Download 9.Implemen Scalability

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?
172.16.14.32/27
172.16.14. 64/27
A
B
HQ
172.16.14.96/27
172.16.0.0/16
C
– 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)
What Is Route Summarization?
172.16.25.0/24
I can route to the
172.16.0.0/16 network.
172.16.26.0/24
A
172.16.27.0/24
Routing Table
172.16.25.0/24
172.16.26.0/24
172.16.27.0/24
B
Routing Table
172.16.0.0/16
– Routing protocols can summarize addresses of
several networks into one address
What Is CIDR?
192.168.8.0/24
192.168.9.0/24
A
B
192.168.9.0/24
192.168.15.0/24
192.168.8.0/21
HQ
H
– Networks 192.168.8.0/24 through 192.168.15.0/24 are
summarized by HQ in one advertisement 192.168.8.0/21
Connecting to ISPs
•
•
•
•
Autonomous Systems
BGP Characteristics
BGP Route Selection Decision Process
BGP Multihoming
Autonomous Systems
IGPs: RIP, IGRP,
OSPF, EIGRP
EGPs: BGP
Autonomous System 100
Autonomous System 200
• 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
172.25.0.0
AS
65500
ISP
AS
65000
E
172.20.0.0
172.30.0.0
B
C
10.10.20.1
10.10.10.2
10.10.20.2
10.10.10.1
A
AS
64500
ISP
AS
65250
Controlling Routing Updates ad
Policies
•
•
•
•
Route Filters with Distribute Lists
Route Maps
Policy-Based Routing
BGP Policy Control
Route Filters with Distribute-List
Routing
update
Determine
interface
Is there a
filter for this
interface?
No
Process packet
normally
End
Yes
Is there an
entry for this
address?
Yes
Process entry
according to filter
configuration
No
End
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
BGP Policy Control
• To restrict routing information to and
from BGP neighbors use
– Distribute lists (using access lists), or
– Prefix lists
Route Redistribution
• When to Use Multiple Routing Protocols
• Redistribution Overview
• Redistribution Implementation Guidelines
When Do You Use Multiple Routing
Protocols?
• 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
What Is Redistribution?
ASBR
AS 200
IGRP
172.16.0.0
IP Routing Table
I 192.168.5.0
I 172.16.1.0
I 172.16.2.0
I 172.16.3.0
S1
C
S0
A
B
S1 Advertises Routes from
EIGRP to IGRP
S0 Advertises Routes from
IGRP to EIGRP
AS 300
EIGRP
192.168.5.0
IP Routing Table
D EX
D
D
D
172.16.0.0
192.168.5.8
192.168.5.16
192.168.5.24
– Routes are learned from another routing protocol
when a router redistributes the information
between the protocols
Redistribution Implementation
Guidelines
IGRP/OSPF
IGRP
Redistribute
OSPF
Default or Static
IGRP
Redistribute
OSPF
Redistribute and Filter or Change
Administrative Distance
Summary