Tag - Split Horizon

Routing Protocol Characteristics

Packets originating from a nonrouting device destined for another network are sent to their default gateway (Layer 3 device on segment). The router consults its routing table to dvcer-mine if the destination network can be reached. If not, the ICMP Destination Unreachable message is sent to the source. If so, packet is forwarded out interface associated with the des­tination network in routing table.

Routing Sources

Default Administrative Distances

Maximum Hop Counts
Routed Source
Default Distance
Static route
EIGRP (internal)
RIPv1 and v2
EIGRP (external)

Static and Default Routes

Static routes are useful in stub networks in which we want to control the routing behavior by manually configuring destination networks into the routing table:

Router(config)#ip route

A floating static route can be configured when redundant connections exist and you want to use the redundant link if the primary fails. This is configured by adding a higher administra­tive distance at the end of a static route:

Router(config)#ip route 2

A default route is a gateway of last resort for a router when there isn’t a specific match for an IP destination network in the routing table (such as packets destined for the Internet):

Router(config)#ip route 0/0

With routing protocols, you can specify a default network, which is a network in the routingtable that routing devices consider to be the gateway of last resort. Using their routing proto­cols, they dvcermine the best path to the default network:

Router(config)#ip default-network

Dynamic Routing Protocols

In complex networks with multiple pathways to destinations, dynamic routing protocols enable routers to advertise their networks to each other and dynamically react to topology changes.

Routing protocols dvcermine the best path based on the lowest metric.

 Routing Metrics

Because one of the core responsibilities of routing protocols is to build routing tables to dvcermineoptimal routing paths, we need to have some means of measuring which routes are preferred whenthere are multiple pathways to a destination. Routing protocols use some measure of metrics toidentify which routes are optimal to reach a destination network. The lowest cumulative metric toa destination is the preferred path and the one that ultimately enters the routing table. Different routing protocols use one or several of the following metrics to calculate the best path.

Routing Metrics

Hop count
The number of routing devices that the packet must travel to reach a destination network
The cumulative bandwidth of the links to the destination in kilobits per second
The length of time (measured in microseconds) a packet takes from source to destination
The consistency of the links and paths toward the destination based on error rates of the interfaces
The cumulative amount of congestion or saturation of the links toward the destination
The maximum frame size that is allowed to traverse the links to the destination
Cost   An arbitrary number typically based on the link’s bandwidth

Interior and Exterior Gateway Routing Protocols

  1. Interior gateway routing protocols: IG routing protocols advertise networks and metrics within an autonomous system.
  2. Exterior gateway routing protocols: EG routing protocols advertise networks in between autonomous systems.

Classful and Classless Routing Updates

  1. Classful routing: The routing updates only contain the classful networks without any subnet mask. Summarization is automatically done when a router advertises a network out an interface that is not within the same major subnet. Classful routing protocols must have a FLSM design and do not operate correctly with discontiguous networks.
  2. Classless routing: The routing updates can contain subnetted networks because the subnet mask is advertised in the updates. Route summarization can be manually config­ured at any bit boundary. Classless routing protocols support VLSM designs and dis­contiguous networks.

all routing protocol

Routing Protocol Classes

  1. Distance vector: The entire routing table is periodically sent to directly connected neighbors regardless of a topology change. These routing protocols manipulate the routing table updates before sending that information to their neighbors and are slow to converge when a topology change occurs.
  2. Link state: All possible link states are stored in an independent topology table in which the best routes are calculated and put into the routing table. The topology table is ini­tially synchronized with discovered neighbors followed by frequent hello messages. These routing protocols are faster to converge than distance vector routing protocols.
  3. Hybrid: By using the best characteristics from link-state and routing protocols, these advanced routing protocols efficiently and quickly build their routing information and converge when topology changes occur.


Redistribution is the method of configuring routing protocols to advertise networks from other routing protocols:

  1. One-way redistribution: Networks from an edge protocol are injected into a more robust core routing protocol, but not the other way around. This method is the safest way to perform redistribution.
  2. Two-way redistribution: Networks from each routing protocol are injected into the other. This is the least preferred method because it is possible that suboptimal routing or routing loops might occur because of the network design or the difference in con­vergence times when a topology change occurs.

Distance Vector Routing Loop Mitigation

Distance vector routing protocols contain several measures to prevent routing loops:

Maximum hop counts: To ensure that routing metrics do not increment until infinity in a routing loop, distance vector routing protocols have a maximum hop count.

Distance Vector/Link State/Hybrid
Maximum Hop Count
RI Pv1
Distance vector
RI Pv2
Distance vector
Link state

Split horizon:

Subnets learned from neighbor routers should not be sent back out the same interface from which the original update came.

Route poisoning with poison reverse:

When a route to a subnet fails, the subnet is advertised with an infinite metric. Routers receiving the poisoned route override the split horizon rule and send a poison reverse back to the source.

Hold-down timers:

The amount of time a router ignores any information about an alternative route with a higher metric to a poisoned subnet.

Flash updates/triggered updates:

When a route fails, the router immediately shoots out an update as opposed to waiting for a normal update interval.

Cisco IOS Revision Commands

Show commands – Cisco routers

Show version – software version, config files, boot images, config register, basic config of router
Show ipx interface – status, ipx rip and sap received/sent, ipx access-lists
Show ipx route – ipx routing table
Show ipx servers – server list (name, port, hops, type of service)
Show ipx traffic – number/type of packets, RIP and SAP packets
Show protocol – ipx addresses of an interface, IP addresses and routing protocol
Show ip protocol – routing update times, networks active on, neighbour routers, administrative distances, AS number
Show access-lists – shows standard and extended access-lists on all interfaces
Show access-list 101
Show ip interface – IP addresses, masks, MTU, access-lists, split horizon, ICMP, CDP status, NAT statistics, LMI info, DLCIs
Show interface serial 0 – shows bandwidth, MTU, keepalives
Show ip route – ip routing table
Show cdp neighbor – deviceID, local intf, holdtime, capability, platform, remote portID
Show cdp entry * – IP address of neighbour, protocol info, platform, capability, portID, holdtime, same info as show version, neighbour device ID, local interface
Show cdp neighbor detail – same as above also IP address, version info
Show cdp traffic – info on traffic and errors
Show cdp interface – encapsulation, timer, holdtime for each intf
Show controllers serial 0 – shows DTE/DCE connections, buffer size, collisions, mac address
Show host(s) – shows hostname to IP resolution of hosts
Show session(s) – shows connections to a remote device
Show users – shows connections to your router, port and time connected
Show frame-relay lmi – lmi errors and lmi type
Show frame-relay pvc – all configured PVC’s, DLCI numbers, status of connections, No. of BECN/FECN
Show frame-relay pvc 16
Show frame-relay map – ip to dlci mappings
Show run – display router config (RAM), configured PVCs, ACLs.

Show commands – 1900 switch

Show mac-address-table
Show run
Show ip – like ipconfig
Show port system – shows switching method
Show vtp
Show trunk A allowed-vlans – shows which VLANS allowed on trunk link
Show vlan-membership
Show vlan 2
Show vlan – verify VLAN config
Show history – shows last 10 commands entered by default
Show terminal – shows terminal config + history buffer size

General router commands

Banner exec – for activation or incoming vty
Banner incoming – displayed on reverse telnet sessions
Banner login – displayed on all connected terminals. Displayed before MOTD banner but before login prompts
Bfe – manual emergency mode setting
Clear counters – clears counters on all interfaces
Clear frame-relay-inarp – clears dynamic mappings
Clear line 1 – clears vty 1 connection to a remote router
Clock set 10.30.10 28 may 2002
Config-register 0x2102 – changes config register
Copy flash tftp
Copy tftp flash – copies IOS image from tftp server default directory to router flash
Disable – to get back to user mode from privileged
Disconnect 1 – clears a connection to a remote router
Enable – to get into privileged mode
Enable last-resort – define enable action if no tacas servers respond
Enable use-tacas – tells the router to authenticate passwords through a tacas server
Encapsulation isl 3 – encapsulates ISL frame-tagging for VLAN 3 on an interface
Exec timeout 0 1 – sets console timeout to 1 second
Exec-timeout 0 0 – sets exec timeout to never timeout
Exec-timeout 1 0 – set exec timeout to 1 minute
Ip domain-lookup – enables DNS (use no form to stop this)
Ip domain-name nikmakris.com – appends domain name to hostname (required for DNS)
Ip host nik – adds to host table (up to 8 ip addr)
Ip name-server – sets ip address(es) of DNS servers
Lock – locks the terminal
Logging synchronous – stops console messages overwriting commands
Logout – to logout of the router
Media-type 100BaseX – sets media type
Mrinfo – request neigbour and version information from a multicast router
No login – sets line, vty ports not to prompt for login info
Passive-inteface s0 – disables routing updates on interface (Global command) – they are not sent out this interface but they are still received.
Ping ipx 40.0000.0c8d.5c9d
Service password-encryption – encrypts passwords (Global command)
Terminal monitor – needed to view debug commands from telnet session
Terminal history size 50 – sets command history to 50 lines
Tftp-server system <image name> – Make router a tftp server

Configure commands

Interface Ethernet 2/0/1 – type slot / port adapter / port number (interface) for VIP cards
Config terminal – configures RAM
Config network (config net) – changes a config on a tftp host
Config memory – configure NVRAM
Ip address secondary – adds a secondary subnet address to an interface
Clock rate 56000 – sets clock rate to 56kbps for DCE interfaces

Access Lists (ACL)

Access-enable – creates a temporary access-list entry
Access-template – creates a temporary access-list entry
Access-profile – apply user profile to interface

Access-list 1 permit – same as next line
Access-list 1 permit any – permits any host
Access-list 1 permit – same as next line
Access-list 1 permit host – permits host
No access list 1 – delvces access list 1
Ip access-group 1 out (out is default)
Access-list 101 deny tcp any eq telnet
Access-list 101 permit ip any any established (TCP connection)
Router(Config)#ip access-list standard nik
Router(Config std-nacl)#deny any log (log – logs packets)

No ip access-group in – won’t delvce inbound lists if you leave the in off the end of the command

Frame relay

Router(Config)#int s0
Router(Config-if)#encapsulation frame-relay cisco (default)
Router(Config)#int s0.16 point-to-point
Router(Config-if)#no inverse-arp
Router(Config-if)#ip address
Router(Config-if)#bandwidth 64 – sets bandwidth to 64kbps (Only used by routing protocols)
Router(Config-if)#frame-relay map ip 16 ietf broadcast – broadcast tells the router to send broadcasts out this intf

IGRP Configuration

Router(Config)#router igrp 50
Router(Config-router)#timers basic 15 45 0 60
Router(Config-router)#variance 1-128 (controls load balancing between best and worse metrics)
Router(Config-router)#traffic-share balanced/min
Balanced – share inversely proportional to metrics
Min – use routers that have only min costs
Router(Config-router)#no metric holddown
Router(Config-router)#metric maximum-hop 50

IPX RIP Configuration

Router(Config)#ipx routing – enables IPX routing, RIP and SAP
Router(Config)#ipx maximum-paths 2 – enables equal cost load balancing over 2 paths
Router(Config)#ipx per-host-load-share – ensures all packets to 1 destination are sent over same line
Router(Config)#int e0
Router(Config-if)#ipx network 10 encapsulation sap secondary – assigns ipx network 10 to a secondary interface on Ethernet 0 with encapsulation Ethernet_802.2
Router(Config-if)#ipx delay 6 – sets tick metric for IPX RIP (LAN = 1, WAN = 6 default)

Cisco ISDN Configuration

Router(Config)#interface bri 0
Router(Config-if)#ip address
Router(Config-if)#no shutdown
Router(Config-if)#dialer-group 1
Router(Config-if)#dialer map ip name 8042 8350661
(Next router ip – hostname of next route (auth) – number to dial)
Router(Config-if)#dialer load-threshold 2 either – makes both interfaces come up immediately
Router(Config-if)#dialer load-threshold 255 either – only brings up 2nd B-channel if first is 100% loaded
Router(Config-if)#hold-queue 75 in – holds 75 packets in the queue during connection
Router(Config-if)#dialer idle-timeout 300 – sets idle time out (120 default)
Router(Config-if)#encapsulation ppp
Router(Config)#dialer-list 1 list 110
Router(Config)#access-list 110 permit tcp any any eq smtp
Router(Config)#access-list 110 permit tcp any any eq telnet

Static/default routes

Ip route = default route
Ip default-network
Ip route serial 1 1 permanent – keeps static route even if router cannot communicate with it, administration distance 1

Cisco Discovery Protocol (CDP) commands

Cdp timer 90
Cdp holdtime 240
Cdp enable (no cdp enable)

1900 Switch Configuration

#config t
(Config)#enable password level 1 nikolas – usermode password
(Config)#enable password level 15 makris – enable password
(Config)#enable secret nik2 – precedes the enable p/w
(Config)#hostname nik
(Config)#vtp pruning enable – enables pruning on whole switch
(Config)#delvce vtp – delvces VTP database
(Config)#vtp server (client/transparent)
(Config)#vtp domain nikolas
(Config)#vtp password cisco
(Config)#ip address
(Config)#ip default-gateway
(Config)#switching-mode store-and-forward (fragment-free)
(Config)#vlan 2 name sales
(Config)#vlan 3 name marketing
(Config)#int Ethernet 0/1 – slot 0/port 1
(Config-if)#vlan-membership static 2
(Config-if)#description Finance_VLAN
(Config-if)#port secure max-mac-count 1 – allows only one mac address per port
(Config)#int f0/26
(Config-if)#trunk on – enables trunking of all VLANS not just VLAN 1
(Config-if)#no trunk-vlan 3 – stops trunking vlan 3
(Config-if)#duplex full – sets to full duplex (auto sense default)
(Config)#copy tftp:// opcode – download into flash memory
(Config)#copy nvram tftp://
(Config)#copy tftp:// nvram
(Config)#delvce nvram – delvces config
(Config)#delvce vtp – delvces VTP database
(Config)#clear mac-address-table [dynamic]

Cisco Debug commands

Debug ipx routing activity – info on RIP updates
Debug ipx sap – info on sap updates
Debug ip igrp events – what’s being rec’d sent, route type (int/ext,system), source, dest
Debug ip igrp transactions – shows contents of routing updates
Debug ip rip – source/contents of updates, network, metrics
Debug frame-relay lmi – enables verification of connection
Debug ppp authentication

Download Telnet Commands 

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