BGP Router Reflection

© 2000, Cisco Systems, Inc. A_BGP_RR

BGP Route ReflectorsBGP Route Reflectors

© 2000, Cisco Systems, Inc. www.cisco.com A_BGP_RR—

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




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


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


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


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


More Route Reflector Split Horizon Rules

More Route Reflector Split Horizon Rules

Autonomous system

Reflector


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


Route Reflectors Reduce the Number of IBGP Sessions

Route Reflectors Reduce the Number of IBGP Sessions

Autonomous system

Reflector


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


Redundant Route ReflectorsRedundant Route Reflectors

Autonomous system

Reflector


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


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


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


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.


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


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





• 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


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


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


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


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


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


Hierarchical Route Reflector Example

Hierarchical Route Reflector Example

Autonomoussystem

Cluster27

Cluster 11 Cluster12

Reflector/Client


EBGP peer

Client

EBGP peer

Client Client

Reflector/Client

Reflector/Client

Reflector Reflector


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


SummarySummary


• 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



• 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





• Plan migration of existing IBGP backbone toward route reflector-based backbone

• Configure BGP route reflectors

• Monitor BGP backbone with route reflectors


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


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


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


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


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)


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


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


SummarySummary


• Plan migration of existing IBGP backbone toward route reflector-based backbone

• Configure BGP route reflectors

• Monitor BGP backbone with route reflectors



• 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?


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

BGP Router Reflection

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