Route Filtering and Route Selection in BGP pptx

© 2008 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialBSCI
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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
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Multi-Homed BGP
Networks
www.cisco.com
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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
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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
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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
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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
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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
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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
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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
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Transit Traffic Issue
 Customers could become transit AS for the Service
Providers
Internet
Multi-homed
Customer
Service Provider #2
Service Provider #1
• Requirement: do not propagate provider
routes to other providers
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Routing Update Reliability Issue
 Customers running BGP could announce any route to
the Service Providers
Internet
Multi-homed
Customer
AS123
21.0.0.0/8
Service Provider #2
Service Provider #1
Network=10.0.0.0/8
AS-Path=123
• Requirement: Service Providers have to filter
IP prefixes in incoming updates

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Return Traffic Issue
Internet
Multi-homed
Customer
Backup ISP
Primary
link
Upstream AS
Another
Customer
Primary ISP
 Customers can only influence their outgoing traffic,
not the return traffic
• Return traffic can take any path - Backup ISP
must also perform proper route selection
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Multi-Homed Customer 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

– Filters on IP prefixes
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Summary
After completing this section, you should be able to
perform the following tasks:
– Describe the issues in multi-homed BGP networks
– Describe the need to influence BGP route selection
– Describe the need for BGP filters
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Review Questions
–Why would a customer want connection to two
service providers?
–What are the technical requirements for multi-
homed customers?
–Which routing protocol needs to be deployed
between multi-homed customers and the ISP?
–Why do we need to influence BGP route selection
rules?
–List three potential customer routing policies
–What are the issues an ISP with multi-homed
customers is facing?
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Transit Autonomous
System Functions
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Objectives
Upon completion of this section, you will be able to
perform the following tasks:
– List the functions of a transit autonomous system
– Describe the external route propagation through
transit AS
– Explain the need for internal BGP inside the transit
AS
– Explain the need for deploying IBGP on all core
routers
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Transit Autonomous System Tasks
 Propagate routes between remote Autonomous
Systems
 Route packets between remote networks
AS 42
AS 12
AS 14
R-14

R-12
Rtr-A Rtr-B
Rtr-DRtr-C
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Route Propagation
AS 42
AS 12
AS 14
R-14
R-12
Rtr-A Rtr-B
Rtr-DRtr-C
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
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Packet Forwarding in an Autonomous
System
AS 42
AS 12

AS 14
R-14
R-12
Rtr-A Rtr-B
Rtr-DRtr-C
Conclusion#1: Rtr-C needs external routes for proper packet forwarding
Conclusion#2: Rtr-C must receive BGP routes
Packet from AS 14 toward AS 12 is received by Rtr-D
Rtr-D forwards the packet toward Rtr-A as dictated
by an IBGP-learned entry in its IP routing table
How will RTR-C forward the packet?
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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
scalable
–Default routing is not applicable in transit AS
core
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Summary
After completing this section, you should be able to
perform the following tasks:
–List the functions of a transit autonomous system
–Describe the external route propagation through

transit AS
–Explain the need for internal BGP inside the transit
AS
–Explain the need for deploying IBGP on all core
routers
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Internal BGP
www.cisco.com
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Objectives
Upon completion of this section, you will be able to
perform the following tasks:
–List the differences between internal BGP and
external BGP
–Describe the 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