Open Scaleable Routing

1
Open Scaleable Routing
Network Professional

• Bob Horne
• Network Consultant
• CCIE 2884 / CVoice
• NNCSS
• 22-Mar-2004

2
Scaleable Routing
3
VLSM

• Variable length is not new
• Ex Classful addressing which used different
  length network address fields determined from
  context
• RFC 760 used only 1st octet
• RFC 791 introduced classful addressing when gt250
  networks threatened the early standard
• Considered the IP standard

4
VLSM

• Subnetting extends prefix to the right using a
  mask to specify prefix length
• Variable-length subnet masking removes
  byte-boundary restriction of classful subnetting

5
Using VLSM

• Subnet with a classful mask at first. For
  example, one might subnet a class-B address with
  an 8-bit subnet using a 24-bit network mask.
• Further subnet one classful subnet with an
  extended mask.
• Keep small subnets together to facilitate
  consolidation of routing information.

6
Prefix-based Routing
Router looks for the longest match
7
DV Quick Review

• Characteristics
• Periodic unreliable updates
• network address
• hop count
• Router checks IP routes each time update is
  received
• Scalability Concerns
• Convergence
• Update traffic
• Metric Limitations
• Max Hop Count
• Hold-down delay

8
Link State Overview

• Topological database
• Complete area topology
• Route selection hierarchy
• Cost metric
• Reliable update technique

9
Link State Concepts

• LSPs
• Exchange link state information
• Flooded within an area
• SPF algorithm SPF tree
• Used to construct a Topology Database
• Scalability
• CPU / Memory utilization
• Complex topological database
• Segmentation
• Areas
• Initial flooding
• Link state traffic
• Convergence

RFC 2328
10
Basic Link State Operation

• Routers Exchange LSPs
• Directly connected networks
• Each router constructs topology database in
  parallel
• SPF tree
• SPF computes paths to each network
• Best paths used in routing table for switching
  packets

11
Link State Routing

• Dampen update frequency
• Settable default of 30 mins
• Updates are multicast
• Designated routers reduce traffic on LANs
• Uses Area hierarchy
• Summarizes routes at area borders
• Scales and stabilizes core
• Timestamps
• Sequence numbering

12
DV and LS Comparison
Distance Vector Link State

• Views net topology from neighbors perspective
• Adds distance vectors from router to router
• Frequent periodic updates
• Slow convergence
• Passes copies of routing table to neighbors

• Gets common view of entire network topology
• Calculates shortest path to other routers
• Event-triggered updates
• Faster convergence
• Passes link-state routing updates to other routers

13
OSPF Concepts

• Link State Packets
• OSPF Router ID
• Link State Advertisement
• Hello protocol
• Designated router on multiaccess networks
• Router classification
• Stub Areas
• Route selection
• Route summarization
• VLSM
• Interoperability

14
OSPF LSP Types

• 1 Hello
• 2 Data Description
• 3 Link State Request
• 4 Link State Update
• aka LSA for Link State Advertisement
• 5 Link State Acknowledge

15
OSPF Router ID

• Number by which the router is known to OSPF
• Default is highest IP address on an active
  interface when OSPF starts
• Default is superceded by (highest) by loopback
  address
• Bogus loopback address
• Not in OSPF topology table
• Real loopback address
• In OSPF table, easier to trace
• Used by NMS for router discovery and management
• int lo 0

16
Link State Advertisement

• Sourced by router directly connected to link
• Flooded by all other routers
• Transmitted at each link state change
• Transmitted every 30 minutes for refresh
• Reliable (acknowledged) messages

17
TYPE 4 (LSA) TYPES

• 1 Router Link Advertisements
• Router's link to the area
• Flooded within the area
• 2 Network Link Advertisements
• Generated by Designated Routers
• Flooded in the area that contains the network
• 3 Summary Link Advertisements
• Routes to networks
• Also used for aggregating routes
• 4 Summary Link Advertisements
• Describes routes to ASBR.
• 5 AS External Link Advertisements
• Originated by ASBR.
• Routes to destinations external to the AS
• Flooded all over except stub areas
• 7 NSSA

18
OSPF Network Types

• Point-to-point
• Neighbors across link using multicast
• Multi-access (Broadcast)
• DR communicates with other OSPF routers on the
  LAN
• Neighbors are formed using multicast hellos
• Non-broadcast Multi-access (NBMA)
• ip ospf net bro
• Point-to-multipoint
• Collection of point-to-point links
• No DR elected
• Packets are multicast
• Virtual links
• Packets are unicast

19
OSPF Hello Protocol

• Neighbor router discovery
• Multicast OSPF Hellos
• Creates and maintains adjacencies
• Elects DR on multiaccess networks
• NBMA use ip ospf pri 0

20
OSPF NEIGHBORS

• Init
• Hello detected
• Two-way
• Own RID seen
• DR/BDR elected
• ExStart
• Sequence number
• Primary/Secondary
• Primary will POLL
• Exchange
• LSAs flooded
• Loading
• Retransmissions
• Full
• Adjacency established

21
OSPF NEIGHBORS

• sh ip ospf neighbor
• Neighbor ID Pri State Dead Time
  Address Interface
• 203.250.12.1 1 2WAY/DROTHER 00037
  203.250.14.3 Ethernet0
• 203.250.15.1 1 FULL/DR
  00036 203.250.14.2 Ethernet0
• 203.250.13.41 1 FULL/BDR
  00034 203.250.14.1 Ethernet0

22
OSPF Designated Router

• OSPF Hello elects BDR
• BDR is promoted to DR
• Receives multicasts to 224.0.0.6
• Advertises to 224.0.0.5
• Minimizes
• OSPF traffic
• adjacency state information

23
OSPF Network Topology

• OSPF is hierarchical by design
• Area 0 backbone required when 2 or more areas are
  implemented

24
OSPF Virtual Links

• Maps to remote RID
• Loopback Interface
• Router OSPF
• area ltarea-idgt virtual-link ltRIDgt
• area ltarea-idgt range ltip-addressgt ltip-maskgt

25
OSPF Router Classification

• Backbone router
• ABR
• Internal area router
• ASBR

Mutually non-exclusive
26
OSPF Stub Areas

• Area 0 has external routes
• All routers attached to stub area
• area ltarea-idgt stub
• Stub ABR originates default
• default-information originate
• May have more than 1 ABR
• Totally Stubby Area
• area ltarea-idgt stub no-summary
• O default route(s) only
• NSSA
• stub area than contains an ASBR that imports
  external routes

27
OSPF Route Selection

• O OSPF derived intra-area route
• IA Inter-Area summary route
• E1 Type-1 external route
• Sum of internal and external metrics
• E2 Type-2 external route
• Does not add internal metrics (default)

28
TROUBLESHOOTING OSPF

• C2513A sh ip ospf interface e 0
• Ethernet0 is up, line protocol is up
• Internet Address 203.250.14.1 255.255.255.0,
  Area 0.0.0.0
• Process ID 10, Router ID 203.250.13.41, Network
  Type BROADCAST, Cost 10
• Transmit Delay is 1 sec, State BDR, Priority 1
• Designated Router (ID) 203.250.15.1, Interface
  address 203.250.14.2
• Backup Designated router (ID) 203.250.13.41,
  Interface address
• 203.250.14.1
• Timer intervals configured, Hello 10, Dead 40,
  Wait 40, Retransmit 5
• Hello due in 00002
• Neighbor Count is 3, Adjacent neighbor count is
  3
• Adjacent with neighbor 203.250.15.1
  (Designated Router)
• Loopback0 is up, line protocol is up
• Internet Address 203.250.13.41 255.255.255.255,
  Area 1
• Process ID 10, Router ID 203.250.13.41, Network
  Type LOOPBACK, Cost 1
• Loopback interface is treated as a stub Host

29
In Summary
30
IP Route Summarization

• Subnetting extends prefix to the right
• Summarization collapses prefix to the left
• IP addresses must share same high-order bits
• Routing protocol must carry prefix length with
  address

RFC 1518
31
IP Summarization Example
Routing Table for B
Routing Table for C
32
OSPF Routing Example
33
OSPF Routing Example
34
Thanks