© 2000, Cisco Systems, Inc. A_BGP_RR
BGP Route ReflectorsBGP Route Reflectors
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 2
ObjectivesObjectives
Upon completion of this lesson, you will be able to perform the following tasks:
• Understand the motivation behind BGP route reflectors
• Describe how BGP route reflectors modify IBGP split horizon rules
• Describe BGP route reflection implementation
• Describe route reflector design rules
• Plan IBGP backbone migration toward route reflector-based backbone
• Configure and monitor BGP route reflector implementation
Introduction to BGP Route Reflectors
Introduction to BGP Route Reflectors
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 3
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 4
ObjectivesObjectives
Upon completion of this section, you will be able to perform the following tasks:
• Understand the need for BGP route reflectors
• Understand and describe modified IBGP split horizon rules
• Describe additional BGP attributes associated with route reflectors
• Describe the concept of route reflector clusters
• Describe route reflector loop-prevention mechanisms
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 5
IBGP Transit AS - ProblemsIBGP Transit AS - Problems
IBGP requires full-mesh between all BGP-speaking routers
• Large number of TCP sessions
• Unnecessary duplicate routing traffic
Solutions• Route reflectors modify IBGP split horizon
rules
• BGP confederations modify IBGP AS Path processing
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 6
Route Reflectors - Modification of Split Horizon Rules
EBGP route
Classic BGP - IBGP routes arenot propagated to other IBGP peers. Full mesh of IBGP peers is therefore required
Route reflector can propagate IBGP routes to other IBGP peers. Full mesh of IBGP peers is nolonger required
Route reflector
EBGP route
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 7
Route Reflector Split Horizon Rules
Route Reflector Split Horizon Rules
Autonomous system
Reflector
Reflector ReflectorClient
Client
Client
ClientEBGP peer
EBGP peer
Client
EBGP peer
Routes received from external peers are propagated to all internal peersRoutes received from external peers are propagated to all internal peers
Routes received from a client are propagated to all other peers
Routes received from a client are propagated to all other peers
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 8
More Route Reflector Split Horizon Rules
More Route Reflector Split Horizon Rules
Autonomous system
Reflector
Reflector ReflectorClient
Client
Client
ClientEBGP peer
EBGP peer
Client
EBGP peer
Routes received from non-client IBGP neighbors are sent to clients and EBGP peers
Routes received from non-client IBGP neighbors are sent to clients and EBGP peers
All IBGP and EBGP routes are sent to EBGP peersAll IBGP and EBGP routes are sent to EBGP peers
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Route Reflectors Reduce the Number of IBGP Sessions
Route Reflectors Reduce the Number of IBGP Sessions
Autonomous system
Reflector
Reflector ReflectorClient
Client
Client
Client
EBGP peer
EBGP peer
Client
EBGP peer
But the reflectorcould be a single
point of failure
But the reflectorcould be a single
point of failure
Design requirement: reflectors must be redundant
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Redundant Route ReflectorsRedundant Route Reflectors
Autonomous system
Reflector
Reflector ReflectorClient
Client
Client
Client
EBGP peer
EBGP peer
Client
EBGP peer
Redundant reflectors solve high availability requirementRedundant reflectors solve high availability requirement
But they might also cause routing loopsBut they might also cause routing loops
The concept of “clusters” is introduced to prevent IBGP routing loops with route reflectors
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 11
Route Reflector ClustersRoute Reflector Clusters
• A group of redundant route reflectors and their clients form a cluster
• Each cluster must have a unique cluster-ID
• Each time a route is reflected, the cluster-ID is added to cluster-list BGP attribute
• The route that already contains local cluster-ID in the cluster-list is not reflected
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 12
A utonomous system
C luster C luster
Ref lec tor
Ref lec tor Ref lec torClient
Client
Client
Client
EBGP peer
EBGP peer
Client
EBGP peer
Redundant Route Reflectors in a Cluster
Redundant Route Reflectors in a Cluster
Route is rejected since the cluster-ID is already in cluster-list
Route is rejected since the cluster-ID is already in cluster-list
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 13
Additional Route Reflector Loop Prevention Mechanisms
Additional Route Reflector Loop Prevention Mechanisms
• Every time a route is reflected, the router-ID of the originating IBGP router is stored in originator-ID BGP attribute
• A router receiving an IBGP route with originator-ID set to its own router-ID ignores that route
• BGP path selection procedure is modified to take in account cluster-list and originator-ID.
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 14
SummarySummary
After completing this section, you should be able to perform the following tasks:
• Understand the need for BGP route reflectors
• Understand and describe modified IBGP split horizon rules
• Describe additional BGP attributes associated with route reflectors
• Describe the concept of route reflector clusters
• Describe route reflector loop-prevention mechanisms
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 15
Review QuestionsReview Questions
• What are the benefits of using route reflectors in an autonomous system?
• Which routes are propagated by a route reflector to its clients?
• Which routes are propagated by a route reflector client to its IBGP neighbors?
• List two loop-prevention mechanisms that support route reflectors?
• Why do you need route reflector clusters?
Network Design with BGP Route ReflectorsNetwork Design with
BGP Route Reflectors
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 16
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 17
ObjectivesObjectives
Upon completion of this section, you will be able to perform the following tasks:
• List the basic route reflector design rules
• Understand minimum requirements for IBGP sessions
• Describe the implications of deviating from the design rules
• Understand the concept of hierarchical route reflectors
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 18
Route Reflectors - DesignRoute Reflectors - Design
• Divide transit AS into smaller areas (called clusters)
• Each cluster contains route reflectors and route reflector clients
• Routers that don’t support route reflector functionality act as one-router cluster or as route reflector client
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 19
Route Reflectors - Sample Network
Autonomoussystem
Redundantcluster
Non-redundantcluster
Reflector
Client Client Client
EBGP peer
EBGP peer
Client
Reflector
Reflector Non-RRrouter
Client
Client
EBGP peer
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 20
Route Reflector IBGP Session Rules
Route Reflector IBGP Session Rules
• All clients in a cluster must have IBGP session with and only with all route reflectors in the cluster
• IGBP full-mesh between all route reflectors within the AS is required
• Non-route reflector capable routers can participate in IBGP full-mesh or be route reflector clients
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 21
Problems you might face when deviating from route reflector rules
Problems you might face when deviating from route reflector rules
• Clients don’t have sessions with all reflectors in a cluster
• Clients have sessions with reflectors in several clusters
• Clients have IBGP sessions to other clients
• Clients will not receive all IBGP routes
• Some routers might receive duplicate copies of the same route
• Clients will receive duplicate copies of the same route
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 22
Hierarchical Route ReflectorsHierarchical Route Reflectors
Problem:• In very large networks, a single layer of route reflectors might not be enough
Solution:• A hierarchy of route reflectors can be established• A route reflector can be a client of another
route reflector
• The hierarchy can be as deep as needed
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 23
Hierarchical Route Reflector Example
Hierarchical Route Reflector Example
Autonomoussystem
Cluster27
Cluster 11 Cluster12
Reflector/Client
Client Client Client
EBGP peer
Client
EBGP peer
Client Client
Reflector/Client
Reflector/Client
Reflector Reflector
Client Client Client
EBGP peer
This router is a reflector in Cluster 11 and client in Cluster 27
This router is a reflector in Cluster 11 and client in Cluster 27
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 24
SummarySummary
After completing this section, you should be able to perform the following tasks:
• List the basic route reflector design rules
• Understand minimum requirements for IBGP sessions
• Describe the implications of deviating from the design rules
• Understand the concept of hierarchical route reflectors
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 25
Review QuestionsReview Questions
• Which IBGP sessions should a route reflector client have?
• What is a hierarchical route reflector design?
• What happens if a route reflector client establishes sessions with route reflectors in two clusters?
• What happens if two route reflectors have each other configured as a client?
Deploying BGP Route ReflectorsDeploying BGP
Route Reflectors
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 26
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 27
ObjectivesObjectives
Upon completion of this section, you will be able to perform the following tasks:
• Plan migration of existing IBGP backbone toward route reflector-based backbone
• Configure BGP route reflectors
• Monitor BGP backbone with route reflectors
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 28
Planning BGP Backbone Migration
Planning BGP Backbone Migration
• Divide your AS in areas (clusters)
• Assign a cluster ID to each area
• On route reflector clients, retain only IBGP sessions with route reflectors in their cluster
• On route reflectors, retain only IBGP sessions with other route reflectors and clients in their cluster
• Configure cluster ID on every route reflector
• Configure clients on every route reflector
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 29
Configuring BGP Route Reflectors
Configuring BGP Route Reflectors
• Configure cluster-ID on route reflectors
• Configure BGP neighbors as route reflectors clients on the route reflectors
• No configuration is needed on the route reflector clients
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 30
Configuring Route Reflectors - Router Configuration Commands
Configuring Route Reflectors - Router Configuration Commands
bgp cluster-id cluster-id
router(config-router)#
• Optionally assigns a cluster-ID to the route reflector (default value is router-ID)
• Required only for clusters with redundant reflectors
• Cluster-ID cannot be changed after the first client is configured
neighbor ip-address route-reflector-client
router(config-router)#
• Configures an IBGP neighbor to be a client of this reflector
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 31
Route Reflector Configuration Example
Route Reflector Configuration Example
Autonomoussystem 123
Cluster 175
1.0.0.1reflector
1.0.0.2reflector
1.0.0.3client
1.0.0.4client
1.2.0.6IBGP peer
2.7.1.1EBGP inAS 222
router bgp 123! cluster IDbgp cluster-id 175! RR clientsneighbor 1.0.0.3 remote-as 123neighbor 1.0.0.3 route-reflectorneighbor 1.0.0.4 remote-as 123neighbor 1.0.0.4 route-reflector! other IBGP neighborsneighbor 1.0.0.2 remote-as 123neighbor 1.2.0.6 remote-as 123! EBGP neighborsneighbor 2.7.1.1 remote-as 222
router bgp 123! cluster IDbgp cluster-id 175! RR clientsneighbor 1.0.0.3 remote-as 123neighbor 1.0.0.3 route-reflectorneighbor 1.0.0.4 remote-as 123neighbor 1.0.0.4 route-reflector! other IBGP neighborsneighbor 1.0.0.2 remote-as 123neighbor 1.2.0.6 remote-as 123! EBGP neighborsneighbor 2.7.1.1 remote-as 222
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 32
Monitoring Route Reflector Operation
Monitoring Route Reflector Operation
show ip bgp neighbor
router#
• Displays whether a neighbor is a route reflector client
show ip bgp network [mask]
router#
• Displays additional path attributes (originator and cluster-list)
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 33
Monitoring Route Reflector Clients
Monitoring Route Reflector Clients
Barney#show ip bgp neighbors 1.0.0.1BGP neighbor is 1.0.0.1, remote AS 213, internal link Index 1, Offset 0, Mask 0x2 Route-Reflector Client BGP version 4, remote router ID 11.0.0.1 BGP state = Established, table version = 5, up for 01:33:24 Last read 00:00:24, hold time is 180, keepalive interval is 60 seconds Minimum time between advertisement runs is 5 seconds Received 257 messages, 0 notifications, 0 in queue Sent 264 messages, 0 notifications, 0 in queue Connections established 5; dropped 4 Last reset 01:33:33, due to : User reset request No. of prefix received 1
Barney#show ip bgp neighbors 1.0.0.1BGP neighbor is 1.0.0.1, remote AS 213, internal link Index 1, Offset 0, Mask 0x2 Route-Reflector Client BGP version 4, remote router ID 11.0.0.1 BGP state = Established, table version = 5, up for 01:33:24 Last read 00:00:24, hold time is 180, keepalive interval is 60 seconds Minimum time between advertisement runs is 5 seconds Received 257 messages, 0 notifications, 0 in queue Sent 264 messages, 0 notifications, 0 in queue Connections established 5; dropped 4 Last reset 01:33:33, due to : User reset request No. of prefix received 1
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Monitoring Reflected BGP RoutesMonitoring Reflected BGP Routes
Routes received from the client as seen on the reflector
Reflected routes as seen on the client
Barney#show ip bgp 11.0.0.0BGP routing table entry for 11.0.0.0/8, version 3Paths: (1 available, best #1, advertised over IBGP) Local, (Received from a RR-client) 1.0.0.1 (metric 40640000) from 1.0.0.1 (11.0.0.1) Origin IGP, metric 0, localpref 100, valid, internal, best
Barney#show ip bgp 11.0.0.0BGP routing table entry for 11.0.0.0/8, version 3Paths: (1 available, best #1, advertised over IBGP) Local, (Received from a RR-client) 1.0.0.1 (metric 40640000) from 1.0.0.1 (11.0.0.1) Origin IGP, metric 0, localpref 100, valid, internal, best
Wilma#sh ip bgp 14.0.0.0BGP routing table entry for 14.0.0.0/8, version 30Paths: (1 available, best #1) Not advertised to any peer Local 1.0.0.3 (metric 41152000) from 1.0.0.2 (14.1.2.3) Origin IGP, metric 0, localpref 100, valid, internal, best Originator: 14.1.2.3, Cluster list: 0.0.2.55
Wilma#sh ip bgp 14.0.0.0BGP routing table entry for 14.0.0.0/8, version 30Paths: (1 available, best #1) Not advertised to any peer Local 1.0.0.3 (metric 41152000) from 1.0.0.2 (14.1.2.3) Origin IGP, metric 0, localpref 100, valid, internal, best Originator: 14.1.2.3, Cluster list: 0.0.2.55
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 35
SummarySummary
After completing this section, you should be able to perform the following tasks:
• Plan migration of existing IBGP backbone toward route reflector-based backbone
• Configure BGP route reflectors
• Monitor BGP backbone with route reflectors
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 36
Review QuestionsReview Questions
• Which BGP parameters have to be configured on a route reflector?
• Which BGP parameters have to be configured on a route reflector client?
• List the migration steps from full-mesh IBGP autonomous system to a backbone based on route reflectors
• How would you identify whether an IBGP route was reflected?
• How would you identify route reflector clients of a router without inspecting its configuration?
© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—Page 37
SummarySummary
After completing this lesson, you should be able to perform the following tasks:
• Understand the motivation behind BGP route reflectors
• Describe how BGP route reflectors modify IBGP split horizon rules
• Describe BGP route reflection implementation
• Describe route reflector design rules
• Plan IBGP backbone migration toward route reflector-based backbone
• Configure and monitor BGP route reflector implementation
© 1999, Cisco Systems, Inc. www.cisco.com Chapter#-38