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Routing Protocols
Classfull Routing Protocols RIP and IGRP
Link state protocols support classfull
addressing as well, but for exam purposes, consider them primarily used in a
classless infrastructure. RIP and IGRP do not pass subnet
information.
RIP
Route Metric HOPS and ticks. The max hop
count is 16. Used in small diameter networks, does not scale well in a very
large enterprise environment. Sends its entire routing table out to all its
interfaces every 30 seconds.
IGRP
Is a more robust than RIP. IGRP is a
Cisco-proprietary classfull routing protocol. IGRP does not support VLSM or
discontiguous subnets. IGRP will send its entire routing table every 90
seconds. Over slow links this can be a problem with the amount of bandwidth
that would be consumed in updates.
IGRP supports load balancing over paths with
unequal bandwidth. The variance command is used to load balance
IGRP.
Router igrp 90
Variance 3
The default variance is 1 for equal
balancing. The variance multiplier can go 1-128.
IGRP uses a “keep alive” timer to
determine if links are still up. Triggered updates cause the IGRP router to
send a routing table update out if its interfaces. Periodic updates still get
broadcasts sent to connected routers every 90 seconds.
Routing loops
Distance vector protocols use the following
to prevent routing loops:
Poison Reverse
|
Split Horizon
|
Holddown Timers
|
Hybrid Protocols
EIGRP
Considered a hybrid protocol. It combines
the best features of both link state and distance vector. It can detect a link
failure within one second. It converges rapidly and scales well into large
networks. EIGRP sends routing updates to directly connected neighbors; only
changes are sent rather than the entire routing table.
Bandwidth is the primary metric. That is why
it is important to specify the bandwidth on an interface. EIRGP has a default
for serial interfaces of 1.5Mbps. So you must set the correct bandwidth with
the bandwidth command.
Advantages to EIGRP:
- VLSM
Support
- Supports
Discontiguous
Subnets
- Multi-Protocol
Support
- Automatic Route
Summarization
- Keeps
copy Neighbors
Routes
- Load Balances up
to 6 Paths
DUAL
Diffusing Update ALgorithm – The
routing engine behind EIGRP. It allows for routers to update at the same time
and allows for multi-protocol routing. It tracks route updates sent by
neighbors and ensures against black holes.
EIGRP Discovery Process
EIRGP sends hello packets out of all of its
interfaces to find its neighbors.
The new router then exchanges update packets
with the adjacent routers. The adjacent routers then send ACKs to the new
router. When this step is completed the new router will then choose its routes
(successors).
Route Summarization for EIGRP
This is on by default, but only at the
network or class boundaries. Manual configuration for route summarization is
primarily done at the core or distribution layers. An example of the syntax for
configuration is as follows:
ip summary-address eigrp 100 10.98.0.0
255.255.0.0
Important! By default, EIRGP does not
support VLSM. You must use the no auto-summary command. Summaries are
done at the interface level rather than at the router level. Use no
auto-summary command to enable support of discontiguous subnets.
Example:
router eigrp 100
network 10.0.0.0
network 192.64.0.0
no auto summary
interface serial 1
ip address 10.98.98.24
255.255.255.0
bandwidth 128
ip summary-address eigrp 100 192.64.0.0
255.255.0.0
EIGRP Tables
EIGRP keeps three active tables in its
database.
Topology Table – Is kept for
each protocol. It is a table of all the route entries the router has learned.
Syntax: show ip eigrp topology shows the number of successors, active or
passive, and distance to destination.
Neighbor Table – EIGRP keeps a
table for each adjacent router and one for each protocol. Syntax: show ip
eigrp neighbors
Routing Table – Each protocol
has its own routing table. EIGRP calculates the best route, or successor, from
the topology table and puts the entry in the routing table. Syntax: show
route eigrp
EIGRP Traffic
You can use the show ip eigrp traffic
to view traffic statistics. This includes hello packets, updates, ACKs and
replies.
EIGRP Configuration
Default routes
You can also configure the route to forward
to a default network if a route is not found in the routers routing table. Use
the following syntax:
router(config)#ip
default-network 10.98.98.1
Static routes
Can be used between two AS’s or in a
DDR situation. You define specific routes between the AS’s so you do not
have to spend bandwidth on routing updates.
router(config)# ip route
10.98.8.0 255.255.255.0 172.16.32.0 e0 permanent
10.98.8.0 255.255.255.0 is the
destination
172.16.32.0 is the next hop
address.
Passive interface
You can configure EIGRP with a passive
interface, which will not allow it send any routing updates including hello
packets. Use the following syntax:
router(config-router)#passive-interface
e0
Route filters
It is also possible to filter out inbound and
outbound routing updates.
Outgoing Syntax:
router(config-router)#
distribute-list 101 out e0 static
101= the access-list-number
out = filters for outbound
e0 = the interface
static = name of route process
Incoming Syntax:
router(config-router)#
distribute-list 101 in e0
in = filters for inbound
e0 = the interface
Route Redistribution with EIGRP and IGRP
Routes can be redistributed within routing
protocols. With EIGRP and IGRP this process is automatic if the AS systems
numbers are the same. If the numbers are not the same then the redistribution
will have to be done manually with the default- metric command:
router(config-router)#
redistribute protocol X metric Y
router(config-router)#default-metric
bandwidth delay reliability loading mtu
Route Redistribution with EIGRP and OSPF
Determine the core routing
protocol.
Locate the router or ASBR where the route
distribution needs to take be configured.
Make the decision of which is your short term
or edge protocol.
Determine the route process where you want
the route distributed.
Example: router(config)#
router ospf X
X= the process id
Use the redistribute command to populate from
the short term or edge protocol to the core routing protocol.
Example:
router(config-router)# redistribute protocol X metric
Y
X= the process id and Y = the metric
value
router(config-router)#
redistribute ospf 213 metric 110 subnets
router(config-router)# default-metric
X
X= the metric value wanted
Define the seed metric to be used in the
calculation of the value of the route before distribution.
For EIGRP
Router(config-router)#
default-metric bandwidth delay reliability loading mtu
Administrative Distance
Advertised Distance
Advertised Distance is the distance a
neighbor router says, or advertises, is the distance to a destination. This is
key in electing a feasible successor or backup route. The lower the distance,
the better. The lower the value the better the route is believable. These
values can be changed with the distance command.
Administrative Distances
Protocols
|
Distance Value
|
Connected Interface
|
0
|
Static routes
|
1
|
EIGRP Summary routes
|
5
|
EBGP
|
20
|
Internal EIGRP
|
90
|
IGRP
|
100
|
OSPF
|
110
|
RIP
|
120
|
EGP
|
140
|
External EIGRP
|
170
|
Internal BGP
|
200
|
EIGRP and Dropped Links
When EIGRP discovers a down link the router
does a lookup in its topology table for a successor. If a suitable one is
found, it changes to the new route. The router then does a recalculation for
the next successor. If one is not found, the router begins a new
process.
The failed route or link is moved into active
status and sends query packets to its adjacent neighbors. It multicasts
out to every interface except the one it was learned from.
The router then waits for a response. If the
adjacent router does not have a route to the destination, nor has a feasible
successor, it sends a unicast packet back to the troubled querying router. If
the neighbor router is using the troubled query router as a successor, it sends
its own query packet to its neighbors. This query will propagate the entire
AS.
If the troubled query router receives a
successor, or feasible successor, the data is changed in the topology table and
the router waits to receive more responses. The routing table is recalculated
and the route is returned to passive state.
If no route updates for a successor or
feasible successor are found, then the router deletes the entry from the
topology database and routing table. If routers do not respond within 180
seconds their routes are also put into active state, and the query router begins
to look for the additional routes it lost though the lost router.
EIGRP and NBMA
Take particular care when configuring EIGRP
in a NBMA environment.
Three NBMA Scenarios:
- Pure point-to-point
configuration – each PVC on
subinterfaces
- Multi-point
configuration – no
subinterfaces
- Hybrid
(mixed) multi-point and
point-to-point
In these scenarios
it is important that:
- The traffic bandwidth given
to EIGRP on each VC must be the same in both
directions;
- The total
EIGRP traffic for the sum of all the VC’s must not exceed the line speed
of the interface;
and
- The EIGRP traffic
allowed on one VC must not exceed the capacity of the
VC.
Link State Protocols
OSPF (Open Shortest Path First)
Is a link state routing protocol that uses
Dijkstra’s algorithm for route calculation.
OSFP has several advantages:
- VLSM
Support
- Fast
Convergence
- No Hop
Count Limit
- Route
Selection Based on
Cost
- Low Bandwidth
Usage
OSPF Commands (Single Area) Setup
router (config)#router ospf
# (#=process ID)
- Enables ospf on the
router
router
(config-router)#network address wildcard-mask area # (#=area
id)
- Address can be a subnet,
network or the address of the
interface
- Selects the
networks that will be in the OSPF
network
router
(config-if)#interface loopback #(#=the loopback
address)
- OSPF has been proven more
reliable with a loopback
number
- Loopback address
can override the highest ip address for the router
id
router
(config-if)#ip ospf priority 0-255
router (config-if)#ip ospf
cost # (#= the cost value 1-65535)
Cost values Ethernet=10, T1=128, 56k
serial=1785
OSPF in a Single Area
Once the loading phase is completed and the
router is ready to join the OSPF network, the router is in Full
State.
Routers exchange hello packets every 10
seconds to verify links are up. The Dead interval is 4X the hello
interval.
SPF hold time - Is the time the router
waits before doing a SPF route calculation. The default hold time is 10
seconds.
Neighbor relationships will vary with the
connection types. Different OSPF configurations are required for each.
NBMA - Non-Broadcast Manual
Configuration needed. Static lists for routes must be created.
Router must poll for the routing information
since broadcasts are not allowed. Use the neighbor command to accomplish
this.
neighbor ip-address X Y [X=priority #]
[Y=poll-interval]
OSPF Multiple-Areas Configuration Commands Enable OSPF on the Router
router(config)# router ospf
X (X=process id)
- Next step is to tell the
router which networks are on the OSPF
network
router(config-router)#network
address wildcard-mask area (area id)
LSA1 – Router Links LSA –
Sends information about the routers links.
LSA2 – Network Link LSA –
Sent by the DR to all routers in the AS. A list of routers in the
segment.
LSA3 – Summary Link LSA –
Sent by ASBR’s list of networks available outside the area.
LSA4 – Summary Link LSA –
Sent by ASBR’s list of networks available outside the area.
LSA5 – External Link LSA –
Sent by ASBR’s list of external network routes.
OSPF recalculates a new table when a route
goes down. So, if you have a link flapping you may want to increase the amount
of time to wait. Use spf holdtime command. If not, it could overload CPU
and cause performance issues.
Moustache
LSA’s are sent and an ACK is expected
as a response. If an ACK is not received it will resend in 5 seconds. This can
be changed with the command: ip ospf retransmit-interval 9 (changes
retransmit time to 9 seconds)
Area Types – The area types
determine what kind of LSA’s the area will receive.
Stub Area – Does not accept
External LSA’s. LSA Type 5’s are rejected. Can except route
summaries.
Totally Stubby Areas – Will not
except any LSA’s with external or summaries.
Internal Routers – Exchange
LSA’s 1 and LSA’s 2. They share the same routing database and all
interfaces are within the same area.
Backbone Routers BBR – Exchange
LSA’s 1 and LSA’s 2. Share at least on interface in the area
0.
Area Border Router ABR– Share an
interface with another OSPF area. This router keeps a database for each
area.
Autonomous System Border Router
ASBR – Have at least one interface in a non-OSPF network. Its uses
LSA 5’s to distribute this routing information into the OSPF
network.
Route Table Updates
Routers take Type 1 and Type 2 LSA’s
and make their own route calculations. These entries are used to modify the
routing table.
The routers will also take Type 3 and Type 4
LSA’s and use them to calculate a route for areas in the internetwork.
When a router has both an inter-area route and intra-area route are always kept.
An easy way to remember this is, “Why go around the block to do next
door?”
Only stub area routers do not expect type 5
LSA’s. All other routers perform path calculations to external
AS’s.
Commands for Stub Area Configuration
router(config-router)#area
area-id stub
- Configures Regular Stub
area
router(config-router)#area
area-id stub no-summary
- Configures A Total Stub
Area
Commands for Route Summarization on OSPF
For ASBR’s:
router(config-router)#summary-address
address mask
- Condenses inter-area routes
into summary
For
ABR’s:
router(config-router)#area
area-id range address mask
- Condenses inter-area routes
into summary
Commands for Troubleshooting OSPF
router#show ip route
- Gives the route information
learned by the
router.
router#show
ip protocol
- Router information along with
metrics and networks. Used to verify how OSPF is
configured.
router#show
ip ospf