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Alcatel-Lucent Routing Protocols
Module 1 — Introduction Module 2 — Static Routing and Default Routes Module 3 — Routing Information Protocol Module 4 – Link-State Protocols Module 5 — Open Shortest Path First Module 6 — Intermediate System–to–Intermediate System Module 7 — Border Gateway Protocol
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IP Addressing — Basic Subnetting
Subnetting allows a network to be subdivided into smaller networks with routing between them.
With basic subnetting, each segment uses the same subnet mask. Potential for wasting IP addresses on links that do not
require high client density Easiest to implement Required for classful routing protocols
VLSM allows the use of different subnet masks for different parts of the network.
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IP Addressing — VLSM
Different subnet masks per network Routing protocols must advertise the subnet mask
with updates More efficient use of IP addressing than basic
subnetting Requires a good understanding of subnetting RFC 1878 defines VLSM Routing protocols that support VLSM are:
RIPv2 OSPF IS-IS BGP
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IP Addressing Review
IP addresses are broken into classes: A, B, C, and D
Class A: 255.0.0.0 or /8 Network Host Host Host
Network Network Host Host
Network Network Network Host
Multicast Multicast Multicast Multicast
Class B: 255.255.0.0 or /16
Class C: 255.255.255.0 or /24
Class D: 255.255.255.255 or /32
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Section Objectives
Introduction to IP routing Review of IP forwarding Control plane vs. data plane functions Common layer 3 routing protocols
—Distance vector—Link state
Classful and classless addressing Variable length subnet masking Classless interdomain routing Private IP addresses Network address translation (NAT/PAT)
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Movement of Data
1.1.1.2 2.2.2.2
1.1.1.1 2.2.2.1
3.3.3.1 3.3.3.2
Data
Source Dest. S D
1.1.1.2 2.2.2.2 A BF
C
S
Data
Source Dest. WAN
1.1.1.2 2.2.2.2 PPPF
C
S
Data
Source Dest. S D
1.1.1.2 2.2.2.2 C DF
C
S
(MAC address = A)
(MAC address = B) (MAC address = C)
(MAC address = D)
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Packet Forwarding
When a router receives a packet, it: Compares the destination IP address of the packet to the
FIB Looks for the longest (most specific) match
If no match is found, the packet is dropped. If the packet is to be forwarded, the next hop and
egress interface must be known. If a match is found, the packet is sent to the next-hop
address via the interface specified in the FIB. The next-hop is the next router in the path toward the
destination. The egress interface is required for encapsulation.
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Common IP Routing Protocols
Legacy routing protocols: RIP version 1 RIP version 2
Modern routing protocols: OSPF IS-IS BGP
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Distance Vector Protocols
Distance = How far away Vector = What direction (interface) RIPv1, RIPv2, and BGP are distance vector protocols
Int 1/1/2
IP – 1.1.1.1Int 1/1/2
IP – 2.2.2.1
IP – 3.3.3.1 IP – 3.3.3.2
Routing Table:1.1.1.0 – Direct 1/1/23.3.3.0 – Direct 1/1/1
2.2.2.0 – 1 hop via 1/1/1
Routing Table:2.2.2.0 – Direct 1/1/23.3.3.0 – Direct 1/1/1
1.1.1.0 – 1 hop via 1/1/1
Int 1/1/1 Int 1/1/1
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Link-State Protocols
Link = An interface State = Active or inactive interface OSPF and IS-IS are link-state protocols More complex than distance vector Faster convergence Triggered updates Three databases:
Adjacency — Neighbor database Topology — Link-state database Routing — Forwarding database
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Link-State Protocols (continued)
Adjacency database
2.2.2.0/24 – via 1/1/1 cost 20– via 1/1/2 cost 40
Link-state database Forwarding database
Adjacency DatabaseRTR-B – on 1/1/1RTR-C – on 1/1/2
Routing Table:2.2.2.0/24 – via 1/1/1
LSDB
RTR - A
RTR - C
RTR - B
Network
2.2.2.0/24
1/1/1
1/1/2
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Routing Table Management
Each routing protocol populates its routes into its RIB.
Each protocol independently selects its best routes based on the lowest metric.
The best routes from each protocol are sent to the RTM.
RTM
RIP
RIB
OSPF
RIB
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Preference
The RTM may have a best route from multiple protocols.
Selection is based on lowest preference value. The RTM sends its best route to the FIB. This route is the active route and is used for
forwarding.
OSPF
BGP
RTM FIB
RIP
RIB
OSPF
RIB
OSPF
BGP
RIB
Alcatel-Lucent Interior Routing Protocols and High Availability Module 0 | 15 All rights reserved © 2006-2007 Alcatel-Lucent
Default Preference Table
Route type Preference Configurable
Direct attached 0 No
Static 5 Yes
OSPF internal 10 Yes
IS-IS Level 1 internal 15 Yes
IS-IS Level 2 internal 18 Yes
RIP 100 Yes
OSPF external 150 Yes
IS-IS Level 1 external 160 Yes
IS-IS Level 2 external 165 Yes
BGP 170 Yes
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IP Addressing — Classful and Classless
10.1.1.0/24
Routing Table:12.1.0.0 – direct 1/1/2
192.1.1.0 – direct 1/1/110.0.0.0 – 1 hop via 1/1/1
12.1.0.0/16
192.1.1.0/24 10.1.2.0/24
10.1.1.0 10.0.0.0
10.1.1.0/24
Routing Table:12.1.0.0/16 – direct 1/1/2
192.1.1.0 /24 – direct 1/1/110.1.1.0/24 – 2 hops via 1/1/110.1.2.0/24 – 1 hop via 1/1/1
12.1.0.0/16
192.1.1.0/24 10.1.2.0/24
10.1.1.0/24
10.1.1.0/24
10.1.2.0/24
Classful
Classless
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IP Addressing — VLSM
Different subnet masks per network Routing protocols must advertise the subnet mask with
updates. High-order bits are not reusable. Routing decisions are made based on the longest match. A more efficient use of IP addressing than basic subnetting Requires a good understanding of subnetting RFC 1878 defines VLSM. Routing protocols that support VLSM are:
RIPv2 OSPF IS-IS BGP
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IP Addressing — VLSM Example
172.16.0.0 – 10101100.00010000.00000000.00000000 – Reserved for WAN segments
172.16.1.0 – 10101100.00010000.00000001.hhhhhhhh – First Ethernet segment
….
172.16.254.0 – 10101100.00010000.11111110.hhhhhhhh – Last Ethernet segment
255.255.255.0 – 11111111.11111111.11111111.00000000 – Ethernet mask
172.16.0.4 – 10101100.00010000.00000000.000001 hh – First WAN segment
172.16.0.252 – 10101100.00010000.00000000.111111 hh – Last WAN segment
255.255.255.252 – 11111111.11111111.11111111.111111 00 – WAN mask
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• Routers need to know where networks are located and how best to access them.
• This can be accomplished statically with administrative commands.
What a Router Needs to Know
1.1.1.1 2.2.2.1
3.3.3.1 3.3.3.2
Routing Table:1.1.1.0/24 – Direct 3.3.3.0/30 – Direct
2.2.2.0/24 – static via 3.3.3.2
Routing Table:2.2.2.0/24 – Direct 3.3.3.0/30 – Direct
1.1.1.0/24 – static via 3.3.3.1
R1 R2
2.2.2.0/241.1.1.0/24
3.3.3.0/30
Alcatel-Lucent Interior Routing Protocols and High Availability Module 0 | 21 All rights reserved © 2006-2007 Alcatel-Lucent
Static Routes — Basic Static Routes
•Configuration of static routes between stub networks and corporate locations
2.2.2.0/24
3.3.3.1 3.3.3.2
Corporate
Headquarters
static-route 2.2.2.0/24 next-hop 3.3.3.2
static-route 0.0.0.0/0 next-hop 3.3.3.1
R1 R2
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Static Routes — Configuration Example
2.2.2.0/24
3.3.3.1 3.3.3.2
Corporate
Headquarters
config>router> static-route 0.0.0.0/0 next-hop 3.3.3.1
config>router> static-route 2.2.2.0/24 next-hop 3.3.3.2
R1 R2
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Default Routes — Basic Default Route
3.3.3.1 3.3.3.2
Corporate
Headquarters
2.2.2.0/24
R2# show router route-table
============================================================================
Route Table
============================================================================
Dest Address Next Hop Type Protocol Age Metric Pref
----------------------------------------------------------------------------
3.3.3.0/24 System Local Local 01d02h 0 0
2.2.2.0/24 System Local Local 08d03h 0 0
0.0.0.0/0 3.3.3.1 Remote Static 01d02h 1 5
----------------------------------------------------------------------------
R1 R2
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Static Routes — Floating Static Routes
2.2.2.0/24
3.3.3.1 3.3.3.2
Primary pathCorporate
Headquarters
Backup
1.1.1.1
1.1.1.2
config>router> static-route 2.2.2.0/24 next-hop 3.3.3.2
config>router> static-route 2.2.2.0/24 next-hop 1.1.1.2 preference 200
• Configuration of a floating static route between stub networks and corporate locations
R1 R2
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Static Route Verification — Show Command
The command below shows static routes configured in the routing table.
Context: show>router>
Syntax: static-route [[ip-prefix [/mask]] | [preference preference] | [next-hop ip-addr] | tag tag
Example: R1# show router route-table protocol static
==============================================================================
Route Table (Router: Base)
==============================================================================
Dest Address Next Hop Type Proto Age Metric Pref
-------------------------------------------------------------------------------
2.2.2.0/24 3.3.3.2 Remote Static 00h01m34s 1 5
2.2.2.0/24 1.1.1.2 Remote Static 00h01m15s 1 200
-------------------------------------------------------------------------------
No. of Routes: 1
==============================================================================
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Static Route Verification — Show Command (continued)
2.2.2.0/24
3.3.3.1 3.3.3.2
Corporate
Headquarters
R1# show router route-table 2.2.2.0/24
==============================================================================
Route Table (Router: Base)
===============================================================================
Dest Address Next Hop Type Proto Age Metric Pref
-------------------------------------------------------------------------------
2.2.2.0/24 3.3.3.2 Remote Static 00h02m54s 1 5
-------------------------------------------------------------------------------
No. of Routes: 1
==============================================================================
R1 R2
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Static Routes — Ping Command
2.2.2.2
2.2.2.0/24
3.3.3.1 3.3.3.2Corporate
Headquarters
R1# ping 2.2.2.2 detail
PING 2.2.2.2: 56 data bytes
64 bytes from 2.2.2.2 via fei0: icmp_seq=0 ttl=64 time=0.000 ms.
64 bytes from 2.2.2.2 via fei0: icmp_seq=1 ttl=64 time=0.000 ms.
64 bytes from 2.2.2.2 via fei0: icmp_seq=2 ttl=64 time=0.000 ms.
64 bytes from 2.2.2.2 via fei0: icmp_seq=3 ttl=64 time=0.000 ms.
64 bytes from 2.2.2.2 via fei0: icmp_seq=4 ttl=64 time=0.000 ms.
---- 2.2.2.2 PING Statistics ----
5 packets transmitted, 5 packets received, 0.00% packet loss
round-trip min/avg/max/stddev = 0.000/0.000/0.000/0.000 ms
R1#
R1# ping 2.2.2.2 detail
PING 2.2.2.2: 56 data bytes
64 bytes from 2.2.2.2 via fei0: icmp_seq=0 ttl=64 time=0.000 ms.
64 bytes from 2.2.2.2 via fei0: icmp_seq=1 ttl=64 time=0.000 ms.
64 bytes from 2.2.2.2 via fei0: icmp_seq=2 ttl=64 time=0.000 ms.
64 bytes from 2.2.2.2 via fei0: icmp_seq=3 ttl=64 time=0.000 ms.
64 bytes from 2.2.2.2 via fei0: icmp_seq=4 ttl=64 time=0.000 ms.
---- 2.2.2.2 PING Statistics ----
5 packets transmitted, 5 packets received, 0.00% packet loss
round-trip min/avg/max/stddev = 0.000/0.000/0.000/0.000 ms
R1#
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Static Routes — Traceroute Command
2.2.2.0/24
3.3.3.1 3.3.3.2
Corporate
Headquarters
R1# traceroute 2.2.2.2
traceroute to 2.2.2.2, 30 hops max, 40 byte packets
1 3.3.3.2 <10 ms <10 ms <10 ms
2 2.2.2.2 <10 ms <10 ms <10 ms
R1# traceroute 2.2.2.2
traceroute to 2.2.2.2, 30 hops max, 40 byte packets
1 3.3.3.2 <10 ms <10 ms <10 ms
2 2.2.2.2 <10 ms <10 ms <10 ms
2.2.2.2
R1 R2
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Learning Assessment
1. Do static routes have a higher or lower preference value than dynamic routes?
2. What is the command syntax to create a static route in the 7750 SR?
3. A router has a default route, a static route to 10.10.8.0/24, and a route to 10.8.0.0/14 learned from RIP. Which route is used for a packet with destination address 10.10.10.10?
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Section Objectives
Distance vector overview Split horizon Route poisoning Poison reverse Hold-down timers
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Distance Vector Overview
100 Mb/s
1 Gb/s
1 Gb/s1 Gb/s
RTR-A RTR-B
RTR-C RTR-D
Routers send periodic updates to physically adjacent neighbors
Updates contain the distance (how far) and vectors (direction) for networks
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Distance Vector Overview (continued)
The router processes and compares the information contained in the routing update received with what is in its routing table.
Update from neighbor
Process
and compare
with routing
table
Periodic update
Sent to neighbor
routers
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Split Horizon
An adjacent router does not advertise networks back to the source of the network information.
RTR-A RTR-B RTR-CX
10.0.0.010.0.0.0 – 1 hop10.0.0.0 – 2 hops
Routing Table:10.0.0.0 – 1 hop
via 1/1/1
Routing Table:10.0.0.0 – 0 hops
via 1/1/1
Routing Table:10.0.0.0 – 2 hops
via 1/1/1
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Route Poisoning
When a network goes away, the sourcing router sets the hop value to infinity and sends a triggered update to its neighbors.
RTR-A RTR-B RTR-C
10.0.0.010.0.0.0 – 16 hops10.0.0.0 – 16 hops
Routing Table:10.0.0.0 – 16 hops
via 1/1/1
Routing Table:10.0.0.0 – 16 hops
via 1/1/1
Routing Table:10.0.0.0 – 16 hops
via 1/1/1
X
Routing Table:10.0.0.0 – 0 hops
via 1/1/1
Routing Table:10.0.0.0 – 1 hop
via 1/1/1
Routing Table:10.0.0.0 – 2 hops
via 1/1/1
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Poison Reverse
Poison reverse is the only time that split horizon is violated. This helps to avoid loop creation when a network fails.
RTR-A RTR-B RTR-C
10.0.0.010.0.0.0 — 16 hops10.0.0.0 — 16 hops
X
10.0.0.0 — 16 hops
Poison reverse
10.0.0.0 — 16 hops
Poison reverse
Routing Table:10.0.0.0 — 16 hops
via 1/1/1
Routing Table:10.0.0.0 — 16 hops
via 1/1/1
Routing Table:10.0.0.0 — 16 hops
via 1/1/1Routing Table:
10.0.0.0 — 0 hopsvia 1/1/1
Routing Table:10.0.0.0 — 1 hop
via 1/1/1
Routing Table:10.0.0.0 — 2 hops
via 1/1/1
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Routing Table:10.0.0.0 – 16 hop –
Via 1/1/1
Routing Table:10.0.0.0 — 0 hops
via 1/1/1
Routing Table:10.0.0.0 – 16 hop –
Via 1/1/0
Routing Table:10.0.0.0 — 1 hop
via 1/1/1
Routing Table:10.0.0.0 – 16 hop –
Via 1/1/1
Routing Table:10.0.0.0 — 2 hops
via 1/1/1
Hold-Down Timers
Hold-down timers provide time for other routers to converge and reduce loops from being created when a network fails.
RTR-A RTR-B RTR-C
10.0.0.010.0.0.0 — 16 hops10.0.0.0 — 16 hops
X
Hold-down timer180 seconds
Hold-down timer180 seconds
Hold-down timer180 seconds
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Routing Table:10.0.0.0 – 16 hop –
Via 1/1/0
Routing Table:10.0.0.0 — 0 hops
via 1/1/1
Routing Table:10.0.0.0 – 16 hop –
Via 1/1/1
Routing Table:10.0.0.0 — 1 hop
via 1/1/1
Routing Table:10.0.0.0 – 16 hop –
Via 1/1/0
Routing Table:10.0.0.0 — 2 hops
via 1/1/1
Combined Loop Avoidance Techniques
Combined, all attributes function as follows:
RTR-A RTR-B RTR-C
10.0.0.010.0.0.0 — 16 hops10.0.0.0 — 16 hops
X
10.0.0.0 — 16 hops
Poison reverse
10.0.0.0 — 16 hops
Poison reverse
Hold-down timer180 seconds
Hold-down timer180 seconds
Hold-down timer180 seconds
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RIP Overview
Uses a hop-count metric Sends updates of the routing table to neighbors Maximum of 15 hops; 16 hops equals infinity 30-second advertisement interval by default Authentication is available in RIPv2 VLSM is supported by RIPv2
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RIP Overview (continued)
100 Mb/s
1 Gb/s
1 Gb/s 1 Gb/s
RTR-A RTR-B
RTR-C RTR-D
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RIPv1 vs. RIPv2
RIPv1 RIPv2
Defined in RFC 1058 Defined in RFCs 1721, 1722, and 2453
Classful routing protocol Classless routing protocol
No subnet mask in updates Sends subnet mask in updates
Does not support VLSM Supports VLSM and CIDR
No manual route summarization Manual route summarization
Does not support authentication Supports authentication
Broadcast updates Multicast or broadcast updates
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RIP – Major Component Configuration
Router Interface (assumed to be already complete) Route policies
RIP Group Neighbor
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Distance vectorDistance vector Link stateLink state
•Views the network topology from the neighbor’s perspective
•Adds distance vectorsfrom router to router
•Frequent, periodic updates:slow convergence
•Passes copies of the routingtable to neighbor routers
•Views the network topology from the neighbor’s perspective
•Adds distance vectorsfrom router to router
•Frequent, periodic updates:slow convergence
•Passes copies of the routingtable to neighbor routers
•Has a common view of theentire network topology
•Calculates the shortestpath to other routers
•Event-triggered updates:faster convergence
•Passes link-state routingupdates to other routers
•Has a common view of theentire network topology
•Calculates the shortestpath to other routers
•Event-triggered updates:faster convergence
•Passes link-state routingupdates to other routers
Distance Vector vs. Link State
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Link State Overview
Classless routing protocol
Sends subnet mask in update
Supports VLSM, CIDR, and manual route summarization
Supports authentication
Maintains multiple databases
Sends updates using multicast addressing
Link state-driven updates, periodic hellos
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Link State Overview (continued)
Link = An interface State = Active or inactive interface, cost IS-IS and OSPF are link-state protocols More complex than distance vector Faster convergence Triggered updates Three databases:
Adjacency – neighbor database Topology – link-state database Routing – forwarding database
Alcatel-Lucent Interior Routing Protocols and High Availability Module 0 | 47 All rights reserved © 2006-2007 Alcatel-Lucent
Link State Overview (continued)
Adjacency database
2.2.2.0/24via 1/1/2 cost 20via 1/1/1 cost 40
Link-state database Forwarding database
Adjacency databaseRTR-B – on 1/1/2RTR-C – on 1/1/1
Routing table2.2.2.0/24 via 1/1/2
LSDB
RTR - A
RTR - C
RTR - B
Network
2.2.2.0/24
1/1/2
1/1/1
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Link State Overview (continued)
Routing table10.0.0.0/8 via 2.2.2.1
…
Routing table10.0.0.0/8 via 2.2.2.1
…
10.0.0.0/8Via 2.2.2.1 Cost 10Via 3.3.3.1 Cost 20
…
10.0.0.0/8Via 2.2.2.1 Cost 10Via 3.3.3.1 Cost 20
…
Step 1 – Updates received from peers
Step 2 – Topology databasecreated
Step 3 – SPF algorithm determines the best
path to destination networksStep 4 – Routing
table created
10.0.0.0/8Via 2.2.2.1 Cost 10 – BEST
Via 3.3.3.1 Cost 20 …
10.0.0.0/8Via 2.2.2.1 Cost 10 – BEST
Via 3.3.3.1 Cost 20 …
10.0.0.0/8
3.3.3.0/30
.1
.2
2.2.2.0/30
.2
.1
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Exchanging Link-State Information
A B C D
R1 Link-state packetR1 Link-state packet
AA 1010
BB 1010
R1 R2 R3
R2 Link-state packetR2 Link-state packet
BB 1010
CC 1010
R3 Link-state packetR3 Link-state packet
CC 1010
DD 1010
Routers exchange LSPs with each other. Each begins with directly connected networks for which it has direct link-state information.
Alcatel-Lucent Interior Routing Protocols and High Availability Module 0 | 50 All rights reserved © 2006-2007 Alcatel-Lucent
Building a Topological Database
A B C DR1 R2 R3
R1 Link-state packetR1 Link-state packet
AA 1010
BB 1010
R2 Link-state packetR2 Link-state packet
BB 1010
CC 1010
R3 Link-state packetR3 Link-state packet
CC 1010
DD 1010
R1 Link-state packetR1 Link-state packet
AA 1010
BB 1010
R2 Link-state packetR2 Link-state packet
BB 1010
CC 1010
R3 Link-state packetR3 Link-state packet
CC 1010
DD 1010
R1 Link-state packetR1 Link-state packet
AA 1010
BB 1010
R2 Link-state packetR2 Link-state packet
BB 1010
CC 1010
R3 Link-state packetR3 Link-state packet
CC 1010
DD 1010
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Calculating the SPF Tree and Populating the Routing Table
A B C DR1 R2 R3
R1 Link-state packetR1 Link-state packet
AA 1010
BB 1010
R2 Link-state packetR2 Link-state packet
BB 1010
CC 1010
R3 Link-state packetR3 Link-state packet
CC 1010
DD 1010
SPF tree
SPF
R1Routing
table
R1Routing
table
1
2
3
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SPF Algorithm
R1
10.0.0.0/8 (net1)
5
10
100
R3
R2
R1 LSDB
R1, R2, 5
R1, R3, 10
R2, R1, 5
R2, R3, 100
R3, R1, 10
R3, R2, 100
R3, net1, 0
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SPF Algorithm (continued)
R1
10.0.0.0/8 (net1)
5
10
100
R3
R2
Step Candidate Cost to root
SPF tree
1 — — R1, R1, 0
2 R1, R2, 5
R1, R3, 10
5
10
R1, R1, 0
3 R1, R3, 10 10 R1, R1, 0
R1, R2, 5
4 R1, R3, 10
R2, R3, 100
10
105
R1, R1, 0
R1, R2, 5
5 R3, net1, 0 10 R1, R1, 0
R1, R2, 5
R1, R3, 10
6 — — R1, R1, 0
R1, R2, 5
R1, R3, 10
R3, net1, 0
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Link State – Topology Change
Run SPFUpdateroutingtable
Run SPFUpdateroutingtable
Run SPFUpdateroutingtable
Run SPFUpdateroutingtable
Run SPFUpdateroutingtable
Run SPFUpdateroutingtable
Topologychange
Topologychange
Link-state updates are driven by topology changes.
Link-state information
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Sequence Numbers
Sequence numbers must be included in the link-state information. Without sequence numbers, the link-state information could
be flooded indefinitely. The sequence number remains the same, router-to-router,
during the flooding process. In a link-state environment, routers use the sequence
numbers for the following decisions when they receive link-state updates: If the sequence number is lower than the one in the
database, the link-state information is discarded. If the sequence number is the same as the one in the
database, an ACK is sent. The link-state information is then discarded.
If the sequence number is higher, the link-state information is populated in the topological database, an ACK is sent, and the link-state information is forwarded to its neighbors.
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Sequence Numbers (continued)
A B C D
R1 Link-state packetR1 Link-state packet
Seq=2Seq=2
R1 R2 R3
R1 Link-state packetR1 Link-state packet
Seq=1Seq=1R1 Link-state packetR1 Link-state packet
Seq=1Seq=1
A B C D
R1 Link-state packetR1 Link-state packet
Seq=2Seq=2
R1 R2 R3
R1 Link-state packetR1 Link-state packet
Seq=2Seq=2R1 Link-state packetR1 Link-state packet
Seq=1Seq=1
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Sequence Numbers (continued)
B C
D
R2 R3
A
F E
R5 R4R6
R1
Z
R1 receives 2 copies of the link-state information for network Z.
—R1 must decide what to do with the second copy of the link-state information it receives.
Cost 20 Cost 20
Cost 10Cost 10
Cost 10 Cost 10
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Link-State Information Aging
Link-state information includes an age field. The age of newly created link-state information is set to
0 for OSPF and 1200 for IS-IS. It is incremented by every hop during the flooding procedure for OSPF and is decremented for IS-IS. The link-state age is also incremented for OSPF and decremented for IS-IS as it is held in the topological database.
Maximum age When the link-state information reaches its maximum
age, it is no longer used for routing. The link-state information is flooded to the neighbors with the maximum age, and the link-state information is removed from the topological database.
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IS-IS – Packet Processing
A router deals with topology changes as follows:
LSU/LSAIs entry in
LSDB?
Sequence No.
same?Ignore
End
NoNo
No
Yes Yes
Yes
Add to LSDB
Send ACK
Flood LSA
Run SPF
Is sequence number higher
than one inLSDB?
Send LSU back with newer information
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Hierarchy in Link-State Networks
Scalability issues exist for link-state networks: The size of the link-state database increases exponentially
with the size of the network. The complexity of the SPF calculation also increases
exponentially. A topology change requires complete recalculation of the
forwarding table on every router. Hierarchy allows a large routing domain to be split into
several smaller routing domains. IS-IS and OSPF both implement hierarchy but use
different techniques. Hierarchy results in suboptimal routing. Hierarchy is less common than in the past due to the
increased capacity of routers.
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IS-IS – Hierarchical View
Backbone (Level 2) links Level 1 linksL1 Level 1L2 Level 2L1/L2 Level 1/Level 2
Area 1
Area 2
Area 3
L1L2
L1/L2
L1/L2
Integrated IS-IS Network
L1
L1/L2 L1
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OSPF – Hierarchical View (continued)
OSPF Hierarchical Routing
Area 0.0.0.0
Area 0.0.0.1 Area 0.0.0.2
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OSPF v1RFC 1131defined
OSPF v1RFC 1131defined
OSPF v2Updated
RFC 1583
OSPF v2Updated
RFC 1583
OSPF v2Updated
RFC 2328
OSPF v2Updated
RFC 2328
OSPF for IPv6
RFC 2740
OSPF for IPv6
RFC 2740
OSPF — RFC History
OSPFworkgroup
formed
OSPFworkgroup
formed
OSPF v2RFC 1247defined
OSPF v2RFC 1247defined
OSPFwork in progress
OSPFwork in progress
OSPF v2Updated
RFC 2178
OSPF v2Updated
RFC 2178
1987
1998
1997
1994
1991
1989
Present
1999
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OSPF — Protocol Overview
Classless routing protocol
Subnet mask sent in update
Support for VLSM, CIDR, and manual route summarization
Support for authentication
Maintenance of multiple databases
Multicast addressing – 224.0.0.5 and 224.0.0.6
Link state-driven updates, periodic hellos
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OSPF — Key Features
Key OSPF features are: Backbone areas Stub areas NSSAs Virtual links Authentication Support for VLSM and CIDR Route redistribution Routing interface parameters OSPF-TE extensions
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OSPF — Protocol Comparison
Feature
Updates
Update type
Transport
Authentication
Metric
Metric type
VLSM / CIDR support
Topology size
Convergence
RIPv2
Periodic
Broadcast/Multicast
UDP
Simple and MD5
Hops
Distance vector
Yes
Small/Medium
Slow
IS-IS
Incremental
L2 Multicast
Layer 2
Simple and MD5
Cost
Link-state
Yes
Large
Fast
OSPF
Incremental
L3 Multicast
IP
Simple and MD5
Cost
Link-state
Yes
Large
Fast
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OSPF — Link-State Protocol Comparison
Feature
Updates
Multicast layer
Authentication
Metric
Metric type
LSA types
Area hierarchy
Area boundaries
Convergence
IS-IS
Incremental
Layer 2
Simple and MD5
Default: all ports cost 10
Link-state
L1 and L2
Not required
On segment
Fast
OSPF
Incremental
Layer 3
Simple and MD5
Auto-calculation on interface
Link-state
Multiple types
Backbone area
At interface
Fast
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OSPF — Path Determination
OSPF uses SPF for path determination. SPF uses cost values to determine the best path to a
destination.
RTR-A
RTR-C
RTR-B
Cost 0 Cost 10
Cost 125 Cost 125
Cost 125
RTR-A
10.0.0.0 – Cost 260 via RTR C
*10.0.0.0 – Cost 135 via RTR B
* = Best path
10.0.0.0
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Calculating Link Cost
Cost = reference-bandwidth ÷ bandwidth The default reference-bandwidth is 100 000 000 kb/s
or 100 Gb/s.
The default auto-cost metrics for various link speeds are as follows:
—10-Mb/s link default cost of 10 000—100-Mb/s link default cost of 1000—1-Gb/s link default cost of 100—10-Gb/s link default cost of 10
The cost is configurable.
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Configuration Basics
Interfaces must be configured in an OSPF area. By default, interfaces in an area are advertised by OSPF. Routes received through OSPF are advertised by OSPF. No other routes are advertised by default.
Verify that adjacencies are formed with neighbors. Verify that routes are in the routing table.
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OSPF — Multicast Addressing
OSPF uses class D multicast addresses in the range 224.0.0.0 to 239.255.255.255.
Specially reserved addresses for OSPF: 224.0.0.5: All routers that speak OSPF on the segment 224.0.0.6: All DR/BDRs on the segment
IP multicast addresses use the lower 23 bits of the IP address as the low-order bits of the MAC multicast address 01-005E-XX-XX-XX. 224.0.0.5 = MAC 01-00-5E-00-00-05 224.0.0.6 = MAC 01-00-5E-00-00-06
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OSPF — Generic Packet
OSPF packets use protocol number 89 in the IP header.
OSPF is its own transport layer.
Link header IP header OSPF packet types Link trailer
IP header protocolID 89 = OSPF
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OPSF — Packet Types
OSPF hello OSPF database descriptor OSPF link-state request OSPF link-state update OSPF link-state ACK
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OSPF — Link Topology Types
Multi-accessMulti-access
Point-to-pointPoint-to-point
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OSPF — Router ID
Each router must have a router ID, the ID by which the router is known to OSPF. The default RID is the last 32 bits of the chassis MAC
address. Configuring a system interface overrides the default.
—Using a system interface is easier to document.
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On point-to-point links, there is no need for a DR or BDR. All packets are sent via IP multicast address 224.0.0.5. Usually a leased-line (i.e., HDLC, PPP) segment Can be configured on point-to-point Ethernets
RTR - A
RTR - C
RTR - B
Network
2.2.2.0/24
OSPF — Point-to-Point Segments
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OSPF — LAN Communication
Election of the DR and BDR in multi-access networks:
C
1.1.1.1
D
1.1.1.2
E
1.1.1.3
A
1.1.1.5
B
1.1.1.4
Each router sends hellos. The router with the highest priority is the DR. If all priorities are the same, the DR is the router with
the highest RID.
RTR-A
Has the highest
RID, so it will be
the DR
RTR-B
Has the second highest
RID, so it will be the BDR
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OSPF — Exchanging Updates in a LAN
Election of the DR and BDR in multi-access networks:
RTR-C
1.1.1.1
D
1.1.1.2
E
1.1.1.3
RTR-A (DR)
1.1.1.5
RTR-B (BDR)
1.1.1.4
Routers use the 224.0.0.6 IP address to send updates to the DRs.
The BDR monitors the DR to ensure that it sends updates.
The DR uses 224.0.0.5 to send updates to all OSPF routers.
RTR-C sends update to
All DRs using IP address
224.0.0.6
RTR-A sends update to
All OSPF routers using
IP address 224.0.0.5
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IS-IS — Protocol Overview
Development began prior to that of OSPF. The U.S. government required ISPs to use IS-IS for
early stages of the Internet. IS-IS supports IPv6. Many large enterprise networks and ISPs use IS-IS
due to the scalability and stability of the protocol.
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RFC 1629NSAP and
Internet
RFC 1629NSAP and
Internet
RFC 33509TLV
code points
RFC 33509TLV
code points
IS-IS — RFC History
RFC 1142Original
RFC
RFC 1142Original
RFC1990
2002
…..
1994
1992
1990
RFC 1195TCP/IPsupport
RFC 1195TCP/IPsupport
ISO 10589released
ISO 10589released
Present
IS-ISwork in progress
IS-ISwork in progress
Other IS-ISRFCs
released
Other IS-ISRFCs
released
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IS-IS — Protocol Overview (continued)
Classless routing protocol
Subnet mask sent in update
Support for VLSM, CIDR, and manual route summarization
Support for authentication
Maintenance of multiple databases
Layer 2 multicast addressing
Link-state driven updates, periodic hellos
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IS-IS — Key Features
Key IS-IS features are: Area hierarchy Authentication Support for VLSM and CIDR Route redistribution Routing interface parameters IS-IS TE extensions
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IS-IS — Protocol Comparison
Feature
Updates
Update type
Authentication
Metric
Metric type
VLSM / CIDR support
Topology size
Summarization
Convergence
RIPv2
Periodic
Broadcast/Multicast
Simple and MD5
Hops
Distance vector
Yes
Small
Manual
Slow
OSPF
Incremental
L3 Multicast
Simple and MD5
Cost
Link-state
Yes
Very large
Manual
Fast
IS-IS
Incremental
L2 Multicast
Simple and MD5
Cost
Link-state
Yes
Very large
Manual
Fast
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IS-IS — Link-State Protocol Comparison
Feature
Updates
Multicast layer
Authentication
Metric
Metric type
Update types
Area hierarchy
Area boundaries
Convergence
IS-IS
Incremental
Layer 2
Simple and MD5
Default: all ports cost 10
Link-state
L1 and L2
Not required
On segment
Fast
OSPF
Incremental
Layer 3
Simple and MD5
Auto-calculation on interface
Link-state
Multiple types
Backbone area
At interface
Fast
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IS-IS — Frequently Used Terms
Area — Corresponds to the level 1 subdomain End system — Typically a computer, printer, or other
attached device Intermediate system — Router in an IS-IS network Neighbor — A physically adjacent router Adjacency — A separate adjacency is created for each
neighbor on a circuit and for each level of routing (level 1 and level 2) on a broadcast circuit.
Circuit — A single locally attached network Link — The communication path between 2 neighbors CSNP — Complete sequence number PDU PSNP — Partial sequence number PDU PDU — Protocol data unit
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IS-IS — Frequently Used Terms (continued)
Designated IS — The intermediate system in a LAN that is designated to generate updates on behalf of the nodes in the LAN
Pseudo node — When a broadcast subnetwork has n connected intermediate systems, the broadcast subnetwork itself is considered to be a pseudo node.
Broadcast subnetwork — A multi-access subnetwork (such as Ethernet) that supports the capability of addressing a group of attached systems with a single PDU
General topology subnetwork — A topology that is modeled as a set of point-to-point links, each of which connects 2 systems
Routing subdomain — A set of intermediate systems and end systems that are located within the same routing domain
Level 2 subdomain — The set of all level 2 intermediate systems in a routing domain
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IS-IS — Protocol Overview
IS-IS uses SPF for path determination. SPF uses cost values to determine the best path to a
destination.
RTR-A
RTR-C
RTR-B
Cost: 10 Cost: 10
Cost: 10 Cost: 10
Cost: 10
RTR-A
10.0.0.0: cost 30 via RTR-C
*10.0.0.0: cost 20 via RTR-B
* = Best path
10.0.0.0
Packet flow
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IS-IS — ISO Network Addressing
IS-IS uses unique addressing (OSI NSAP addresses) compared to that of other IP routing protocols.
Each address identifies the area, system, and sector. Routers with common area addresses form L1
adjacencies. Routers with different area addresses form L2
adjacencies, if capable. 2-layer hierarchy:
Level 1: Builds the local area topology and forwards traffic to other areas through the nearest L1/L2 router
Level 2: Exchanges prefix information and forwards traffic between areas
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IS-IS — ISO Network Addressing (continued)
Layer 2 multicast addressing is implemented to support IS-IS.
On Ethernet, the following multicast addresses are reserved: L1 updates use 01-80-C2-00-00-14. L2 updates use 01-80-C2-00-00-15.
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IS-IS — Link-State Overview
Backbone (level 2) link Level 1 linkL1 Level 1L2 Level 2L1/L2 Level 1/level 2
Area 49.0001
Area 49.0002
Area 49.0003
L1L2
L1/L2
L1/L2
L1
L1/L2 L1
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IS-IS — NSAP Addressing
IDP DSP
AFI System ID SELHigh Order-DSP
variable 6 1
Area ID System Address
NSAP — Network service access point
IDP — Initial domain part DSP — Domain specific part
AFI — Authority and format indicator IDI — Initial domain identifier (e.g., 49 is local assigned, binary)
High Order-DSP — High Order Domain Specific Part
SEL — N-selector (NSEL)
IDI
NSEL
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IS-IS — Protocol Characteristics
Item
Value
Maximum metric value assignable to a link
16 777 215
Maximum metric value for a path
4 261 412 864
All L1 IS multicast address
01-80-C2-00-00-14
All L2 IS multicast address
01-80-C2-00-00-15
SAP for IS-IS on 802.3 LANs
FE
Protocol discriminator for IS-IS
83
NSAP selector for IS-IS
00
Sequence modulus
232
Size of LSP, which all IS routers must be able to handle
1492
Maximum age
1200
Zero life age
60
Maximum number of area addresses in a single area
3
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IS-IS — Packet Format
IS-IS packets use layer 2 encapsulation of the media. The Ethernet type field is set to 0xFEFE to denote an
IS-IS packet instead of an IP packet. The TLV identifies the type of information in the IS-IS
packet. IS-IS packets are called PDUs.
Ethernet header
Type = 0xFEFEIS-IS header IS-IS TLV Link trailer
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IS-IS — Packet Format Details
Ethernet destination address: 01-80-C2-00-00-14 – L1 updates 01-80-C2-00-00-15 – L2 updates
Ethernet source address: source router interface MAC address
802.3 LLC DSAP and SSAP = FE:FE Layer 3 protocol discriminator: 83
Ethernet header
Type = 0xFEFEIS-IS header IS-IS TLV Link trailer
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IS-IS — Packet Format Details (continued)
IS-IS sends PDUs. PDUs are encapsulated directly into the layer 2
frame. There are 4 types of PDUs:
Hello (ESH, ISH, and IIH) — Maintain adjacencies LSP (link-state packet) — Information about neighbors
and links, generated by all L1 and L2 routers PSNP (Partial Sequence Number PDU) — Specific
requests and responses about links, generated by all L1 and L2 routers
CSNP — Complete list of LSPs exchanged to maintain database consistency
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BGP Scope
Enables the exchange of routing information between autonomous systems (AS)
An AS is a collection of routers that are under a single administration, which presents a consistent routing policy.
Enables the implementation of administrative policies BGP has already scaled to:
Large number of ASs Large number of neighbors Large volume of table entries High rate of change
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Autonomous Systems in BGP
AS-65001
AS-65002
AS-65003
• An AS is a group of networks and network equipment under a common administration.
• IGP protocols such as OSPF, IS-IS, and RIP run in an AS.
• BGP is used to connect ASs.
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Autonomous Systems in BGP (continued)
Public autonomous systems: Assigned by ARIN or another authority Must be used when connecting to other ASs on the
Internet. Range from 0 to 64 511
Private autonomous systems: Assigned by ISPs (for some clients) and local
administrators Not allowed to be advertised to other ISPs or on the
Internet Range from 64 512 to 65 535
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BGP Features
Path vector protocol: Neighbor is any reachable device Unicast exchange of information Reliability using TCP Uses well-known TCP port 179 Periodic keepalive for session management Event-driven Robust metrics Authentication
Similar behavior as other TCP/IP applications Because BGP peers are not always directly
connected, BGP relies on IGP to route between peers.
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eBGP vs. iBGP Overview
2 types of BGP sessions are possible. The routers may be in different ASs:
Called external BGP or eBGP Typically directly connected, but not mandatory Different administrations
The routers may be in the same AS: Called internal BGP or iBGP Typically remote, but could be directly connected Same administration
Recommended