Transcript Lesson 10
IP Routing
Dynamic Routing Protocols
Problem
• How are routing tables kept
up to date in a changing
network?
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Definitions
• Routing protocol
– ex: OSPF, RIP, IGRP, BGP
• Routed protocol
– ex: IP, IPX, AppleTalk
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Goals of a routing protocol
•
•
•
•
•
Dynamic learning of subnets
Dynamic updating of the routing table
Choosing the best route if more than one
Notice when a route is no longer valid
Remove invalid routes + add new ones if they
exist
• Small convergence time
• Prevent routing loops
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Routing issues
•
•
•
•
Path determination
Metrics
Convergence
Load balancing
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Path Determination
1.
2.
3.
4.
5.
RouterA deduces it’s connected
networks
RouterA enters the networks in
the table
RouterA places the networks in
a packet
RouterA announces this packet
to RouterB & C
RouterB & RouterC perform the
same process
Why is it more complicated
than this ?
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Path Determination- the questions
• What should routerA do with
received route packets
– Pass on from B to C ? And vice
versa?
• What if there is no link between B
and C ?
– They need to pass on the
information.
• What if both routers announce the
same network?
– Which is valid/best?
• What guarantees all routers receive
info, but prevents infinite loops?
• Should routers announce networks
that they share?
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Metrics
•
•
•
•
•
•
Hop count
Bandwidth
Load
Delay
Reliability
Cost
Network
Next Hop Router
192.168.1.0
Directly connected
192.168.2.0
Directly connected
192.168.3.0
Directly connected
192.168.4.0
B,C
192.168.5.0
B,C
192.168.6.0
B,C
192.168.7.0
B,C
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Convergence
• Routing loops
• Convergence time
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Load Balancing
• Load balancing
– Per packet
– Per destination
• Load sharing
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Overview
• Static routing <--> Dynamic routing
– How routing table is calculated
• Exterior Gateway Protocols <--> Interior Gateway
Protocols
– Where the protocol is applied
• Interior Gateway Protocols
– Distance Vector <--> Link State
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Distance Vector Routing Protocols
• Routes are announced as vectors of
– (distance, direction) or (metric, next hop)
• RIP, IGRP, EIGRP, AppleTalk’s RTMP
• Common caracteristics
– Periodic updates
– Neighbors
– Broadcast updates
– Full routing table updates
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Routing by Rumor – an example
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Refinements 1 – Invalidation timers
• Flag the route as unreachable
– Network 10.1.5.0 down
– Router D down -> timer
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Refinements 2 – Split Horizon
Router C
s0
E0
1
162.11.10.0
162.11.6.0
E0
162.11.6.0
0
162.11.7.0
1
162.11.10.0 0
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Router B
s0
2
162.11.6.0
0
162.11.7.0
0
162.11.10.0 1
162.11.7.0
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Refinements 2 – Split Horizon
E0 Fails !!!
Router C
s0
E0
2
162.11.10.0
Router B
162.11.6.0
E0
162.11.6.0
0
162.11.7.0
1
162.11.10.0 0
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s0
1
0
162.11.7.0 16
162.11.10.0 1
162.11.7.0
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Refinements 2 – Split Horizon
Router C
s0
E0
1
162.11.10.0
Router B
s0
162.11.6.0
E0
162.11.6.0
0
162.11.7.0
3
162.11.10.0 0
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Direct cost = 16
Cost via C = 2 !!
2
0
162.11.7.0 2
162.11.10.0 1
162.11.7.0
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Refinements 2 – Split Horizon
Direct cost = 16
Cost via C = 3 !!
Router C
s0
E0
1
162.11.10.0
Router B
s0
E0
162.11.6.0
0
162.11.7.0
4
162.11.10.0 0
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2
162.11.6.0
0
162.11.7.0
3
162.11.10.0 1
162.11.7.0
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Refinements 2 – Split Horizon
• Reverse routes is a waste of resources
• It creates a route loop
Split Horizon
Split Horizon with
poisoned reverse
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Refinements 3 – Counting to infinity
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Refinements 4
• Triggered updates
• Holddown timers
• Asynchronous updates
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Link State Routing Protocols
•
•
•
•
All routers have a complete picture of the network
Routers have first hand information
Based on Dijkstra’s shortest path algorithm
Examples
– OSPF – ISIS – DEC’s DNA Phase V – Novell’s
NLSP
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Link state protocol – the process
1. Establish adjacency with neighbors
• Hello protocol
2. Send LSA to neighbors + flood received LSA’s
• The links connected to the router
• The state of the link (up or down)
• The metric of the router to the link
• Neighbors on the link
3. Store the LSA in a database
• Topological database
4. Compute the shortest path to each router using
Dijkstra’s SPF algorithm
• Enter information in routing table
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When a Link Changes State
Router 2, Area 1
Router 1, Area 1
LSA
ACK
• Every router in area hears
a specific link LSA
• Each router computes
shortest path routing table
• LSA has sequence number
+ age
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Link State Table
Dijkstra Algorithm
Old Routing Table
New Routing Table
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Link state database
1. Router link information
2. Stub network information
(not shown here)
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Router ID
Neighbor/network
Cost
RA
RB
2
RA
RD
4
RA
RE
4
RB
RA
2
RB
RC
1
RB
RE
10
RC
RB
5
RC
RF
2
RD
RA
4
RD
RE
3
RD
RG
5
RE
RA
5
RE
RB
2
RE
RD
3
RE
RF
2
RE
RG
1
RE
RH
8
RF
RC
2
RF
RE
2
RF
RH
4
RG
RD
5
RG
RE
1
RH
RE
8
RH
RF
6
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Dijkstra SPF overview
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Dijkstra basics
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LSP data
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Dijkstra example (1/7)
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Dijkstra example (2/7)
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Dijkstra example (3/7)
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Dijkstra example (4/7)
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Dijkstra example (5/7)
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Dijkstra example (6/7)
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Dijkstra example (7/7)
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Area’s – creating hierarchy
• Common concerns
about link state
protocols
1. More memory
2. More CPU
3. Flooding in
unstable networks
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Interior and Exterior Gateway protocols
• Networks and Routers under a single
administrative authority
• Each AS is assigned a number
• AS numbers range form 1 to 65,535
– Documented in RIPE, ARIN, APNIC,
AfriNIC and LACNIC
– http://www.ripe.net/whois?form_type
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Static or dynamic routing
• Static
– In small networks
– Simple topologies
• Dynamic
– In large networks
– Complex topologies
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RIP v1
• Directly connected subnets are known
• Routing updates are broadcasted to neighbors on
UDP port 520
• Listen to routing updates
• Hop count is used as Metric
• Routing info consists of subnet and metric
• Periodic updates (25 - 30 sec)
• A route is learned via neighbors
• Failed route has a metric of infinite (=16)
• Expiration timer 180 seconds
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RIP Timers
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RIP message format
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RIP classfull routing table
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Configuring RIP
Andy's RIP configuration.
router rip
network 172.17.0.0
network 192.168.12.0
network 192.168.83.0
Goober's RIP configuration.
router rip
network 172.17.0.0
Barney's RIP configuration.
router rip
network 10.0.0.0
network 192.168.83.0
Opie's RIP configuration.
router rip
network 172.17.0.0
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Debug RIP
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RIPv2
Routing Protocols and Concepts
Objectives
• Encounter and describe the limitations of RIPv1’s
limitations.
• Apply the basic Routing Information Protocol Version
2 (RIPv2) configuration commands and evaluate
RIPv2 classless routing updates.
• Analyze router output to see RIPv2 support for
VLSM and CIDR
• Identify RIPv2 verification commands
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Introduction
• Chapter focus
-Difference between RIPv1 & RIPv2
RIPv1
RIPv2
-A classful distance vector routing protocol
-Does not support discontiguous subnets
-Does not support VLSM
-Does not send subnet mask in routing update
-Routing updates are broadcast
-A classless distance vector routing protocol that is an
enhancement of RIPv1’s features.
-Next hop address is included in updates
-Routing updates are multicast
-The use of authentication is an option
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Introduction
• Similarities between RIPv1 & RIPv2
-Use of timers to prevent routing loops
-Use of split horizon or split horizon with poison
reverse
-Use of triggered updates
-Maximum hop count of 15
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RIPv1 Limitations
• Lab Topology
• Scenario - Overview:
3 router set up
Topology is discontiguous
There exists a static
summary route
Static route information
can be injected into routing
table updates using
redistribution.
Routers 1 & 3 contain
VLSM networks
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RIPv1 Limitations
• Scenario Continued
• VLSM
-Recall this is sub netting
the subnet
• Private IP addresses are
on LAN links
• Public IP addresses are
used on WAN links
• Loopback interfaces
-These are virtual
interfaces that can be
pinged and added to
routing table
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RIPv1 Limitations
• Route redistribution
– Redistribution command is way to disseminate a
static route from one router to another via a routing
protocol
– Example
R2(config-router)#redistribute static
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RIPv1 Limitations
• Verifying and Testing Connectivity
–
Use the following commands:
show ip interfaces brief
ping
traceroute
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RIPv1 Limitations - Discontiguous
• RIPv1 – a classful routing protocol
-Subnet mask are not sent in updates
-Summarizes networks at major network boundaries
-if network is discontiguous and RIPv1 configured
convergence will not be reached
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RIPv1 Limitations
Examining the routing
tables
-To examine the contents of
routing updates use the
debug ip rip command
-If RIPv1 is
configured then
Subnet masks will not be
included with the
network address
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RIPv1 Limitations - VLSM
• RIPv1 does not support VLSM
Reason: RIPv1 does not send
subnet mask in routing updates
• RIPv1 does summarize routes to
the Classful boundary
Or
uses the Subnet mask of the
outgoing interface to determine
which subnets to advertise
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RIPv1 Limitations - CIDR
• No CIDR Support
• In the diagram R2 will not
include the static route in
its update
Reason: Classful routing
protocols do not support
CIDR routes that are
summarized with a smaller
mask than the classful
subnet mask
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Configuring RIPv2
• Comparing RIPv1 & RIPv2 Message Formats
-RIPv2 Message format is similar to RIPv1 but has 2
extensions
1st extension is the subnet mask field
2nd extension is the addition of next hop
address
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Configuring RIPv2
• Enabling and Verifying RIPv2
• Configuring RIP on a Cisco router
By default it is running RIPv1
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Configuring RIPv2
• Configuring RIPv2 on a
Cisco router
-Requires using the
version 2 command
-RIPv2 ignores RIPv1
updates
• To verify RIPv2 is
configured use the
show ip protocols
command
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Configuring RIPv2
• Auto-Summary &
RIPv2
• RIPv2 will
automatically
summarize routes at
major network
boundaries and can
also summarize routes
with a subnet mask
that is smaller than the
classful subnet mask
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Configuring RIPv2
• Disabling AutoSummary in RIPv2
• To disable automatic
summarization issue
the no auto-summary
command
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Configuring RIPv2
• Verifying RIPv2 Updates
• When using RIPv2 with automatic summarization turned
off
Each subnet and mask has its own specific entry, along with the exit
interface and next-hop address to reach that subnet.
• To verify information being sent by RIPv2 use the
debug ip rip command
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VLSM & CIDR
• RIPv2 and VLSM
• Networks using a VLSM
IP addressing scheme
Use classless routing
protocols (i.e.
RIPv2)
to disseminate network
addresses and their
subnet
masks
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VLSM & CIDR
• CIDR uses Supernetting
Supernetting is a bunch of contiguous classful
networks that is addressed as a single
network.
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VLSM & CIDR
• To verify that
supernets are
being sent and
received use the
following
commands
-Show ip route
-Debug ip rip
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Verifying & Troubleshooting RIPv2
• Basic Troubleshooting steps
-Check the status of all links
-Check cabling
-Check IP address & subnet mask configuration
-Remove any unneeded configuration commands
• Commands used to verify proper operation of RIPv2
–
–
–
–
Show ip interfaces brief
Show ip protocols
Debug ip rip
Show ip route
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Summary
Routing
Protocol
Distance
Vector
Classless
Routing
Protocol
Uses
HoldDown
Timers
Use of
Split
Horizon
or
Split
Horizon
w/
Poison
Reverse
Max
Hop
count
= 15
Auto
Summary
Support
CIDR
Supports
VLSM
Uses
Authentication
RIPv1
Yes
No
Yes
Yes
Yes
Yes
No
No
No
RIPv2
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
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OSPF
Routing Protocols and Concepts
Introduction
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Introduction to OSPF
Background of OSPF
• Began in 1987
• 1989 OSPFv1 released in RFC 1131
This version was experimental & never deployed
• 1991 OSPFv2 released in RFC 1247
• 1998 OSPFv2 updated in RFC 2328
• 1999 OSPFv3 published in RFC 2740
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Introduction to OSPF
OSPF Message Encapsulation
• OSPF packet type
– There exist 5 types
• OSPF packet header
– Contains
Router ID , area ID and
Type code for
OSPF packet type
• IP packet header
Contains - Source IP
address, Destination IP
address, & Protocol field
set to 89
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Introduction to OSPF
OSPF Message Encapsulation
• Data link frame header
Contains - Source MAC address and Destination
MAC address
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Introduction to OSPF
OSPF Packet Types
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Introduction to OSPF
Hello Protocol
• OSPF Hello Packet
– Purpose of Hello Packet
Discover OSPF neighbors & establish adjacencies
Advertise guidelines on which routers must agree to
become neighbors
Used by multi-access networks to elect a designated
router and a backup designated router
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Introduction to OSPF
• Hello Packets continued
– Contents of a Hello Packet
– router ID of transmitting router
• OSPF Hello Intervals
– Usually multicast (224.0.0.5)
– Sent every 30 seconds for NBMA segments
• OSPF Dead Intervals
– This is the time that must transpire
– before the neighbor is considered
– down
– Default time is 4 times
– the hello interval
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Introduction to OSPF
• Hello protocol packets contain information that is
used in electing
-Designated Router (DR)
DR is responsible for updating all other OSPF
routers
-Backup Designated Router (BDR)
This router takes over DR’s responsibilities if DR
fails
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Introduction to OSPF
OSPF Link-state Updates
• Purpose of a Link State Update (LSU)
– Used to deliver link state advertisements
• Purpose of a Link State Advertisement (LSA)
– Contains information about neighbors & path costs
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Introduction to OSPF
OSPF Algorithm
• OSPF routers build &
maintain link-state
database containing LSA
received from other
routers
– Information found in
database is utilized
upon execution of
Dijkstra SPF algorithm
– SPF algorithm used to
create SPF tree
– SPF tree used to
populate routing table
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Introduction to OSPF
Administrative Distance
• Default Administrative Distance for OSPF is 110
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Introduction to OSPF
• OSPF Authentication
– Purpose is to encrypt & authenticate routing
information
– This is an interface specific configuration
– Routers will only accept routing information
from other routers that have been
configured with the same password or
authentication information
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Basic OSPF Configuration
Lab Topology
• Topology used for this chapter
– Discontiguous IP addressing scheme
– Since OSPF is a classless routing protocol the
subnet mask is configured in
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Basic OSPF Configuration
The router ospf command
• To enable OSPF on a router use the following
command
– R1(config)#router ospf process-id
– Process id
A locally significant number between 1 and 65535
-this means it does not have to match other OSPF
routers
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Basic OSPF Configuration
• OSPF network command
-Requires entering: network address
wildcard mask - the inverse of the subnet
mask
area-id - area-id refers to the OSPF area.
OSPF area is a group of routers that
share link state information
-Example: Router(config-router)#network network-address
wildcard-ask area area-id
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Basic OSPF Configuration
• Router ID
– This is an IP address used to identify a router
– 3 criteria for deriving the router ID
Use IP address configured with OSPF router-id
command
-Takes precedence over loopback and
physical interface addresses
If router-id command not used then router
chooses highest IP address of any loopback
interfaces
If no loopback interfaces are configured then the
highest IP address on any active interface is used
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Basic OSPF Configuration
OSPF Router ID
• Commands used to verify current router ID
– Show ip protocols
– Show ip ospf
– Show ip ospf interface
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Basic OSPF Configuration
OSPF Router ID
• Router ID & Loopback addresses
-Highest loopback address will be used as router ID if router-id
command isn’t used
-Advantage of using loopback address
the loopback interface cannot fail OSPF stability
• The OSPF router-id command
– Introduced in IOS 12.0
– Command syntax
Router(config)#router ospfprocess-id
Router(config-router)#router-idip-address
• Modifying the Router ID
– Use the command Router#clear ip ospf process
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Basic OSPF Configuration
Verifying OSPF
• Use the show ip ospf command to verify & trouble
shoot OSPF networks
– Command will display the following:
Neighbor adjacency
–
-No adjacency indicated by Neighboring router’s Router ID is not
displayed
A state of full is not displayed
–
-Consequence of no adjacency No link state information exchanged
Inaccurate SPF trees & routing tables
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Basic OSPF Configuration
Verifying OSPF - Additional Commands
Command
Show ip protocols
Show ip ospf
Show ip ospf interface
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Description
Displays OSPF process ID, router
ID, networks router is advertising
& administrative distance
Displays OSPF process ID, router
ID, OSPF area information & the
last time SPF algorithm
calculated
Displays hello interval and dead
interval
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Basic OSPF Configuration
Examining the routing table
• Use the show ip route command to display the routing
table
-An “O’ at the beginning of a route indicates that the
router source is OSPF
-Note OSPF does not automatically summarize at
major network boundaries
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OSPF Metric
• OSPF uses cost as the metric for determining the best route
-The best route will have the lowest cost
-Cost is based on bandwidth of an interface
Cost is calculated using the formula
108 / bandwidth
-Reference bandwidth
defaults to 100Mbps
can be modified using
auto-cost reference-bandwidth command
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OSPF Metric
• COST of an OSPF route
– Is the accumulated value from one router to the
next
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OSPF Metric
• Usually the actual speed of a link is different than the default
bandwidth
– This makes it imperative that the bandwidth value reflects
link’s actual speed
Reason: so routing table has best path information
• The show interface command will display interface’s bandwidth
– -Most serial link default to 1.544Mbps
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Basic OSPF Configuration
Modifying the Cost of a link
• Both sides of a serial link should be configured with the same
bandwidth
– Commands used to modify bandwidth value
Bandwidth command
– Example: Router(config-if)#bandwidthbandwidth-kbps
ip ospf cost command – allows you to directly specify
interface cost
– -Example:R1(config)#interface serial 0/0/0
–
R1(config-if)#ip ospf cost 1562
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Basic OSPF Configuration
Modifying the Cost of the link
• Difference between bandwidth command & the ip
ospf cost command
– Ip ospf cost command
Sets cost to a specific value
– Bandwidth command
Link cost is calculated
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OSPF and Multiaccess Networks
Challenges in Multiaccess Networks
• OSPF defines five network types:
– Point-to-point
– Broadcast Multiaccess
– Nonbroadcast Multiaccess (NBMA)
– Point-to-multipoint
– Virtual links
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OSPF in Multiaccess Networks
• 2 challenges presented by multiaccess networks
– Multiple adjacencies
– Extensive LSA flooding
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OSPF in Multiaccess Networks
• Extensive flooding of LSAs
– For every LSA sent out there must be an acknowledgement of
receipt sent back to transmitting router.
– consequence: lots of bandwidth consumed and chaotic
traffic
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OSPF in Multiaccess Networks
• Solution to LSA flooding issue is the
use of
– Designated router (DR)
– Backup designated router (BDR)
• DR & BDR selection
– Routers are elected to send &
receive LSA
• Sending & Receiving LSA
– DRothers send LSAs via
multicast 224.0.0.6 to DR & BDR
– DR forward LSA via multicast
address 224.0.0.5 to all other
routers
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OSPF in Multiaccess Networks
DR/BDR Election Process
• DR/BDR elections DO NOT occur in point to point
networks
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OSPF in Multiaccess Networks
• DR/BDR elections will take place on multiaccess
networks as shown below
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OSPF in Multiaccess Networks
• Criteria for getting elected DR/BDR
1. DR: Router with the highest OSPF
interface priority.
2. BDR: Router with the second highest
OSPF interface priority.
3. If OSPF interface priorities are equal, the
highest router ID is used to break the tie.
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OSPF in Multiaccess Networks
• Timing of DR/BDR Election
– Occurs as soon as 1st router has its interface
enabled on multiaccess network
When a DR is elected it remains as the DR until
one of the following occurs
-The DR fails.
-The OSPF process on the DR fails.
-The multiaccess interface on the DR fails.
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OSPF in Multiaccess Networks
• Manipulating the election process
-If you want to influence the election of DR & BDR
then do one of the following
Boot up the DR first, followed by the BDR, and
then boot all other routers,
OR
Shut down the interface on all routers, followed
by a no shutdown on the DR, then the BDR, and
then all other routers.
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OSPF in Multiaccess Networks
OSPF Interface Priority
• Manipulating the DR/BDR election process continued
– Use the ip ospf priority interface command.
– Example:Router(config-if)#ip ospf priority {0 - 255}
Priority number range 0 to 255
– 0 means the router cannot become the DR or BDR
– 1 is the default priority value
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More OSPF Configuration
Redistributing an OSPF Default Route
• Topology includes a link to ISP
– Router connected to ISP
Called an autonomous system border router
Used to propagate a default route
Example of static default route
R1(config)#ip route 0.0.0.0 0.0.0.0 loopback 1
Requires the use of the default-information originate
command
Example of default-information originate command
R1(config-router)#default-information originate
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More OSPF Configuration
Fine-Tuning OSPF
• Since link speeds are getting faster
it may be necessary to change
reference bandwidth values
– Do this using the auto-cost
reference-bandwidth
command
– Example:
R1(config-router)#auto-cost
reference-bandwidth 10000
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More OSPF Configuration
Fine-Tuning OSPF
• Modifying OSPF timers
– Reason to modify timers
Faster detection of network failures
– Manually modifying Hello & Dead intervals
Router(config-if)#ip ospf hello-interval seconds
Router(config-if)#ip ospf dead-interval seconds
– Point to be made
Hello & Dead intervals must be the same between
neighbors
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Summary
• RFC 2328 describes OSPF link state concepts and
operations
• OSPF Characteristics
– A commonly deployed link state routing protocol
– Employs DRs & BDRs on multi-access networks
DRs & BDRs are elected
DR & BDRs are used to transmit and receive
LSAs
– Uses 5 packet types:
1: HELLO
2: DATABASE DESCRIPTION
3: LINK STATE REQUEST
4: LINK STATE UPDATE
5: LINK STATE ACKNOWLEDGEMENT
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Summary
• OSPF Characteristics
– Metric = cost
Lowest cost = best path
• Configuration
– Enable OSPF on a router using the following
command
R1(config)#router ospf process-id
– use the network command to define which
interfaces will participate in a given OSPF process
Router(config-router)#network network-address
wildcard-mask area area-id
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Summary
• Verifying OSPF configuration
– Use the following commands
show ip protocol
show ip route
show ip ospf interface
show ip ospf neighbor
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Dynamic Routing protocols
Border Gateway Protocol
Fundamentals of BGP
Autonomous System
• Networks and Routers under a single
administrative authority
• Each AS is assigned a number
• AS numbers range form 1 to 65,535
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Internal BGP
• When BGP neighbors belong to the same AS
A
B
AS 200
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External BGP
• When BGP neighbors belong to
different AS
• Neighbors should be directly
connected
131.108.0.0
• Configuration
AS 109
.1
A
131.108.10.0
– Router B
B
– router bgp 110
.2
network 150.10.0.0
neighbor 131.108.10.1 remote-as 109
150.10.0.0
– Router A
– router bgp 109
network 131.108.0.0
neighbor 131.108.10.2 remote-as 110
AS 110
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IBGP, EBGP Example
AS 1
EBGP
AS 3
AS 2
EBGP
IBGP
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Advertising Networks
• Using the Network command
• Redistributing static routes
• Redistributing Dynamic routes
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Advertising Networks
Using Network Command
Router A
AS1
11.0.0.0
12.0.0.0
router bgp 1
neighbor 1.1.1.2 remote-as 2
network 11.0.0.0
network 12.0.0.0
Router B
router bgp 2
neighbor 1.1.1.1 remote-as 1
network 92.0.0.0
network 93.0.0.0
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A
EBGP
92.0.0.0
93.0.0.0
B
AS2
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Advertising Networks
By redistributing Static Routes
11.0.0.0
12.0.0.0
A
AS1
Router A
EBGP
router bgp 1
neighbor 1.1.1.2 remote-as 2
redistribute static
ip route 11.0.0.0 255.0.0.0 null 0
ip route 12.0.0.0 255.0.0.0 null 0
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92.0.0.0
93.0.0.0
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B
AS2
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Advertising Networks
By Redistributing Dynamic Routes
11.0.0.0
12.0.0.0
A
AS1
Router A
EBGP
router bgp 1
neighbor 1.1.1.2 remote-as 2
redistribute ospf 1
92.0.0.0
93.0.0.0
router ospf 1
network 11.0.0.0 0.255.255.255 area 0
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B
AS2
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BGP Attributes
•
•
•
•
•
•
AS-path
Next-hop
Local preference
MED
Origin
Communities
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BGP Attributes
AS-Path
1883
193.0.32/24
1881
193.0.33/24
• Path traversed one or more
members of a set
–{1880, 1881, 1882} (as-set)
• A list of AS’s that a route has
1880
traversed
193.0.34/24
–1880 1883 (sequence)
1882
193.0.35/24
193.0.33/24 1880 1881
193.0.34/24 1880
193.0.35/24 1880 1882
193.0.32/22
{1880,1881,1882}
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BGP Attributes
Local Preference
690
1755
1880
A
Needs to go to 690
666
• Preference sent to all routers in local AS
• Where do I want to leave the traffic ?
102
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© 1998, Cisco Systems, Inc.
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BGP Attributes
Multi-Exit Discriminator (MED)
690
1883
1755
1880
209
200
• Preference sent to all routers in remote AS
• Where do I want to receive the traffic ?
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VRRP—Virtual Redundancy
Router Protocol (HSRP)
10.1.1.33
10.1.1.2
00:10:7B:04:88:CC
10.1.1.3
00:10:7B:04:88:BB
10.1.1.1
00:10:7B:04:88:AA
default-gw = 10.1.1.1
• Transparent failover of default router
• “Phantom” router created
• One router is active, responds to
phantom addresses
• Others monitor and take over
– phantom addresses
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