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IP routing commands Essay

Displayed with:
#show ip route connected
Only shows directly connected routes in the routing table
2 versions of commands to modify MTU
(config-if)# ip mtu
This commands sets MTU for current interface for IP

(config-if)# mtu
This command sets MTU only for all protocols

Setting a secondary IP address on an interface
(config-if)# ip address “ip” “mask” secondary
Removing default allowance for zeroth subnet in IOS 12.0 and its effect
(config)# no ip subnet-zero
a. This will reject IP address combo for a zero subnet on local router only.
b. Will not affect other routers, routing protocols
c. Zero subnet is the first subnet after subnetting a network.
Series of commands to configure ISL trunking on router interface.
(config)# interface fa0/1.1
(config-sub-if)# ip address
(config-sub-if)# encapsulation isl 1

*encapsulation number must match sub interface number which all match VLAN

Series of commands to configure 802dot1q trunking on router interface.
(config)# interface fa0/1.2
(config-sub-if)# ip address
(config-sub-if)# encapsulation dot1q 2
(config-if)# encapsulation dot1q 1 native

*last command sets native vlan to 1 on the interface
*can configure physical interface as native with no encapsulation and just an ip

Cconfigure static routes
(config)# ip route “network” “mask” ” [next hop IP | exit interface] [“AD”] [permanent]
2 ways to set default routes
(config)# ip route 0.0.0.0 0.0.0.0 {“next hop IP” | “exit interface”}
(config)# ip default-network “classful IP”
*this command tells router to consider route learned about this classful network as a default route candidate
Configuring a summary address
(config-if)# ip summary-address rip “network” “mask”
(config-if)# ip summary -address eigrp “AS” “ip” “mask”
//Applies to the interface

(config-router)# area “AID” range “ip” “mask” [advert | not-adver | cost “cost”]
//OSPF only, in OSPF process config mode

Tracking ICMP information
#debug ip icmp
Series of Commands for IP configuration for Switches
(config)# interface vlan 1
(config-if)# ip address
(config-if)# no shut
(config-if)# ip default-gateway
Define most specific match
Most specific match is when routers use the route with the longest prefix length if a destination IP matches more than one route in the table
How is MTU calculated
MTU is calculated based on maximum size of the data portion of the data-link frame, wherever it will be placed.

*default MTU of ethernet is 1500 bytes

Define fragmentation
Breaking packet into sizes smaller than MTU using minimum number of fragments.
Process of re-assembly of fragmented packets
a. Reassembly done by endpoint host
b. based on ID and offset value in each fragmented IP packet
2 requirements for router to add directly connected routes
a. interface is in up/up state
b. interface has an IP address assigned manually or via dhcp
Pro and con of using secondary addressing to expand a subnet
a. does not require labor intensive remapping or cause overlaps

b. but, packets destined for hosts on the same LAN may be inefficiently routed through L3 device

Define Ping and Extended Ping
a. regular pings by default uses address of the output interface as the source of icmp echo request
b. echo reply reverses source and destination
c. we can set parameters like source ip, target ip, packet size, etc on extended ping
Define default route
Default route is a special route that matches all packet destinations
Define gateway of last resort
When routers know many default route, best one is chosen based on AD. Then that route is set as the gateway of last resort
See chart about comparing classless and classful networks
Classful networks only use default routes if it doesn’t already have a route to that classful network
See chart about comparing classless and classful networks
Define VLSM
VLSM is when a network uses more than one mask for different subnets of a single classful network
See Chart about classful and classless IGP
When VLSM is in use, the router lists the masks per route under show ip route.
By default, the mask is only listed at the classful network headline
See Chart about classful and classless IGP
Process of finding VLSM network overlaps
1. Calculate subnet ID and broadcast address of each subnet
2. List subnets in order
3. Look at adjacent subnets and find overlaps
Strategy assigning vlsm subnets
Generally recommended to assign largest subnets first.
Define route-summarization
Route summarization allow engineers to advertise one route that replaces several smaller routes with new route matching the same range of addresses
Advantages of route-summarization
a. Reduce sizing of routing table, so improves routing performance
b. Status of components of summary route does not trigger re-convergence every time, improving convergence time.
Notes on route-summarization and subordinate routes
a. router stop advertising subordinate routes
b. summary uses best metric among all subordinate routes
c. Advertise summary route out on the interface it was applied to only
d. Add entry to local routing table with destination null0 when advertising summary route.
e. nothing in the show ip route output line that tell of it being summary and not a subnet
Process of finding best summary route
Best summary route is the summary route with smallest range of addresses aside from the intended range of addresses.

1. List all network to be summarized in numeric order
2. Note low network ID and high broadcast
3. Pick shortest prefix of all networks and subtract 1
4. figure on if that prefix and lowest subnet id combined can include all addresses.
5. If not, less 1 on the prefix and redo.

Define Contiguous networks
A classful network where packets sent between a pair of subnets can only pass through subnets of the same classful network w/o passing through subnets of any other classful network
Define discontiguous networks
A classful network where packets are send through subnets of a different classful network
Define ICMP, RFC 792
Part of TCP/IP’s Network Layer, ICMP define set of messages and procedures about operation of IP. It provides information about network health and operational status.

ICMP sits inside IP packet without transport layer header

4 Types of ICMP Messages
a. Destination Unreachable
b. Time Exceeded
c. Redirect
d. Echo Request/Reply
Define ICMP Destination Unreachable
Tells source host that there is problem delivering packet
Define ICMP Time Exceeded
TTL counter has hit 0, the packet was discarded by the router sending the message
Define ICMP Redirect
Router sending this message is telling the host that another router has a better route for specific destinations and to use that route.
Especially for networks with multiple default gateways
Define ICMP Echo Request/Reply
This is used by Ping command to verify connectivity
Define 5 different ICMP Unreachable Codes
Most security policies filter these messages.
Define 5 different ICMP Unreachable Codes
Define Ping Codes in reponse to ICMP Echo Request
Define Ping Codes in reponse to ICMP Echo Request
Mechanism of Traceroute
Host sends ICMP Echo Request with increasing TTL counts and random UDP ports.
Routers respond with IP and time exceeded messages.
End host sends back Port Unreachable message response to random UDP port.
Microsoft tracert command uses ICMP echos and not UDP. ACL’s could cause IOS traceroute to fail while tracert succeed or vice versa.
Mechanism of Traceroute
What to configure for LAN Switch IP support
Switches have no NIC, uses a virtual interface in VLAN 1

Configure: IP, mask, default gateway, DNS

See Chart about #show ip route
See Chart about #show ip route
VLSM Overlap IP address
a. The router will not accept IP address command if interface is not shutdown.
b. If interface is shutdown, the router accepts the address but will not leave shutdown state.
c. Router detects overlap with interfaces on the same router only.
Troubleshooting VLSM
1. Determine if design uses VLSM, note whether routing protocol is classless
2. Overlapping subnets can be configured
3. Some hosts may work well while overs can’t send packets out of subnet
4. Traceroute command to look for routes that direct packets to wrong network
5. Easier to troubleshoot math if VLSM overlap is detected

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