Dynamic Routing
Distance Vector and Link State
RIP
OSPF
Internet Routing
• IP implements datagram forwarding
• Both hosts and routers
• Have an IP module
• Forward datagrams
• IP forwarding is table-driven
• Table known as routing table
Routing Tables
• Static routing
• Fixes routes at boot time
• Useful only for simplest cases
• Dynamic routing
• Table initialized at boot time
• Values inserted/updated by protocols that propagate
route information -> Routers use protocols to learn
new information and update their routing table
dynamically
• Necessary in large internets
Interdomain and Intradomain
Routing
Intradomain Routing
• Routing within an AS
• Ignores the Internet outside
the AS
• Protocols for Intradomain
routing are also called
Interior Gateway Protocols
or IGP’s.
• Popular protocols are
• RIP (simple, old)
• OSPF (better)
4
Interdomain Routing
• Routing between AS’s
• Assumes that the Internet
consists of a collection of
interconnected AS’s
• Normally, there is one
dedicated router in each AS
that handles interdomain
traffic.
• Protocols are collectively
called Exterior Gateway
Protocols or EGP’s.
• Routing protocols:
• Border Gateway Protocol
(BGP) v4 current
Routing Domains
Components of a Routing
Algorithm
• A procedure for sending and receiving
reachability information about a network to
other routers
• A procedure for calculating optimal routes
• Routes are calculated using a shortest path
algorithm (least “cost”)
• A procedure for reacting to and advertising
topology changes
6
Two Basic Shortest Path Routing
Algorithms
Distance Vector Routing
• Each node knows the distance (cost) to its directly connected
neighbors
• A node sends periodically a list of routing updates to its
neighbors.
• If all nodes update their distances to destinations using neighbor
information, the routing tables eventually converge
• New nodes advertise themselves to their neighbors.
Link State Routing
• Each node knows the distance (cost) to its directly connected
neighbors
• The distance information is broadcast to all nodes in the network
• Each node calculates the routing tables independently using
global information.
7
Summary of Differences
Internet Routing Algorithms
Distance Vector
Link State
• Routing Information
Protocol (RIP)
• Intermediate System Intermediate System (IS-IS)
• Gateway-to-Gateway Protocol
(GGP)
• Open Shortest Path First
(OSPF)
• Exterior Gateway Protocol
(EGP)
• Interior Gateway Routing
Protocol (IGRP)
9
Distance Vector Algorithm
• Initialize routing table with one entry for each directly
connected Network
• Periodically run a distance-vector update to exchange
information with routers that are reachable over directly
connected networks
Distance Vector Dynamic
Updates
• Every router sends list of its routes to all its neighbors
• List contains pairs of destination network and distance
• Receiver replaces entries in its table if routing through the
sender (i.e., router that just sent an update) is less
expensive than the current route in its table
• Receiver propagates new routes next time it sends out an
update
• Update Algorithm has well-known shortcomings (we will
see an example later)
Example
Assume:
• link cost is 1 on all hops
• all updates occur simultaneously
• initially each router only knows the cost of
connected interfaces = 0.
Rip Convergence Example
After First Update
After Second Update
After Third Update
Last Update for Convergence
The Count-to-Infinity Problem
1
1
Network 4.0.0.0 goes down
19
Event: Update from B to C occurs
Node C uses that Update to
Update its table
This same process repeats when B sends again its update to
C, and vice versa. The metric will increase to infinity so this
phenomenon is called “counting to infinity”
Count-to-Infinity
• The reason for the count-to-infinity
problem is that each node ONLY has a
“next-hop-view”
• For example, in the first step, C did not
realize that its route (with cost 1) to
network 4.0.0.0 went through node B
and B did not realize that C’s update
was based on its (B’s) connectivity
information.
• How can the Count-to-Infinity problem
be
solved?
22
How to Prevent Count to Infinity
• ROUTE POISONING Step 1:
• Marking a down link as a distance of infinity away instead of
just being down. For example, when network 4 goes down,
router C starts route poisoning by advertising the metric (hop
count) of this network as 16, which indicates an unreachable
network. When B gets update, it knows not to use that route.
• SPLIT HORIZON Step 2:
• A router never sends information about a route back in the
same direction from which the original information came,
routers keep track of where the information about a route
came from. Means when router A sends update to router B
about a particular network failure, router B does not send any
update for that same network to router A as A is on the path to
that network destination.
More….
• OR……
• POISON REVERSE:
• The poison reverse rule overwrites split horizon rule. For
example, if router B receives a route poisoning of network 4
from router C then router B will send an update back to router
C (which breaks the split horizon rule) with the same poisoned
hop count of 16. This ensures all the routers in the domain
receive the poisoned route update. Notice that every router
performs poison reverse when learning about a downed
network. In the above example, router A also performs poison
reverse when learning about the downed network from B
More
• OR…….
• HOLD DOWN TIMERS:
• After hearing a route poisoning, router starts a hold-down timer for that route. If
it gets an update from the router with a better metric than the originally
recorded metric within the hold-down timer period, the hold-down timer is
removed and the table is updated. Also within the hold-down timer, if an update
is received from a different router than the one who performed route poisoning
with an equal or poorer metric, that update is ignored. During the hold-down
timer, the “downed” route appears as “possibly down” in the routing table.
• For example, in the above example, when B receives a route poisoning update
from C, it marks network 4 as “possibly down” in its routing table and starts the
hold-down timer for network 4. In this period if it receives an update from C
informing that the network 4 is recovered then B will accept that information,
remove the hold-down timer and allow data to go to that network. But if B
receives an update from A informing that it can reach network 4 in 1 (or more)
hops, that update will be ignored and the hold-down timer keeps counting.
• Note: The default hold-down timer value = 180 second.
More
• TRIGGERED UPDATE :
• If any route goes down in the network, do not wait for the next
periodic update, instead send an immediate update for that
route using route poisoning.
• COUNTING TO INFINITY:
• Maximum count 15 hops after that destination is declared not
reachable.
Characteristics of Distance Vector
Routing
• Periodic Updates: Updates to the routing tables are
sent at the end of a certain time period. A typical value
is 90 seconds.
• Triggered Updates: If a metric changes on a link, a
router immediately sends out an update without
waiting for the end of the update period.
• Full Routing Table Update: Most distance vector
routing protocol send their neighbors the entire routing
table (not only entries which change).
• Route invalidation timers: Routing table entries are
invalid if they are not refreshed. A typical value is to
invalidate an entry if no update is received for 3-6
update periods.
27
Link State Algorithm
• Alternative to distance-vector
• Distributed computation
• Broadcast information
• Allow each router to compute shortest paths
• Avoids problem where one router can damage
the entire internet by passing incorrect
information
• Also called Shortest Path First (SPF)
Link State Update
• Participating routers learn internet topology
• Think of routers as nodes in a graph, and
networks connecting them as edges or links
• Pairs of directly-connected routers periodically
• Test link between them
• Propagate (broadcast) status of link
• All routers
• Receive link status messages
• Re-compute routes from their local copy of
information
RIP - Routing Information Protocol
• A simple intradomain protocol (Interior Gateway Protocol IGP)
• Straightforward implementation of Distance Vector Routing
• Each router advertises its distance vector every 30 seconds (or
whenever its routing table changes) to all of its neighbors
(destination address, distance)
• Uses hop count metric and uses 1 as link metric
• Maximum hop count is 15, with “16” equal to “”
• Routes are timed out (set to 16) after 3 minutes if they are not
updated
• Uses split horizon and poison reverse techniques to solve
``Inconsistencies’’
• Current standard is RIPv2
30
Two Forms of RIP
Active
• Used by routers
• Broadcasts routing updates periodically
• Uses incoming messages to update routes
Passive
• Used by hosts
• Uses incoming update messages to change route
table – changes eliminate ICMP redirects
• Does not send updates
Changes to RIP1  RIPv2
• Update includes subnet mask
• Authentication supported
• Explicit next-hop information
• Messages can be multicast (optional)
• IP multicast address is 224.0.0.9
RIPv2 Update Format
Route Tag: Used to carry information from other
routing protocols (e.g., autonomous system number)
RIP Messages
• Dedicated port for RIP is UDP port 520.
• Two types of command messages:
• Request messages
• used to ask neighboring nodes for an update
• Response messages
• contains an update
34
Routing with RIP
• Initialization: Send a request packet on all interfaces
requesting routing tables from neighboring routers:
• RIPv1 uses broadcast if possible,
• RIPv2 uses multicast address 224.0.0.9, if possible
• Request received: Routers that receive above request send
their entire routing table
• Response received: Update the routing table
• Regular routing updates: Every 30 seconds, send all or part of
the routing tables to every neighbor in a response message
• Triggered Updates: Whenever the metric for a route changes,
send entire routing table.
35
RIP Summary
• Slow convergence
• Limited to 16 hops
• Only uses local information for routing decisions
(from neighbors) - relies on others (propagation)
for global information
Open Shortest Path First (OSPF)
• Uses Link State routing
• Each node acquires complete topology information
using link state updates
• Link-state - what it means:
• Link: That’s the interface of a router.
• State: Description of that interface and how it’s connected to
neighbor routers.
• Link state information must be flooded to all nodes
(uses multicasting)
• Cost metric used to calculate shortest paths. Metric
can be any link or network parameter (time,
congestion, bandwidth, $$, distance) or a function that
combines several weighted parameters
• Guaranteed to converge
Link State Routing: Basic principles
1. Routers establish a relationship (“adjacency”) with neighbors.
Two types:
1.
2.
full neighbors: allows exchange of routing information
2way neighbor: no routing information exchange
2. Each router generates link state advertisements (LSAs) which are
distributed to all “adjacent” routers (after all routers have established
adjacencies).
LSA = (link id, state of the link, cost, neighbors of the link)
3. Each router maintains a database (LSDB) of all received LSAs
(topological database or link state database), which describes the
network as a graph with weighted edges
4. Each router uses its link state database to run a shortest path
algorithm (Dijikstra’s algorithm) to produce the shortest path to each
network
38
Operation of a Link State
Routing protocol
Received
LSAs
Dijkstra’s
Algorithm
Link State
Database
LSAs are flooded
to other interfaces
39
IP Routing
Table
Features of OSPF
• Provides authentication of routing messages
• Enables load balancing by allowing traffic to
be split evenly across routes with equal cost
• Type-of-Service routing allows setup of
different routes dependent on the TOS (DS)
field in IP header
• Uses AREAs to subdivide large networks,
providing a hierarchical structure and limits the
multicast LSAs within routers of the same
area. Area 0 is called the backbone area and
all other areas connect directly to it. All OSPF
networks must have a backbone area
40
OSPF Areas
Area Border Routers (ABR) are any routers that have one interface in
one area and another interface in another area
Link State Advertisements (LSA)
• OSPF routers use LSAs to describe the link state
of all its interfaces.
• An LSDB stores all received LSAs on a router.
• A router uses Router LSA to describe its interface
IP addresses.
• After OSPF is started on a router, it creates an
LSDB that contains entries of this router’s Router
LSAs
OSPF Operation
• HELLO messages are used to maintain adjacent
neighbors.
• By default, OSPF routers send Hello packets every 10
seconds on multiaccess and point-to-point segments and
every 30 seconds on non-broadcast multiaccess (NBMA)
segments (e.g. frame relay).
• It is a classless routing protocol. It sends the subnet
mask in the routing updates.
OSPF Operation contd.
• Link-state routing protocols generate routing updates only when a
change occurs in the network topology.
• When a link changes state, the device that detected the change
creates a link-state advertisement (LSA) concerning that link and
sends it to all neighboring devices using a special multicast
address.
• Each routing device reads the LSA, and updates its link-state
database (LSDB).
• The LSA has a sequence number that allows the router to check
to see if it has already seen that update. If old, it is discarded, if
new, LSDB info updated and LSA passed along to next
neighbors.
• The entire routing table (LSDB) is transmitted once every 30
minutes
Types of OSPF Messages
• There are five types of OSPF Link-State Packets (LSPs).
1. Hello: are used to establish and maintain adjacency
with other OSPF routers. They are also used to elect
the Designated Router (DR) and BackupDesignated
Router (BDR) on multiaccess networks (like Ethernet
or Frame Relay).
2. Database Description (DBD or DD): contains an
abbreviated list of the sending router’s link-state
database and is used by receiving routers to check
against the local link-state database
LSPs contd.
3. Link-State Request (LSR): used by receiving
routers to request more information about any
entry in the DBD
4. Link-State Update (LSU): used to reply to LSRs
as well as to announce new information. LSUs
can contain seven different types of Link-State
Advertisements (LSAs)
5. Link-State Acknowledgement (LSAck): sent to
confirm receipt of an LSU message
OSPF Packet Format
OSPF Message
IP header
OSPF Message
Header
OSPF packets are not
carried as UDP or TCP
payload!
OSPF has its own IP
protocol number: 89
Body of OSPF Message
Message Type
Specific Data
TTL: set to 1 (in most cases)
LSA
LSA
Header
LSA
... ...
LSA
LSA
Data
Destination IP: neighbor’s IP address or 224.0.0.5
(ALLSPFRouters) or 224.0.0.6 (AllDRouters:
(designated and backup designated only)
47
OSPF Packet Format
OSPF Message
Header
2: current version
is OSPF V2
Message types:
1: Hello (tests reachability)
2: Database description
3: Link Status request
4: Link state update
5: Link state acknowledgement
Standard IP checksum taken
over entire packet
Body of OSPF Message
version
type
message length
source router IPI address
D
ID of the Area
from which the
packet originated
Area ID
checksum
authentication type
authentication
authentication
32 bits
0: no authentication
1: Cleartext
password
2: MD5 checksum
(added to end
packet)
48
OSPF Hello Message
Example of OSPF
• Suppose OSPF has just been enabled on R1 & R2. Both
R1 and R2 are very eager to discover if they have any
neighbors nearby but before sending Hello messages
they must first choose an OSPF router identifier (routerid) to tell their neighbors who they are. The Router ID
(RID) is an IP address used to identify the router and is
chosen using the following sequence:
• The highest IP address assigned to a loopback (logical)
interface.
• If a loopback interface is not defined, the highest IP address of
all the active router’s physical interfaces will be chosen.
• The router ID can be manually assigned if necessary
Example contd.
• In this example, suppose R1 has 2 loopback interfaces &
2 physical interfaces:
• Loopback 0: 10.0.0.1
• Loopback 1: 12.0.0.1
• eth0/0: 192.168.1.1
• eth0/1: 200.200.200.1
• The loopback interfaces are preferred to physical
interfaces (because they are never down) so the highest
IP address of the loopback interfaces is chosen as the
router-id -> Loopback 1 IP address is chosen as the
router-id.
Router 1
Router 2
Next Step – Hello Msgs
• Now both the routers have the Router-ID so they
will send Hello packets on all OSPF-enabled
interfaces to determine if there are any neighbors
on those links.
• The information in the OSPF Hello includes the
OSPF Router ID of the router sending the Hello
packet.
Hello Packet Exchange
Hello Packet Content
Indicates values that have to be the same for both routers if they are
to establish an adjacency, i.e., become neighbors
Description of Hello Values
• Router ID: Each OSPF router needs to have an unique ID which is the highest IP
• address on any active interface. More about this later.
• Hello / Dead Interval: Every X seconds we are going to send a hello packet, if we
don’t hear any hello packets from our network for X seconds we declare you “dead”
and we are no longer neighbors. These values have to match on both sides in order
to become neighbors.
• Neighbors: All other routers who are your neighbors are specified in the hello packet.
• Area ID: This is the area you are in. This value has to match on both sides in order to
become neighbors.
• Router Priority: This value is used to determine who will become designated or
backup designated router.
• DR and BDR IP address: Designated and Backup Designated router for multiple
access networks such as an Ethernet segment.
• Authentication password: You can use clear text and MD5 authentication for OSPF
which means every packet will be authenticated. Obviously you need the same
password on both routers in order to make things work.
• Stub area flag: Besides area numbers OSPF has different area types. Both routers
have to agree on the area type in order to become “neighbors”.
Hello Msg R1 to R2
• R1 wants to find out if it has any neighbor running OSPF
it sends a Hello message to the multicast address
224.0.0.5.
• This is the multicast address for all OSPF routers and all
routers running OSPF will process this message.
Discovery of Neighbors
• Routers multicasts OSPF Hello packets on
all OSPF-enabled interfaces.
• If two routers share a link, they can become
neighbors, and establish an adjacency.
• Certain conditions have to be met.
• In broadcast environments, adjacency is only
established with Designated and
BackupDesignated Routers.
59
Establishing adjacency
• If an OSPF router receives an OSPF Hello packet
that satisfied all its requirements (all * values are
the same) then it will establish adjacency with the
router that sent the Hello packet. In this example,
if R1 meet R2′s requirements, meaning it has:
• the same Hello/Dead interval,
• AREA number,
• Password
• Stub Area Flag
 R2 will add R1 to its neighbor table.
Hello Msg Adjacency Parameters
• Hello interval: indicates how often it sends Hello packets.
• Dead interval: number of seconds this router should wait
between receiving hello packets from a neighbor before
declaring the adjacency to that neighbor down
• AREA number: the area it belongs to
Establishing Adjacency
Before establishing an adjacency, OSPF routers need to go through
several state changes.
• Init state – router has received Hello message from other OSFP router
• 2-way state – neighbor has received Hello message and replied with a
Hello message of his own
• Exstart state – beginning of the LSDB exchange between both
routers.
• Exchange state – DBD (Database Descriptor) packets are exchanged.
DBDs contain LSAs headers. Routers see what LSAs they need.
• Loading state – one neighbor sends LSRs (Link State Requests) for
every network it doesn't know about. The other neighbor replies with
the LSUs (Link State Updates) which contain information about
requested networks. After all the requested information have been
received, other neighbor goes through the same process
• Full state (adjacency) - both routers have the synchronized database
Exchange DD or DBD packets
• R1 and R2 are neighbors but they don’t exchange LSAs
immediately. Instead, they send Database Description
(DD or DBD) packets which contain an abbreviated list of
the sending router’s link-state database.
• The neighbors also determine who will be the master
and who will be the slave. The router with higher RouterID will become master and initiates the database
exchange.
• The receiver acknowledges a received DD packet by
sending an identical DD packet back to the sender.
• Each DD packet has a sequence number and only the
master can increment sequence numbers.
DD Msg Exchange
Neighbor discovery and
database synchronization
10.1.10.1
Discovery of
adjacency
10.1.10.2
OSPF Hello
OSPF Hello: I heard 10.1.10.2
After neighbors are discovered the nodes exchange their databases
Database Description: Sequence = X
Sends database
description.
(description only
contains LSA
headers)
Acknowledges
receipt of
description
65
Database Description: Sequence = X, 5 LSA headers =
Router-LSA, 10.1.10.1, 0x80000006
Router-LSA,
10.1.10.2, 0x80000007
Router-LSA,
10.1.10.3, 0x80000003
Router-LSA,
10.1.10.4, 0x8000003a
Router-LSA,
10.1.10.5, 0x80000038
Router-LSA,
10.1.10.6, 0x80000005
Database Description: Sequence = X+1, 1 LSA header=
Router-LSA,
10.1.10.2, 0x80000005
Database Description: Sequence = X+1
Sends empty
database
description
Database
description of
10.1.10.2
LSA Request
R1 or R2 can send Request to get missing LSA from its neighbors
LSA Exchange
R2 sends back an
LSAck packet to
acknowledge the
packet
LSA exchanges – Request
and Response
10.1.10.1
Link State Request packets, LSAs =
Router-LSA,
10.1.10.1,
Router-LSA,
10.1.10.2,
Router-LSA,
10.1.10.3,
Router-LSA,
10.1.10.4,
Router-LSA,
10.1.10.5,
Router-LSA,
10.1.10.6,
10.1.10.1 sends
requested LSAs
Link State Update Packet, LSAs =
Router-LSA, 10.1.10.1, 0x80000006
Router-LSA, 10.1.10.2, 0x80000007
Router-LSA, 10.1.10.3, 0x80000003
Router-LSA, 10.1.10.4, 0x8000003a
Router-LSA, 10.1.10.5, 0x80000038
Router-LSA, 10.1.10.6, 0x80000005
Link State Update Packet, LSA =
Router-LSA,
10.1.1.6, 0x80000006
68
10.1.10.2
10.1.10.2 explicitly
requests each LSA
from 10.1.10.1
10.1.10.2 has more
recent (higher
sequence number)
value for 10.0.1.6 and
sends it to 10.1.10.1
Creating LSDBs
• Note that routers first exchange DD msgs that only list
the content of the LSDB but no details.
• Once a router gets that info, it can then check to see if it
has that information in its LSDB.
• If it doesn’t it requests an LSA to fill in the details.
• Reliable transmission: when a router receives an
Update, it sends an Ack to the Update sender.
• If the sender does not receive Ack within a specific
period, it times out and retransmits Update.
• OSPF uses Update-Ack to implement reliable
transmission. It does not use TCP!
Routing Data Distribution
• LSA-Updates are distributed to all other
routers via Reliable Flooding using
multicast addresses.
• Example: Flooding of LSA from 10.10.10.1
10.10.10.1
10.10.10.2
LSA
ACK
LSA
Update
database
Update
database
70
10.10.10.4
10.10.10.2
LSA
ACK
Update
database
LSA
10.10.10.6
Update
database
10.10.10.5
Update
database
Dissemination of LSA-Update
• A router sends and re-floods LSA-Updates,
whenever the topology or link cost changes. (If a
received LSA does not contain new information,
the router will not flood the packet)
• Exception: Infrequently (every 30 minutes), a
router will flood LSAs even if there are no new
changes.
• Acknowledgements of LSA-updates:
• explicit ACK, or
• implicit via reception of an LSA-Update from neighbor.
71
Filling the LSDB
Flow Chart
• In this example a new LSA is arriving at the router and OSPF has to decide
what to do with it:
1. If the LSA isn’t already in the LSDB it will be added and a LSAck
(acknowledgement) will be sent to the OSPF neighbor. The LSA will be
flooded to all other OSPF neighbors and we have to run SPF to update our
routing table.
2. If the LSA is already in the LSDB and the sequence number is the same
then we will ignore the LSA.
3. If the LSA is already in the LSDB and the sequence number is different then
we have to take action:
1.
2.
If the sequence number is higher it means this information is newer and we have
to add it to our LSDB.
If the sequence number is lower it means our OSPF neighbor has an old LSA
and we should help them. We will send a LSU (Link state update) including the
newer LSA to our OSPF neighbor. The LSU is an envelope that can carry
multiple LSAs in it.
LSAs
• It’s not just the sequence number that OSPF will
look at to determine if a LSA is more recent. It will
consider the LSA to be more recent if it has:
• A higher sequence number.
• A higher checksum number.
• An age equal to the maximum age.
• If the link-state age is much younger.
LSA Sequence Numbers
• What do the sequence numbers look like for OSPF LSAs?
• There are 4 bytes or 32-bits.
• Begins with 0x80000001 and ends at 0x7FFFFFFF.
• Every 30 minutes each LSA will age out and will be flooded and the
sequence number will increment by one.
• With 32-bits we have a LOT of sequence numbers and every 30
minutes it will increase. If we make it to the last sequence number
0x7FFFFFFF it will wrap around and start again at 0x80000001.
Every 30 minutes OSPF will flood a LSA to make sure the LSDB
stays up to date and when it does this the sequence number will
increase and OSPF will reset the max age when it receives a new
LSA update.
Broadcast Environments: Designated
and Backup Designated Router
• To minimize OSPF traffic (LSAs) on broadcast networks, OSPF elects a
Designated Router (DR) and a Backup DR (BDR)
• How do we select a DR/BDR? During the process of becoming OSPF
neighbors, right after the two-way state that’s where routers elect who will
become DR or BDR. Who is going to win the election?
•
•
•
•
The router with the highest priority will become DR.
The router with the second highest priority will become BDR.
If the priority is the same the OSPF router ID is the tiebreaker. Higher wins.
DR/BDR election is non-preemptive. This means if you change the priority or router
ID you have to reset OSPF in order to select a new DR/BDR.
• Routers that are not DR or BDR show up as DROTHER.
• Only DR and BDR have adjacencies (full neighbor) with all routers on the
broadcast segment. The other routers are two-way neighbors. If a non
designated router has an update, the LSA is sent to the designated routers
using the 224.0.0.6 address. The LSA is then sent by the designated router to
all the routers on the broadcast segment using multicast address 224.0.0.5.
Example
Full neighbor state
Router Status
And router Susan (the BDR) sees the DR and DROTHER.
Two-way neighbor state
Choosing DR and BDR
• We can change which router becomes the DR/BDR by playing
with the priority.
• You change the priority if you like by using the ip ospf priority
command:
• The default priority is 1.
• A priority of 0 means you will never be elected as DR or BDR.
• You need to use clear ip ospf process before this change takes effect.
• Let’s turn router Nancy in the DR:
• Donna is still the DR, we need to reset the OSPF neighbor
adjacencies so that we’ll elect the new DR and BDR.
By Multiple Access not By Area
• Something you need to be aware of is that the DR/BDR election is
per multi-access segment…not per area!
• In the example below we have 2 multi-access segments. Between
router Donna and Nancy, and between router Donna and Susan.
For each segment there
will be a DR/BDR election.
You can see that router Nancy
is the DR for the 192.168.12.0/24
segment and router Susan is the
DR for the 192.168.23.0/24 segment.
Point to Point Links
• For a point-to-point link running say HDLC. You can see that we
have a neighbor but we didn’t do an election for DR or BDR.
Makes sense because there is always only one router on the
other side.
192.168.12.0
.1
.2
Link Cost and Path Choice
• What about the link metric? OSPF uses a metric called cost which is based on
the bandwidth of an interface, it works like this:
• Cost = Reference Bandwidth / Interface Bandwidth
• The reference bandwidth is a default value on Cisco routers which is a 100Mbit
interface.
• You divide the reference bandwidth by the bandwidth of the interface and you’ll
get the cost.
• Example: If you have a 100 Mbit interface what will the cost be?
• Cost = Reference bandwidth / Interface bandwidth
• 100 Mbit / 100 Mbit = COST 1
• Example: If you have a 10 Mbit interface what will the cost be?
• 100 Mbit / 10 Mbit = COST 10
• Example: If you have a 1 Mbit interface what will the cost be?
• 100 Mbit / 1 Mbit = COST 100
• The lower the cost the better the path is.
• If we have links that are > 100M the reference bandwidth is changed to always
have a link cost that is >1
OSPF LSA Types
• OSPF has many different types of LSAs:
• LSA Type 1: Router LSA
• LSA Type 2: Network LSA
• LSA Type 3: Summary LSA
• LSA Type 4: Summary ASBR LSA
• LSA Type 5: Autonomous system external LSA
• LSA Type 6: Multicast OSPF LSA (NOT USED)
• LSA Type 7: Not-so-stubby area LSA
• LSA Type 8: External attribute LSA for BGP
Router LSA
• Each router within the area will flood a type 1
router LSA within the area.
• In this LSA you will find a list with all the directly
connected links of this router.
• The router LSA will always stay within the area.
Network LSA
• The network LSA or type 2 is
created for multi-access network
that have a DR/BDR.
• If this is the case you will see
these network LSAs being
generated by the DR.
• The other routers in the segment
generate a type 1 LSA to inform
the DR of an update.
• In the type 2 LSA we will find all
the routers that are connected to
the multi-access network, the DR,
BDR, and the prefix and subnet
mask.
• The network LSA always stays
within the area.
Multi Area LSAs
• Type 1 router LSAs always stay within the area. OSPF however
works with multiple areas and you probably want full connectivity
within all of the areas. Router Nancy is flooding a router LSA within
the area so router Donna will store this in her LSDB.
• Router Mary and Susan also need to know about the topology in
Area 2.
• Router Donna is going to create a Type 3 summary LSA and
flood it into area 0. This LSA will flood into all the other areas of our
OSPF network. This way all the routers in other areas will know about
the prefixes from other areas.
An outside RIP Router
• In this example we have router Nancy who is redistributing information from the
RIP router into OSPF. This makes router Nancy an ASBR (Autonomous
System Border Router).
• Router Nancy will flip a bit in her router LSA to identify herself as an ASBR.
• When router Donna who is a ABR receives this router LSA she will create a
type 4 summary ASBR LSA and flood it into area 0.
• This LSA will also be flooded in all other areas and is required so all OSPF
routers know where to find the ASBR.
Outside Network
• Same topology but we’ve added a prefix (5.5.5.0 /24) at our RIP router. This
prefix will be redistributed into OSPF.
• Router Nancy (our ASBR) will take care of this and create a type 5 external
LSA for this that will contain the external network prefix.
• We still need type 4 summary ASBR LSA to locate router Nancy.
Special LSA Type: Not So Stubby Area (NSSA)
• NSSA areas do not allow type 5 external LSAs. They are pseudo
stubs, limited external traffic.
• Router Nancy is still our ASBR redistributing information from RIP
into OSPF.
• Since type 5 is not allowed we have to think of something else.
That’s why we have a type 7 external LSA that carries the exact
same information but is not blocked within the NSSA area.
• Router Donna will translate this type 7 into a type 5 and flood it
into the other areas.
OSPF Tables
• There are 3 type of tables stored at a Router:
• Neighbor
• Topology
• Routing
Neighbor Table
• Contain information about the neighbors
• Neighbor is a router which shares a link on same
network
• Another relationship is adjacency
• Not necessarily all neighbors
• LSA updates are only when adjacency is
established
Topology Table
• Contains information about all network and paths
to reach any network
• All LSA’s are entered into the topology table
• When topology changes, LSA’s are generated
and router sends new LSA’s
• Using the topology table a shortest path
connectivity graph is created (routing table), the
algorithm is known as SPF or Dijkstra’s algorithm
Routing Table
• Also known as forwarding database
• Generated when an algorithm is run on the
topology database
• Routing table for each router is unique
Examples
• A simple setup with 3 routers and 2 areas. I’ve added a
couple of loopbacks so we have prefixes to look at.
1
Susan
Show ip OSPF DB
1.1.1.1
1.1.1.1
1.1.1.1
1.1.1.1
Explanation
• By using the show ip ospf database we can look at the LSDB
and we can see the type 1 router LSAs, type 2 network LSAs and
the type 3 summary LSAs here.
• Link ID: This is what identifies each LSA.
• ADV router: the router that is advertising this LSA.
• Age: The maximum age counter in seconds. The maximum is 3600
seconds or 1 hour.
• Seq#: Here you see the sequence number which starts at 0x80000001 and
will increase by 1 for each update.
• Checksum: There is a checksum for each LSA.
• Link count: This will show the total number of directly connected links and
is only used for the router LSA.
Adding an ASBR
• On router Nancy we created an additional loopback interface and configured an IP
address. Then telling OSPF to redistribute the directly connected interfaces into
OSPF.
1.1.1.1
1.1.1.1