Topic Links
Classful
and Classless Protocols
Routing
Algorithms and Features
An administrative distance is a measure of the trustworthiness of a
route.The router uses this to determine
the best path when there is information from more than one routing
protocol.The lower the administrative
distance the greater the reliability of the information. The EIGRP summary
route can only be seen on the table where the summary was configured.
|
Source
of Route |
Administrative
Distance (default value) |
|
Connected
Interface |
0 |
|
Static
Route out an interface |
0 |
|
Static
Route to next hop |
1 |
|
|
5 |
|
External
BGP |
20 |
|
Internal
EIGRP |
90 |
|
IGRP |
100 |
|
OSPF |
110 |
|
IS-IS |
115 |
|
RIP(Version
1 and Version 2) |
120 |
|
EGP |
140 |
|
External
EIGRP |
170 |
|
Internal
BGP |
200 |
|
Unknown |
255 |
Each routing
protocol uses different methods to determine the best path.In order to determine the best path a routing
metric is created.Depending on the
protocol it may be as simple as hop count (RIP v1) or much more complex as is
the case with IGRP which uses up to 5 factors in determining the final
metric.The standard metric formula for
IGRP is:
Metric =
(K1*Bandwidth) + [(
IF K5 {MTU} is not
zero, Metric = Metric * (K5/ (reliability + K4))
Given default constant values: Metric = bandwidth + delay
Bandwidth is in Kbps
and delay is in microseconds and can be determined with the show interfaces
command.
Classfull and Classless Protocols
IGRP and RIP v1 are Classfull protocols and will only do route summarization at the class boundaries.
OSPF, EIGRP, RIP v2, BGP-4, and
IS-IS are classless protocols.Route summarization is manual and selected
boundaries.
Classless protocols
use different subnet masks within a network, this is called variable-length
subnet masking or VLSM.This permits
more selective summarization that would be possible with Classfull
routing.
TCP Port- 6UDP Port-17TFTP
- 69
RIP Port-520IGRP Protocol 9
DNS Port-53SNMP Port- 161
Routing Algorithms and Features
RIP and IGRP use Bellman-Ford
for route calculation
EIGRP uses Diffusing Update
Algorithm (DUAL)
Distance Vector Protocols
|
Feature |
RIPv1 |
RIPv2 |
IGRP |
EIGRP |
|
Count to Infinity |
YES |
YES |
YES |
NO |
|
|
YES |
YES |
YES |
YES |
|
Hold Down Timer |
YES |
YES |
YES |
NO |
|
Triggered Updates
w/ Rt Poisining |
YES |
YES |
YES |
YES |
|
Load Bal Equal
Paths |
YES |
YES |
YES |
YES |
|
Load Bal UnEqual Paths |
NO |
NO |
YES |
YES |
|
VSLM Support |
NO |
YES |
NO |
YES |
|
Metric |
HOPS |
HOPS |
Composite |
Composite |
|
Hop CountLimit |
15 |
15 |
100 |
100 |
The default for IGRP
and EIGRP on hop count limit is 100 but it can be configured to be 255.For IGRP that makes it
scalable for medium
size enterprises and for EIGRP for large enterprises.
|
Feature |
OSPF |
IS-IS |
EIGRP |
|
Hierarchical
Topology Required |
YES |
YES |
NO |
|
Retains
Information on all possible routes |
YES |
YES |
YES |
|
Route
Summarization (Manual) |
YES |
YES |
YES |
|
Route
Summarization (Automatic) |
NO |
NO |
YES |
|
Event-Triggered
Announcements |
YES |
YES |
YES |
|
Load Balancing
(Equal-Paths) |
YES |
YES |
YES |
|
Load Balancing
(Unequal Paths) |
NO |
NO |
YES |
|
VSLM Support |
YES |
YES |
YES |
|
Routing Algorithm |
Dijkstra
|
IS-IS |
DUAL |
|
Hop-Count limit |
Unlimited |
1024 |
100 |
|
|
|
|
|
5
Election and Exchange Process
8)
Description of Update Process
9) Keys to Understanding OSPF in Larger Network
Open
Shortest Path First version 2 (OSPF) is:
an Interior Gateway
Protocol (IGP)
is described inRFC
2328
has faster convergence than
RIP
floods routing changes
throughout network
supports Variable length
subnet masks (VSLM)
OSPF was written for larger networks, specifically 50 + routers.
It uses protocol number 89 with:
6- TCP
17- UDP
OSPF uses cost metrics assigned to the interface output side, called:
interface output cost and based on
the speed of the media (bandwidth)
OSPF Routers form adjacencies with neighbors.To have an adjacency means
that the databases for the two routers have been synchronized.
Link States are advertised to other routers with LSAs
Neighbors are defined as two routers that have interfaces on a
common network.These are discovered and maintained with
Hello packets (TOP)
Neighbor database bi-directional communication
Link-State database (topology database)
Routing table - (forwarding database) which is created using the
SPF algorithm (Dijkstra algorithm)
OSPF Topologies
Broadcast Multiaccess
Point-to-Point
Nonbroadcast multiaccess (NBMA) [Frame or X.25] (TOP)
Hello packets
are sent out to an address called the AllSPFRouter
address as
multicast
on 224.0.0.5 on MAC address: 010005E 0000005 it also uses
multicast on 224.0.0.6 on MAC address: 010005E 0000006 (TOP)
Router ID:This is the highest IP address on an active interface
Used to break ties in DR and BDR elections if priority is tied
Hello and
dead intervals:Time between sends (10 sec default) and the time before a link is considered down is the dead interval, generally 4 time the hello interval. (NBMA is 30 seconds and 120 seconds respectively.)
Neighbors:Has bi-directional communication meaning that the router sees itself in the other routers hello packet.
Area ID:Routers that share a common segment have the same subnet and mask and have the same link-state information.
Router Priority:8-bit number that indicates the priority of the router during DR and BDR elections.The higher priority the more likely the selection. Each link has its own election process set at the interface configuration mode.
BDR and DR
IP Addresses:The addresses of the BDR and DR
Authentication
PasswordSet if authentication is used
Stub area flag:Two routers must agree on this in hello packets
OSPF Packet Header
Includes:
Version Number
Type
Hello
Database Description (adjacency)
Link-state request
Link- state update
Link-state Ack
Packet Length
Router ID (source)
Checksum
Authentication Type:
0 no authentication
1 clear text
2- MD5
Authentication Information
Data routing information
On multiaccess networks the DR and BDR get routing updates but only the DR sends out
the updates, unless the DR goes down.Each
router must set up an
adjacentcy
with the DR and the BDR.After the initial
election new routers form an adjacency with the DR and the BDR.
(TOP)
Router with highest priority is the DR and the next highest is the BDR
The default interface priority is 1.
The highestrouter ID breaks ties.
A router with the priority of 0 cannot be a BDR or DR.
A router that is not the DR or BDR is called a Drother.
If a DR goes down, the BDR becomes the DR and a new BDR is elected
Higher priority routers added to the network do not cause a new election.
BDR uses a reliability timer if no LSA is heard then it assumes the DR is down.
OSPF Exchange/Startup Process Upon All Routers Coming Up at
the Same Time
Step 1:Router is in down state and sends a hello packet our all participating interfaces on 224.0.0.5.
Step 2: Router that get packet from (1) add the sending router to their list of
neighbors. This is the init state.
Step 3: All routers that got the first packet send a reply hello packet that includes
all other neighboring routers.
Step 4: The first router sees its own ID in the packet and adds the other routers to its neighbor database. This creates the two-way state.
Step 5: Election for DR and BDR is held. Once this is held the routers are in an
exstart state where the exchange protocol movesthe routers toward a full state. With the exchange protocol the DR and the BDR establish adjacencies witheach router on the network, then based on the higher router ID they decide on a master/slave relationship with the master as the higher router ID.Link state info is exchanged between the DR and BDR and the router with which they have adjacencies.Database Description Packets (DBDs or DDPs (Database Descriptor Packets)) are exchanged with sequence numbers defined by the master.The slave gets the DBD, sends a LSAck, compares the DBD to see if it is more up to date. If it is more up to date, it asks for a Link-State Update (LSU) by sending a Link-State Request(LSR). This is called the Loading state. When all the LSRs are satisfied, the routers are in the full state.
Step 6: Hello packets are sent each 10 sec. including the names of the DR and BDR.
OSPF works on cost metrics, the higher the bandwidth, the lower the cost.
OSPF keeps up to 6 equal cost route entries in the table for load balancing.
The default is 4 but the maximum-paths router configuration command.
will allow up to 6. paths to same destination.
Flapping causes new LSUs, so each time a LSU is received the router waits for a
period of time before recalculating the routing table (5 seconds is the default.) the
times spf spf-delay spf-holdtime command allows this to be configured.
The SPF algorithm
forms an SPF Tree which is a loop free shortest path to all
networks with the
router as the root.
MultiaccessLink state process
Step 1:Router notices a change floods it to 224.0.0.6 the all DR and BDR address with an LSU packet that includes one or many LSAs.
Step 2:The DR acknowledges this and floods the LSU to the other routers on the network on 224.0.0.5.Each router responds with an LSAck.
Step 3 : If the router is connected to other networks it forwards the LSU to the DR of those networks or to the adjacent routers if point-to-point.Those DRs then multicast the LSUs
Step 4: When a router receives the LSU is takes the changed LSAs and updates its link-state database. This is then used to run the SPF algoithm and update the routing database.
LSA age field aging timer = default of 30 minutes.When this expires the router that originated the LSU sends an update to say the link is still valid.
Entries that already exist are discarded.
Point-to-Point links have no election. Both routers automatically become adjacent
routers.
debug ip ospf adj -Lookat the point-to-point adjacency election
For BRI/PRI and Asynchronous
BRI/PRI use dialer-map along with OSPF configuration + Broadcast to indicate that broadcasts should be forwarded to the protocol address.
For Asynchronous use:
async
default routing
(TOP)
Non-Broadcast MultiAccess Topologies (NBMA)
1. Broadcast option must be enable for all Virtual Circuits
2. Full mesh (n(n-1))/2 where n is the number of sites
Two modes of OSPF
in NBMA Networks
Non-Broadcast MultiAccess All broadcast packets are replicated and sent to all routers, usually in a fully meshed topology.Possible configuration to be sure all adjacencies are set. RFC-defined
Point-to-Multipoint treats the NBMA network as a set of point-to-point links, no election, usually used in partially meshed networks. RFC defined.
NBMA topology
often uses subinterfaces.
Router(config)#
interface serial number.subinterface-number {multipoint | point-to-point}
Default OSPF mode for point-to-point subinterface is point-to-point mode
Default OSPF mode for point-to multipoint subinterface mode is non-broadcast MultiAccess mode.
In NBMA mode sometimes a subinterface can fail while the main link or keep alives from another subinterface continue meaning that OSPF fails to notice the down link.
Point-to-multipoint does not require full mesh, but all routers are on one IP subnet. It does not require the static neighbor configuration.
Other Modes:
Point-to-multipoint non-broadcast requires static definition of neighbors
Broadcast mode adjacency is automatic Cisco standard
Point-to-point NBMA mode
(TOP)
router ospf process-id
router(config-router)# network address wildcard-mask area area-id
{note on wildcard masks 1 means dont care and 0 means match}
To set a higher router ID use loopback command:
router(config)# interface loopback number
{Since loopback addresses are always active a loopback address can be more reliable for key routers}
To determine the router ID of a router:
show
ip ospf interface
To change the ospf priority on a router:
Router(config-if) ip ospf priority number (Can be 1-255)
The highest priority is the DR, 1 is the default and 0 is a drother.
To change the link cost:
ip ospf cost cost
Some default costs
are:
56
kbps serial link 1785
T1
- 64
Ethernet10
16
Mbps Token Ring6
To change the cost
use the:
router(config-router) #auto-cost reference-bandwidth reference-bandwidth
The reference
bandwidth is 100Mbps or 10^8 bps.Also
ip ospf cost overrides the
calculation
process.
Setting mode
type:
router(config-if)#
ip ospf network command-mode
nonbroadcast
point-to-multipoint
point-to-multipoint
non-broadcast
broadcast
point-to-point
The neighbor command is used to configure ospf neighbors:
router(config-router)#neighbor ip-address [priority number] [poll-interval sec] [cost number]
Verification commands
show ip route
show ip protocols
show ip route ospf
show ip ospf interface e0
show ip ospf
show ip ospf neighbor
show ip ospf neighbor detail
show ip ospf database
clear ip route * (resets routing table)
clear ip route destination network
debug ip ospf events displays information about protocol related events such as flooding, DR/BDR election and spf calculations
debug ip ospf packet displays information about each OSPF packet received
Other parameters for debug ip ospf include:
Adjacent
flood
lsa-generation
retransmission
spf
tree
Description of update process:
A router receives an LSA and first checks if the LSA is from an external network or if
the router itself is a stub router.If either of these items is true then the router acknowledges
the LSA then discards the LSA.Next the router checks if the LSA timer (MAXAGE) has
expired or if the neighbor is in a loading or exchange state.If either of these items is true
then the LSA is acknowledged and discarded.If this false, then the router check to see if
the LSA is in its topological database.If it is in the topological database and the LSA received
is more recent than the one in the database then a new LSA is sent to the sender of the old LSA,
and then the arrival of the LSA is checked against the last run of the SPF algorithm and if the
minimum timer after the last run of the algorithm has expired, then the LSA is flooded out all
interfaces except for the arriving interface.If the LSA is less recent than LSA already in the
database, then the packet is discarded and acknowledged.The more recent LSA is installed
in the topological database, time stamped with the arrival time and acknowledged.
OSPF Larger Networks
For an OSPF network with Multiple Areas there are a set ofkey concepts:
AREA 0 This is the backbone area.
ABRs - Area Border Routers
ASBRs - Autonomous System Border Routers
Stub Areas -Only accept internal route updates
Totally Stubby Areas -Only accepts default route to other networks
Not So Stubby Areas -
Accepts limited summary routes to other
networks
LSA Types - Internal and External LSAs are key to working with area types.
Summarization at area boundaries and the definition of area types control the size of routing tables.
1.)First In First Out (FIFO) This is the default queuing method for all interfaces over E1(2.048 Mbps).Packets come in and they are buffered and go out in the order in which they arrived. This is the fastest method but makes no distinction between types of packets.
2.)Priority Queuing Each packet type is assigned a priority of high, medium, normal or low.The high queue is emptied first then medium, normal then low. It is possible that lower priority traffic can never get sent.
3.)Custom Queuing 16 queues are allowed and bandwidth assigned to each.More sensitive traffic an be assigned larger bandwidth.
4.)Weighted-Fair Queuing A complex algorithm determines the distribution of packets assigning them precedence and breaking up large packet streams so all traffic can get through. This is the only dynamic queuing method.
Key advantages are:
Rapid convergence
Only network change information is propagated
Normal operation in a stable network yields only hello traffic
EIGRP uses DUAL (Diffused Update Algorithm) to determine the best path.
EIGRP chooses the best path as the successor path and next best path as the feasible successor. Knowledge about the network is derived from hello packets sent on a 5 second basis for high bandwidth links and every 60 seconds for lower bandwidth links. These hellos generate neighbor information. When a router sees another router's hello packets it becomes a neighbor.
Examples of low bandwidth circuits are multipoint frame T1 or less circuits or ISDN BRI circuits.
Hello Interval - the time between hello packets
Hold Time - the amount of time where a router does not receive a hello packet. This is usually 3 times
the Hello interval. By default this would be 15 and 180 seconds.
The hello interval can be adjusted with the ip hello-interval eigrp
The hold time can be adjusted with the ip hold-time eigrp
To see eigrp neighbors type - show ip eigrp neighbor
Eigrp does not build peers over secondary addresses.
In a point to multi-point topologies the broadcast key work must be used in the frame-relay map command.
Eigrp installs routes in a topology table which can be seen with the show ip eigrp topology
statement. This table has the information needed to build a set of vectors and distances needed to reach each
network.
EIGRP Metrics:
The base formula for EIGRP metrics is:
metric = [K1 * bandwidth + (K2 * bandwidth) / (256 − load) + K3 * delay] * [K5 / (reliability + K4)]
Using the default values for K1 -> K5 this reduces down to:
metric = bandwidth + delay calculated as [(10000000/(bandwidth) + SUM(Delays)) *256]
The lowest configured bandwidth is used an the delays in Microseconds / 10 are summed
to get to the total delay.
PATHS
The feasible distance is the best metric along the path toward a destination network including the
metric to the neighbor advertising that path.
The reported distance is the metric to the destination network as advertised by an upstream
neighbor.
A feasible successor is a path whose reported distance is less than the feasible distance (best path)
Note a feasible successor will only be designated if the successor metric is less than the reported
distance for that route. If no successor is in place then new queries are sent when a route goes down.