© 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

© 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

© 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.

© 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]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.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-5

show ip bgp neighbors [address]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

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-6

Monitoring the BGP Maximum-Prefix Function (Cont.)

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

Monitoring the BGP Maximum-Prefix Function (Cont.)

© 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.

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