© 2008 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialBSCI 1 Route Filtering and Route Selection in BGP Route Filtering and Route Selection in BGP © 2008 Cisco Systems, Inc. All rights reserved.BSCI 2 CCNP – BSCI Bachkhoa Networking Academy Học viện mạng Bách Khoa - www.bkacad.com Objectives Upon completion of this lesson, you will be able to perform the following tasks: – Describe Multi-Home BGP Networks – Describe Transit BGP Networks – Filter incoming and outgoing BGP updates route- maps – Influence BGP route selection – Monitor and troubleshoot BGP filters – Implement non-disruptive BGP policy changes – Limit the number of routes received from a BGP neighbor © 2008 Cisco Systems, Inc. All rights reserved.BSCI 3 CCNP – BSCI Bachkhoa Networking Academy Học viện mạng Bách Khoa - www.bkacad.com Multi-Homed BGP Networks www.cisco.com © 2008 Cisco Systems, Inc. All rights reserved.BSCI 4 CCNP – BSCI Bachkhoa Networking Academy Học viện mạng Bách Khoa - www.bkacad.com Multi-homed Customers Business Requirements Some customers need redundant Internet access for their mission-critical applications Full redundancy is only achieved by connecting to two independent Service Providers Internet Multi-homed Customer Service Provider #2 Service Provider #1 © 2008 Cisco Systems, Inc. All rights reserved.BSCI 5 CCNP – BSCI Bachkhoa Networking Academy Học viện mạng Bách Khoa - www.bkacad.com Multi-homed Customers Technical Requirements Multi-homed customers have to run BGP with the Internet Service Providers They usually need public AS-number and provider- independent address space Internet Multi-homed Customer Service Provider #2 Service Provider #1 © 2008 Cisco Systems, Inc. All rights reserved.BSCI 6 CCNP – BSCI Bachkhoa Networking Academy Học viện mạng Bách Khoa - www.bkacad.com Multi-homed Customers Simple-Minded Approach Configure two BGP sessions and announce your address space Internet Multi-homed Customer Service Provider #2 Service Provider #1 router bgp my-AS-number neighbor provider-A remote-as ISP-A neighbor provider-B remote-as ISP-B network my-network © 2008 Cisco Systems, Inc. All rights reserved.BSCI 7 CCNP – BSCI Bachkhoa Networking Academy Học viện mạng Bách Khoa - www.bkacad.com The Results of Simple-Minded Approach BGP routes are selected based on AS-path length The default BGP route selection does not always result in optimum routing as123#show ip bgp BGP table version is 16, local router ID is 1.2.3.4 Status codes: s suppressed, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 1.0.0.0 0.0.0.0 0 32768 i * 21.0.0.0 3.4.5.6 0 37 21 i *> 2.3.4.5 0 0 21 i *> 37.0.0.0 3.4.5.6 0 0 37 i * 2.3.4.5 0 21 37 i * 40.0.0.0 3.4.5.6 0 0 37 40 i *> 2.3.4.5 0 21 40 i © 2008 Cisco Systems, Inc. All rights reserved.BSCI 8 CCNP – BSCI Bachkhoa Networking Academy Học viện mạng Bách Khoa - www.bkacad.com Routing Policies for Multi-Homed Customers Multi-homed customers could require a number of routing policies, for example: – One provider is primary, the other is backup – Traffic to direct customers of the ISPs go direct, all other traffic goes through the primary provider – All transatlantic traffic goes through one ISPs – Traffic toward a specific destination goes only through one of the ISPs © 2008 Cisco Systems, Inc. All rights reserved.BSCI 9 CCNP – BSCI Bachkhoa Networking Academy Học viện mạng Bách Khoa - www.bkacad.com Primary/Backup Provider Internet traffic always flows over primary ISP Routes received from primary ISP should be preferred over routes received from backup ISP A route selection tool is needed in BGP - weights or local preference Internet Multi-homed Customer Backup ISP Primary link Upstream AS Another Customer Primary ISP © 2008 Cisco Systems, Inc. All rights reserved.BSCI 10 CCNP – BSCI Bachkhoa Networking Academy Học viện mạng Bách Khoa - www.bkacad.com Local Traffic Goes Direct Internet traffic flows over primary ISP, traffic to customers of backup ISP goes direct Route selection has to be performed based on AS- numbers in the AS-path Internet Multi-homed Customer Backup ISP Primary link Upstream AS Another Customer Primary ISP [...]... AS path processing in internal BGP –Explain the need for BGP split horizon and its implications –Understand the next-hop processing in internal BGP and its implications BSCI © 2008 Cisco Systems, Inc All rights reserved 25 CCNP – BSCI Bachkhoa Networking Academy AS Path Processing in IBGP Network X is announced as coming from AS 12 X 12 … Rtr-A Rtr-B IBGP session R-12 AS 12 X 42 12 … EBGP session Rtr-C... all BGP- speaking routers in the AS for proper IBGP route propagation The IBGP full-mesh is only a logical mesh of TCP sessions, physical full mesh is not required BSCI © 2008 Cisco Systems, Inc All rights reserved 28 CCNP – BSCI Bachkhoa Networking Academy IBGP Full-mesh EBGP update AS 12 IBGP update R-12 Rtr-A IBGP update Rtr-B Rtr-C AS Incoming EBGP update is directly 42 propagated from ingress router... needs external routes for proper packet forwarding Conclusion#2: Rtr-C must receive BGP routes BSCI © 2008 Cisco Systems, Inc All rights reserved Học viện mạng Bách Khoa - www.bkacad.com 21 CCNP – BSCI Bachkhoa Networking Academy Packet Forwarding in an Autonomous System All core routers must have all external routes Core routers must receive BGP routes –Redistribution of BGP routes into IGP is not... to all BGP- speaking routers in the AS AS 14 EBGP update Rtr-D R-14 Incoming IBGP update is further propagated to next AS BSCI © 2008 Cisco Systems, Inc All rights reserved 29 CCNP – BSCI Bachkhoa Networking Academy IBGP Neighbors Rtr-A Rtr-B IBGP session EBGP session R-12 AS 12 Physical connections (for example, WAN links) AS 14 Rtr-C AS 42 Rtr-D R-14 Due to IBGP full-mesh requirements, IBGP neighbors... interfaces into IGP at the edge routers or –Include links to EBGP neighbors into IGP and make them passive interfaces BSCI © 2008 Cisco Systems, Inc All rights reserved 33 CCNP – BSCI Bachkhoa Networking Academy Transit Network Using Edge Routers as Next-hops Alternate design: Next-hop processing is modified at the edge routers –Edge routers announce themselves as the next-hop in IBGP updates –No redistribution... Bachkhoa Networking Academy Route Propagation Rtr-A Rtr-B R-12 AS 12 AS 14 Rtr-C AS 42 Rtr-D R-14 Routes between autonomous systems are always exchanged via External BGP (EBGP) The only protocol that can transport all BGP attributes across the backbone is BGP inside autonomous system, called Internal BGP (IBGP) IBGP session must be established between transit AS border routers to propagate EBGP routes BSCI... changed on IBGP sessions Next-hop is set to local IP address on EBGP sessions BSCI © 2008 Cisco Systems, Inc All rights reserved Rtr-D AS 14 R-14 3.0.0.1 3.0.0.2 32 CCNP – BSCI Bachkhoa Networking Academy Transit Network Using External Nexthops All EBGP peers must be reachable by all BGPspeaking routers within the AS EBGP next hops shall be announced using IGP: –Redistribute connected interfaces into... deploying IBGP on all core routers BSCI © 2008 Cisco Systems, Inc All rights reserved 23 CCNP – BSCI Bachkhoa Networking Academy Internal BGP BSCI © 2008 Cisco Systems, Inc All rights reserved www.cisco.com 24 CCNP – BSCI Bachkhoa Networking Academy Objectives Upon completion of this section, you will be able to perform the following tasks: –List the differences between internal BGP and external BGP. .. ? Which interfaces shall you choose as the source and destination addresses of IBGP TCP sessions BSCI © 2008 Cisco Systems, Inc All rights reserved 30 CCNP – BSCI Bachkhoa Networking Academy IBGP Neighbor Sessions Always run your IBGP sessions between loopback interfaces –IBGP sessions can always be established, even if some physical interfaces are down –IBGP sessions are stable - physical interface... result in suboptimal routing if you have multiple paths to a neighbor AS Use default next-hop processing if at all possible BSCI © 2008 Cisco Systems, Inc All rights reserved 35 CCNP – BSCI Bachkhoa Networking Academy Change the Next-hop Processing at Edge Routers router(config-router)# neighbor ip-address next-hop-self Bypass the BGP next-hop processing and announce the local IP address as the BGP . Cisco Systems, Inc. All rights reserved. Cisco ConfidentialBSCI 1 Route Filtering and Route Selection in BGP Route Filtering and Route Selection in BGP © 2008 Cisco Systems, Inc. All rights. Describe Transit BGP Networks – Filter incoming and outgoing BGP updates route- maps – Influence BGP route selection – Monitor and troubleshoot BGP filters – Implement non-disruptive BGP policy changes –. Requirement Summary BGP must support the following mechanisms: – Route selection based on BGP neighbors – Route selection based on AS-numbers in the AS- path – Filters based on AS-numbers in the AS-path –