warning-only][restart restart-interval] router(config-router)# This command controls how many prefixes can be received from a neighbor. The optional threshold parameter specifies the percentage where a warning message is logged (default is 75%). The optional warning-only keyword specifies the action on exceeding the maximum number (default is to drop the neighbor relationship). The optional restart keyword instructs the router to try to re- establish the session after the specified interval in minutes.

5. © 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-5 show ip bgp neighbors [address] router> For neighbors with the maximum-prefix function configured, displays the maximum number of prefixes and the warning threshold For neighbors exceeding the maximum number of prefixes, displays the reason that the BGP session is idle Monitoring the BGP Maximum-Prefix Function

6. © 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-6 Monitoring the BGP Maximum-Prefix Function (Cont.)

7. © 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-7 Monitoring the BGP Maximum-Prefix Function (Cont.)

8. © 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-8 Summary An improperly configured filter in a customer router may accidentally cause a large number of Internet routes to be received by the customer. The neighbor maximum-prefix command allows you to configure a maximum number of prefixes that a BGP router is allowed to receive from a peer. When the number of received prefixes exceeds the maximum number configured, the router either terminates the peering (by default) or sends a log message but continues peering with the sender. You can use the show ip bgp neighbors command to monitor the status of BGP neighbors, displaying information about the number of prefixes that a BGP router has received from a neighbor and if any limits have been configured.

9. © 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-9