Route Selection Using Policy Controls Implementing Changes in BGP Policy

Outline Overview Traditional Filtering Limitations BGP Soft Reconfiguration Cisco IOS Commands for Soft Reconfiguration Monitoring Soft Reconfiguration BGP Soft Reset Enhancement Route Refresh Using Route Refresh Monitoring Route Refresh Why Use Route-Maps as BGP Filters? Summary

Traditional Filtering Limitations All filters apply only to new incoming and outgoing updates. To change outbound routing policy, you have to resend BGP updates to your neighbors. To change inbound routing policy, you have to force your neighbor to resend the updates to you. The traditional mechanism is to clear BGP sessions.

Traditional Limitations of Clearing the BGP Session router# clear ip bgp {* | ip-address | peer-group-name} This command tears down the BGP session with all neighbors, a specific neighbor, or all neighbors in a peer group. All BGP routes are lost after the session is torn down; connectivity through the BGP neighbor is lost. A new session is re-established within 30 to 60 seconds. A full routing update is exchanged once the session is re-established, resulting in enforcement of new routing policy. Processing the full Internet routing table can take a long time. Clearing the BGP session is a very disruptive way to implement routing policies.

BGP Soft Reconfiguration Soft reconfiguration was introduced in Cisco IOS Software Release 11.2 to facilitate nondisruptive changes in BGP routing policies. Outbound soft reconfiguration resends the complete BGP table. Always enabled, not configurable Inbound soft reconfiguration stores the complete BGP table of your neighbor in router memory.

Soft Reconfiguration and Memory Use

Inbound Soft Reconfiguration Cisco IOS Commands router(config-router)# neighbor ip-address soft-reconfiguration inbound

Inbound Soft Reconfiguration Cisco IOS Commands (Cont.) router# clear ip bgp ip-address soft in

Outbound Soft Reconfiguration Cisco IOS Commands router# clear ip bgp ip-address soft out

Monitoring Soft Reconfiguration

BGP Soft Reset Enhancement Provides automatic support for dynamic soft reset of inbound BGP routing table updates that is not dependent upon stored routing table update information Requires no preconfiguration (as with the neighbor soft-reconfiguration command) Requires much less memory than the previous soft reset method for inbound routing table updates

Route Refresh Route refresh is a new BGP capability. It is used to request a neighbor to resend routing information. It is typically used after configuration changes to update the BGP table (route-map, distribute-list, prefix-list, filter-list, weight, local preference, MED, and so on). The traditional way of accomplishing this function is to clear the BGP session.

Route Refresh (Cont.) Inbound soft reconfiguration consumes memory on the receiving router. It is needed only because there is no mechanism in standard BGP to request retransmission of BGP routes. BGP route refresh is an optional BGP capability that allows a BGP router to request retransmission of BGP routes from a neighbor.

Route Refresh (Cont.) Step 1 - Route refresh is negotiated when the BGP session is established. Step 2 - Inbound routing policy is changed on RTR-B. Step 3 - Operator requests inbound route refresh. Step 4 - RTR-B sends route refresh message to RTR-A. Step 5 - RTR-A resends all BGP routes to RTR-B.

Using Route Refresh router# clear ip bgp {* | ip-address | peer-group-name } in Sends a route refresh message to the neighbor or neighbors Only works if the neighbor has previously advertised the route refresh capability

Monitoring Route Refresh show ip bgp neighbor neighbor Verifies the support for route refresh capability

Monitoring Route Refresh (Cont.) Debug output after BGP session reset

Monitoring Route Refresh (Cont.) Debug output after route refresh

Why Use Route-Maps as BGP Filters? Some scenarios require complex filters. Filters on IP prefixes coming from specific AS number Filters on other BGP attributes In some cases, network administrators even need to modify BGP attributes. Route-maps provide a solution to both requirements.

Summary Because of the huge volumes of routing information that BGP is capable of handling and the effects of a mass routing update, BGP cannot use traditional routing update methods. Soft reconfiguration provides the possibility to run all routes through filters without tearing down the sessions. The Cisco IOS commands that are required to configure and perform a soft reconfiguration include the neighbor soft-reconfiguration router configuration command, which configures Cisco IOS software to start storing updates and the clear ip bgp EXEC command, which resets a BGP connection using BGP soft reconfiguration.

Summary (Cont.) The Cisco IOS tools that are available to monitor the operation of a soft reconfiguration include the show ip bgp command, which displays the local BGP table, the show ip bgp neighbor ip-address routes command, which checks incoming information that is received from a neighbor, and the show ip bgp neighbor ip-address received command, which displays the information that is saved in the extra copy outside the filters. The BGP Soft Reset Enhancement feature provides automatic support for dynamic soft reset of inbound BGP routing table updates that is not dependent upon stored routing table update information. This method requires no preconfiguration and needs much less memory than the previous soft reset method for inbound routing table updates.

Summary (Cont.) Route refresh is a new BGP capability that is used to request a neighbor to resend routing information after configuration changes. The clear ip bgp ip-address soft in command sends a route refresh message to the neighboring router and executes if the neighbor has previously advertised the route refresh capability. To verify that a neighbor supports route refresh, you can use the show ip bgp neighbor command. To display the negotiation process, you can use the debug ip bgp command. Network administrators cannot achieve certain complex filtering goals by using a prefix-list only or by using an AS-path filter list only. A route-map is a powerful filtering tool that can also modify routing information.