ip route[address][subnet
mask][next hop IP]
Routers can only forward packets to subnets
in its routing table
ip route[address][subnet
mask][next hop IP]
the new routes can be verified with
ip route [address][subnet
mask][interface]
show ip route (displays all of router’s ip routes)
-> Disadvantage to doing this is that
static routing works one-way
(add route on Router A to Router B, but
Router B still doesn’t have route to Router A)
Extended Ping Command: Simulates a ping from ethernet host, but actually comes from
router itself. When a ping from a router works, but a ping from a host does
not, the extended ping could help you re-create the problem without needing to
work with the end user on the phone.
1. ping
2. Target Address (IP)
3. Extended Commands = y
4. Source Address (IP)
Distance Vector Concepts
Advertise entire routing table (subnet
number and metric) to directly connected neighbors.
Key Points:
- Updates are sent to all neighboring routers through active interfaces, once every so many seconds.
- If a router learns multiple routes to the same subnet, it chooses the best route based on the metric (number of hops).
- Failure to receive updates after x amount of time results in removal of that route
- Routers add directly connected subnets to their routing tables, even without a routing protocol
Distance Vector Loop Avoidance Features eliminate roundabout "ghost" routes to failed or
cyclically-linked routers.
Route Poisoning: Router notices link is down, continues to advertise the route, but
with very large metric (view as infinite and invalid). Other routers remove
their routes to the downed subnet.
RIP uses 16 as the infinite metric
:
Split Horizon:If 2 routers advertise tables at about the same time, with one link
down, they would continually exchange incorrect routing metrics (counting to infinity).
Split horizon doesn't allow this because all routes with outgoing
interface x are not included in updates sent out that same interface x.
In other words, if route to subnet comes in
through interface 1, don’t send update of the same route out interface 1
Ex) Router A’s Ethernet goes down, set
metric to 16 and send to neighbor. At the same time, Router B sends update to
Router A, using the old metric that the cost is 2. Now Router A = 2, Router B =
16. After x amount of time, the two routers exchange routing table and in turn
switch the metrics. This process would repeat indefinitely.
*Split Horizon with Poison Reverse (or
Poison Reverse):Cisco’s proprietary distance vector routing protocols use
this (used by default). Spit horizon used when network links up, but when link
fails, allows broadcast of infinite metric (including previously blocked port
from Split Horizon).
*Hold-Down Timer:Defeats the
counting-to-infinity problem when multiple/redundant links exist between
routers. Routers must wait the duration of the hold-down timer before believing
any "good" information about that route.
Triggered/Flash Updates: Sends new update as soon as route fails.
Summary
|
Issue
|
Solution
|
|
Multiple routes to the same subnet have
equal metrics
|
Either use the first route learned or put
multiple routes to the same subnet in the
routing table. |
|
Routing loops occur due to updates
passing each other over a single link
|
Split horizon—The routing protocol advertises routes out an
interface only if they were not learned from updates entering that interface. Split horizon with poison reverse—The routing protocol uses split-horizon rules unless a route fails. In that case, the route is advertised out all interfaces, including the interface in which the route was learned, but with an infinite-distance metric. |
|
Routing loops occur because routing
information loops
through alternative paths |
Route poisoning—When a route to a subnet fails, the subnet is
advertised with an infinite-distance metric. This term specifically applies to routes that are advertised when the route is valid. Poison reverse refers to routes that normally are not advertised because of split horizon but that are advertised with an infinite metric when the route fails. |
|
Counting to infinity
|
Hold-down timer—After finding out that a route to a subnet
has failed, a router waits a certain period of time before believing any other routing information about that subnet. Triggered updates—When a route fails, an update is sent immediately rather than waiting on the update timer to expire. Used in conjunction with route poisoning, this ensures that all routers know of failed routes before any hold-down timers can expire. |
Comparing RIP and IGRP
|
Feature
|
RIP
|
IGRP
|
|
Update Timer
|
30 seconds
|
90 seconds
|
|
Metric
|
Hop count
|
Bandwidth/delay (also, reliability, MTU,
and load)
|
|
Hold-Down Timer
|
180
|
280
|
|
Flash Updates
|
Yes
|
Yes
|
|
VLSM
|
No
|
No
|
|
Infinite-Metric Value
|
16
|
4,294,967,295
|
Configuring RIP and IGRP
|
Command
|
Configuration Mode
|
|
|
router rip
|
Global
|
|
|
router igrp [as number]
|
Global
|
|
|
router igrp [as number]
|
Global
|
enable IGRP for the AS and enter
(config-router) mode.
|
|
network [net
number]
|
Router subcommand
|
advertise to other routers that this
router has a path to the given network.
|
|
passive-interface[default]
{interface type interface number} |
Router subcommand
|
don't send routing updates out through
the given interface.
|
|
maximum-pathsnumber
|
Router subcommand
|
keep at most number'''different
routes to the same destination in the routing table.
|
|
traffic-share {balanced | min}
|
Router subcommand
|
|
|
traffic-share {balanced | min}
|
Router subcommand
|
share traffic between routes, either
proportionally or just between those of the minimum metric value.
|
EXEC Commands
|
Command
|
Description
|
|
show ip route
|
Shows the entire routing table, or a
subset if parameters are entered.
|
|
show ip protocols
|
Shows routing protocol parameters and
current timer values.
|
|
debug ip rip
|
Issues log messages for each RIP update.
|
|
debug ip igrp transactions [ip address]
|
Issues log messages with details of the
IGRP updates.
|
|
debug ip igrp transactions [ip address]
|
Issues log messages with details of the
IGRP updates.
|
|
debug ip igrp events [ip address]
|
Issues log messages for each IGRP packet.
|
|
trace
|
Sends a series of ICMP echoes with
increasing TTL values to verify the current route to a host.
|
RIP Configuration
router rip
network [network
address1] -> Use network number (address w/ normal class
address)
network [network address2]
network [network address2]
IGRP ConfigurationIGRP Configuration
router igrp [as
number] -> Note: All routers should use the same
AS number
network [network
address1]
network [network address2]
network [network address2]
show running-config -> I =
address found by IGRP, C = directly connected
Example
I 10.1.4.0
[100/8539] via 10.1.2.14, 00:00:50, Ethernet0
-> The
[100/8359] can be broken into two separate
parts:
- 100 = administrative distance
- 8539 = metric (function of bandwidth and delay)
- The higher the bandwidth, the lower the metric
- The lower the cumulative delay, the lower the metric
Bandwidth Defaults
LAN Interfaces = default reflects the correct bandwidth
Serial Interfaces = defaults to 1544 kbps (T1 speed)
-> Configure using the bandwidth
[kbps] interface command
To migrate from RIP to IGRP (Commands)
- no router rip
- router igrp [as number]
- network [network ID]
Debug/show commands include
- debug ip rip
- show ip route
- debug ip igrp transactions (detailed info on updates)
- debug ip igrp events (summary that states updates received)
- show ip protocol (Update timer, elapsed time since update received)
Additional Notes
- If multiple route exist, router chooses best metric route
- If routes tie, keep the
first/pre-existing route
|
|
|
Command: maximum-paths 1 (default is
maximum-paths 4)
|
|
|
|
|
|
|
- When RIP places multiple routes,
router balances traffic
|
|
|
|
|
Command to use lowest-cost: traffic-share
min
|
|
|
||
variance allows
metrics to be considered equal, since IGRP/EIGRP metrics are calculated through
formula and often won’t be exactly the same.
Example:
metric = 100, variance = 2, If value > (lowest metric *
variance), add route
Administrative Distance: In order to compare metrics between different routing protocols,
use administrative distance to denote how believable an entire routing protocol
is on a single router.
-> The lower the number, the better
Default Administrative Distances
|
Route Type
|
Administrative Distance
|
|
Connected
|
0
|
|
Static
|
1
|
|
EIGRP summary route
|
5
|
|
EBGP
|
20
|
|
EIGRP (internal)
|
90
|
|
IGRP
|
100
|
|
OSPF
|
110
|
|
IS-IS
|
115
|
|
RIP
|
120
|
|
EIGRP (external)
|
170
|
|
iBGP (external)
|
200
|
http://www.proprofs.com/mwiki/index.php/RIP%2C_IGRP%2C_and_Static_Route_Concepts_and_Configuration
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