CCNA -A Short Notes - 5 - IP Routing

Part V – IP Routing

Routing: process involving the selection of the best path and the transmission of the data in the chosen direction.

Static Routing: process by which the administrator manually inputs all routing table information.
[no] ip route destnet netmask nexthop [admindist] [permanent]: nexthop is the pingable IP address of the next router or the exit interface for a WAN link. The permanent option will keep the route in memory even if the link goes down. Use the no keyword to remove a route entered.

Default Routing: by replacing destnet and netmask with the 0.0.0.0 wildcard, you can configure a default route on a stub network.
ip classless: required when using default routing since Cisco routers expect by default to know the subnet of all remote networks

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Dynamic Routing: process of using protocols to find and update routing tables.

Routing Protocol	Definition	Example Protocols	Default Admin Distances	Maximum Hop Count
		Directly connected	0
Static		Static routing	1
Distance vector	uses a distance to a remote network to find the best path. Uses hop counts, tick counts (1/18 sec) or bandwidth of links. This type of routing protocol typically has a slow convergence time. Updates are more frequent than link state.	RIP (Routing Information Protocol) IGRP (Interior Gateway Routing Protocol)	120 100	15 255
Link state	maintains three tables (directly attached neighbors, topology of entire network, and routing table)	OSPF (Open Short Path First) – uses the Dijkstra algorithm NLSP (Netware Link State Protocol)	110
Hybrid	Uses aspects of distance vector and link state.	EIGRP	90	224
		External EIGRP	170

RIP (Routing Information Protocol): RIP only uses hop count and is capable of performing round-robin load balancing to up to six equal-cost links. Pinhole congestion happens when two equal-cost links are of different bandwidth, which is disregarded by RIP. RIP does not support AppleTalk.

Routing information messages including the complete routing table are sent every 30 sec by default.

(config)#router rip: enables RIP.

(config-router)#network network: limits the propagation of the RIP messages to the network. For example, if subnet 172.16.40.0 is to be used by RIP, then network should be 172.14.0.0.

(config-router)#passive-interface type number: the interface will not send RIP messages but still receive them. 

Routing loops: is due to the slow convergence of RIP and occurs when conflicting update information is received from different routers.

Maximum Hop Count: will set any network beyond a certain distance to be unreachable with the max hop count +1.

Split Horizon: enforces the rule that information cannot be sent back in the direction from which it was received.

Route Poisoning: sets down links to the unreachable value. It is followed by a poison reverse.

Hold-downs: timer that prevents conflicting rapid updates of the routing tables. Once a value is changed, the router will wait the hold-down timer prior accepting another change.

Triggered Updates: resets the hold-down timer if the timer expires, the router receives a processing task proportional to the number of links or another update is received indicating the network topology has changed. Creates a new routing table sent immediately to neighbour routers. 

IGRP (Interior Gateway Routing Protocol): Cisco proprietary distance-vector routing protocol. Uses bandwidth, and delay as default metrics, and can also use reliability, load and Maximum Transmission Unit (MTU). IGRP can load-balance up to six unequal links. Routing information messages are sent every 90 sec by default.

(config)#router igrp ASnumber: enables RIP, but only shares information between the routers on the same autonomous system (AS).

(config-router)#network network: limits the propagation of the RIP messages to the network.

(config-router)#variance multiplier: number between 1 and 28 which controls the load balancing between the best and worst metric.

(config-router)#traffic-share balanced/min: share inversely proportional to metric or only routes that have minimum cost. 

Other Routing Commands: 

sh ip route: shows the routing tables. Also shows the administrative distance of each link, the hop count, the next hop and exit interface.

sh [ip] protocols: network layer address of each interface or (with ip) the routing protocols on the router and timers used.

debug ip rip/(igrp events/transactions): rip and igrp transactions send routing updates to the console. igrp events only sends a summary, including the destination and provenance, and the number of routers included in each message.

undebug all: turns off debugging. show debug will show what debug options are turned on.

ping address: verify connectivity with remote host.