chapter 9. implementing scalability features in your internetwork
TRANSCRIPT
Chapter 9.Implementing Scalability
Features in Your Internetwork
Table of Contents
• Routing Principles• Extending IP Addressing Space• Connecting to ISPs• Controlling Routing Updates and
Policies• Route Redistribution• Summary
Routing Principles
• Routing Defined• Classful Routing• Classless Routing
What Is Routing?
•Routing is the process of forwarding an item from one location to another
•Routers forward traffic to a logical destination in a computer network
•Routers perform two major functions:– Routing
• Learning the logical topology of the network
– Switching• Forwarding packets from an inbound interface to an
outbound interface
Classful Routing
•Classful routing protocols are a consequence of the distance vector method of route calculation– RIPv1– IGRP
•Subnet masks are not carried within the routing updates
•Summary routes are automatically created at Class A, B, or C network boundaries
Classless Routing
•Classless routing protocols include the routing mask with the route advertisement– Open Shortest Path First (OSPF)– EIGRP– RIPv2– IS-IS– BGP
• Summary routes can be manually controlled within the network
Extending IP Addressing Space
• IP Addressing Solutions• VLSM Overview• Route Summarization Overview• CIDR Overview
IP Addressing Solutions
• Subnet masking, RFCs 950, 1812• Address allocation for private Internets,
RFC 1918
• Network Address Translation, RFC 1631
• Hierarchical addressing
• Variable-length subnet masks, RFC 1812
• Route summarization, RFC 1518
• Classless interdomain routing, RFCs 1518,
1519, 2050
What Is a Variable-Length Subnet Mask?
BB172.16.14. 64/27
– Subnet 172.16.14.0/24 is divided into smaller subnets:
• Subnet with one mask at first (/27)• Further subnet one of these subnets not used
elsewhere (/30)
172.16.0.0/16
172.16.1.0/24
172.16.2.0/24
HQHQ
CC
172.16.14.32/27
172.16.14.96/27
AA 172.16.14.132/30
172.16.14.136/30
172.16.14.140/30
What Is Route Summarization?
– Routing protocols can summarize addresses of several networks into one address
I can route to the 172.16.0.0/16 network.
Routing Table172.16.0.0/16Routing Table
172.16.25.0/24172.16.26.0/24172.16.27.0/24
172.16.27.0/24
172.16.26.0/24
172.16.25.0/24
A B
What Is CIDR?
192.168.8.0/24
192.168.9.0/24
192.168.15.0/24
– Networks 192.168.8.0/24 through 192.168.15.0/24 are summarized by HQ in one advertisement 192.168.8.0/21
A
192.168.8.0/21 192.168.8.0/21
192.168.15.0/2
4 192.168.15.0/2
4
192.168.8.0/
24 192.168.8.0/
24
192.168.9.0/24 192.168.9.0/24
H
HQ
B
Connecting to ISPs
• Autonomous Systems• BGP Characteristics• BGP Route Selection Decision Process• BGP Multihoming
Autonomous Systems
Autonomous System 100 Autonomous System 200
IGPs: RIP, IGRP, OSPF, EIGRP EGPs: BGP
• An autonomous system (AS) is a collection of networks under a a single technical administration
• IGPs operate within an autonomous system• EGPs connect different autonomous systems
BGP Characteristics
•BGP is a distance vector protocol with enhancements:– Reliable updates—BGP runs on top of TCP (port
179)– Incremental, triggered updates only– Periodic keepalives to verify TCP connectivity– Rich metrics (called path vectors or attributes)– Designed to scale to huge internetworks
BGP Route Selection Decision Process
• Consider only (synchronized) routes with no AS loops and a valid next-hop, and then:
– Prefer highest weight (local to router)– Prefer highest local preference (global within AS)– Prefer route originated by the local router– Prefer shortest AS-path– Prefer lowest origin code (IGP < EGP < incomplete)– Prefer lowest MED (from other AS)– Prefer EBGP path over IBGP path– Prefer the path through the closest IGP neighbor– Prefer the path with the lowest neighbor BGP
router ID
Multihoming Example
AS 64500
10.10.10.110.10.20.2
AS 65500
172.25.0.0
E
ISP
AS 65000 10.10.10.2
172.20.0.0
A
10.10.20.1
172.30.0.0ISP
AS 65250
CB
Controlling Routing Updates ad Policies
• Route Filters with Distribute Lists• Route Maps• Policy-Based Routing• BGP Policy Control
Route Filters with Distribute-List
Yes
No
Process packet normally
Determineinterface
Process entry according to filter
configuration
Is there a filter for thisinterface?
End
End
Is there anentry for this
address?
No
Routingupdate
Yes
Drop packet
Route Maps
– Filter for network advertisements– Offer detailed control over
advertisements– Complex access lists
•Complex conditional advertisement via match command
•Changes routing table parameters via set command
Policy-Based Routing
– Allows you to implement policies that selectively cause packets to take different paths
– Can also mark traffic with different TOS
– Since Cisco IOS Release 11.0
– Applied to incoming packets– Implemented using route maps
• To restrict routing information to and from BGP neighbors use
– Distribute lists (using access lists), or
– Prefix lists
BGP Policy Control
Route Redistribution
• When to Use Multiple Routing Protocols• Redistribution Overview• Redistribution Implementation
Guidelines
•Interim during conversion•Application-specific protocols
•One size does not always fit all
•Political boundaries•Groups that do not work and play nicely
with others
•Mismatch between devices•Multivendor interoperability•Host-based routers
When Do You Use Multiple Routing Protocols?
– Routes are learned from another routing protocol when a router redistributes the information between the protocols
IP Routing TableI 192.168.5.0I 172.16.1.0I 172.16.2.0I 172.16.3.0
IP Routing Table
D EX 172.16.0.0D 192.168.5.8D 192.168.5.16D 192.168.5.24
S1 Advertises Routes from EIGRP to IGRP
S0 Advertises Routes from IGRP to EIGRP
AS 200IGRP172.16.0.0
S0S1 AS 300EIGRP192.168.5.0
What Is Redistribution?
ASBR
ABC
Redistribution Implementation Guidelines
IGRP/OSPF
IGRP OSPFRedistribute
Default or Static
IGRP OSPFRedistribute
Redistribute and Filter or Change Administrative Distance
Summary