Ch.16/Mod.7 – Distance Vector Routing Protocols Part 2 of 2: Distance Vector Routing and IGRP

IGRP Features IGRP is a distance vector routing protocol developed by Cisco. IGRP sends routing updates at 90 second intervals, advertising networks for a particular autonomous system. Key design characteristics of IGRP are a follows: –The versatility to automatically handle indefinite, complex topologies –The flexibility needed to segment with different bandwidth and delay characteristics –Scalability for functioning in very large networks

IGRP Features By default, the IGRP routing protocol uses bandwidth and delay as metrics. Additionally, IGRP can be configured to use a combination of variables to determine a composite metric. Those variables include: –Bandwidth –Delay –Load –Reliability

IGRP Metrics

The metrics that IGRP uses are: –Bandwidth – The lowest bandwidth value in the path –Delay – The cumulative interface delay along the path –Reliability – The reliability on the link towards the destination as determined by the exchange of keepalives –Load – The load on a link towards the destination based on bits per second –NO… MTU – The Maximum Transmission Unit value of the path. MTU has never been used by IGRP or EIGRP as a routing metric. IGRP has an administrative distance of 100, more “trustworthy” than RIP at 120. This means a Cisco router will prefer an IGRP learned route over a RIP learned route to the same network.

Administrative Distances

IGRP Metrics

IGRP Routes Interior “Interior routes are routes between subnets of a network attached to a router interface. If the network attached to a router is not subnetted, IGRP does not advertise interior routes.” Clarification IGRP also advertises three types of routes: –interior, system, and exterior. Interior routes are routes between subnets in the network attached to a router interface. If the network attached to a router is not subnetted, IGRP does not advertise interior routes.

IGRP Routes System “System routes are routes to networks within an autonomous system. The Cisco IOS software derives system routes from directly connected network interfaces and system route information provided by other IGRP-speaking routers or access servers. System routes do not include subnet information.”

IGRP Routes Exterior “Exterior routes are routes to networks outside the autonomous system that are considered when identifying a gateway of last resort. The Cisco IOS software chooses a gateway of last resort from the list of exterior routes that IGRP provides. The software uses the gateway (router) of last resort if a better route is not found and the destination is not a connected network. If the autonomous system has more than one connection to an external network, different routers can choose different exterior routers as the gateway of last resort.”

IGRP Timers IGRP has a number of features that are designed to enhance its stability, such as: –Holddowns –Split horizons –Poison reverse updates

IGRP Timers The update timer specifies how frequently routing update messages should be sent. The IGRP default for this variable is 90 seconds. Update timer

IGRP Timers The invalid timer specifies how long a router should wait in the absence of routing-update messages about a specific route before declaring that route invalid (unreachable), but still in the routing table. The IGRP default for this variable is three times the update period or 270 seconds. Then placed in the holddown state. “If I haven’t heard from you in 270 seconds, I am considering this route as unreachable, I will start the holddown timer, but I will keep it in the routing table until the flush timer expires.” Invalid timer

IGRP Timers The holddown timer specifies the amount of time for which information about poorer routes are ignored. Zinin: “Holddown specifies the number of seconds that a route must spend in holddown state after expiration of the Invalid Timer.” The IGRP default for this variable is three times the update timer period plus 10 seconds = 280 seconds. The original route is still in the routing table but marked as unreachable, until the flush timer expires. Holddown timer

IGRP Timers Finally, the flush timer indicates how much time should pass before a route is flushed from the routing table. The IGRP default is seven times the routing update timer or 630 seconds. Zinin: “Flush specifies the number of seconds that a route must remain in the routing table in the garbage collection state after it exits the holddown state.” Each time an update is received the invalid and flush timers are reset. If the invalid timer expires before another update is heard, the route is marked as unreachable, but remains in the routing table. If the flush timer then expires before another update is heard, the route will be deleted from the routing table. Flush timer

IGRP Timers All timers begin at the same time. –Update timer = 90 seconds –Invalid timer = 270 seconds –Holddown timer = 280 seconds –Flush timer = 630 seconds Today, IGRP is showing its age, it lacks support for variable length subnet masks (VLSM). Enhanced IGRP (EIGRP) supports VLSM.

Configuring IGRP Same network commands as RIP. IGRP “AS” number must be the same on all routers.

Configuring IGRP timers basic (IGRP) To adjust Interior Gateway Routing Protocol (IGRP) network timers, use the timers basic router configuration command. To restore the default timers, use the no form of this command. Router(config-router)#router igrp 100 Router(config-router)#timers basic update invalid holddown flush [sleeptime] Router(config-router)# no timers basic

Migrating from RIP to IGRP Router(config)#router rip Router(config-router)#network Router(config-router)#network Router(config-router)#exit Router(config)#router igrp 10 Router(config-router)#network Router(config-router)#network Router(config-router)#exit Router(config)#no router rip Enable IGRP Suggestion: Remove RIP configuration from routers even though the administrative distance will prefer RIP

Verifying IGRP

Troubleshooting IGRP

Summary But there is still more!

IGRP Metric Information

Router> show interfaces s1/0 Serial1/0 is up, line protocol is up Hardware is QUICC Serial Description: Out to VERIO Internet address is /30 MTU 1500 bytes, BW 1544 Kbit, DLY usec, rely 255/255, load 246/255 Encapsulation PPP, loopback not set Keepalive set (10 sec) bandwidthdelay reliabilityload Metric Calculation

Displaying Interface Values Router> show interface s0/0 Serial0/0 is up, line protocol is up Hardware is QUICC Serial Description: Out to VERIO Internet address is /30 MTU 1500 bytes, BW 1544 Kbit, DLY usec, rely 255/255, load 246/255 Encapsulation PPP, loopback not set Keepalive set (10 sec) BandwidthDelay ReliabilityLoad Routing Table Metric Default: Slowest of bandwidth plus the sum of the delays of all outgoing interfaces from “this router” to the destination network.

Bandwidth Expressed in kilobits (show interface) This is a static number and used for metric calculations only. Does not necessarily reflect the actual bandwidth of the link. It is an information parameter only. You cannot adjust the actual bandwidth on an interface with this command. Use the show interface command to display the raw value The default values: Default bandwidth of a Cisco interface depends on the type of interface. Default bandwidth of a Cisco serial interface is 1544 kilobits or 1,544,000 bps (T1), whether that interface is attached to a T1 line (1.544 Mbps) or a 56K line. IGRP metric uses the slowest bandwidth of all of the outbound interfaces to the destination network. Metric Calculation

Changing the bandwidth informational parameter: The bandwidth can be changed using: Router(config-if)# bandwidth kilobits To restore the default value: Router(config-if)# no bandwidth Metric Calculation

Delay Like bandwidth, delay it is a static number. Expressed in microseconds, millionths of a second (Uses the Greek letter mu with an S,  S, NOT “ms” which is millisecond or thousandths of a second) Use the show interface command to display the raw value It is an information parameter only. The default values: The default delay value of a Cisco interface depends upon the type of interface. Default delay of a Cisco serial interface is 20,000 microseconds, that of a T1 line. IGRP metric uses the sum of all of the delays of all of the outbound interfaces to the destination network. Metric Calculation

Changing the delay informational parameter: The delay can be changed using: Router(config-if)# delay tens-of-  S (microseconds) Example of changing the delay on a serial interface to 30,000 microseconds: Router(config-if)# delay 3000 To restore the 20,000 microsecond default value: Router(config-if)# no delay Metric Calculation

IGRP bandwidth = (10,000,000/bandwidth) delay = delay/10 Metric Calculation

IGRP Metrics Values displayed in show interface commands and sent in routing updates.

Router> show interfaces s1/0 Serial1/0 is up, line protocol is up Hardware is QUICC Serial Description: Out to VERIO Internet address is /30 MTU 1500 bytes, BW 1544 Kbit, DLY usec, rely 255/255, load 246/255 Encapsulation PPP, loopback not set Keepalive set (10 sec) bandwidthdelay reliabilityload Metric Calculation

From Casablanca to /24

Using BW IGRP and DLY IGRP to calculate the IGRP metric: The slowest bandwidth has the highest BW IGRP value. IGRP metric = highest BW IGRP + total of the DLY IGRP = 19,531 + ( , , ) = 19, ,200 = 23,731 From Casablanca to /24

Calculating the IGRP Metric Using the Raw Values: Bandwidth and Delay

From Casablanca to /24

So how is Bandwidth, BW IGRP, calculated? The bandwidth metric is calculated by taking 10 7 (10,000,000) and dividing it by the slowest bandwidth metric along the route to the destination. This is known as taking the inverse of the bandwidth scaled by a factor of 10 7 (10,000,000) The lowest bandwidth on the route is 512K or 512 (measured in kilobits), the outgoing interface of the Quebec router. Divide 10,000,000 by 512 and you get the bandwidth! Bandwidth = 10,000,000/512 = 19,531 Which is the lowest BW IGRP along the route Calculating Bandwidth

So how is Delay, DLY IGRP, calculated? Delay is the total sum of delays on the outgoing interfaces, in 10- microsecond units The sum of the delays on each of the outgoing interfaces between Casablanca and Yalta, from /24 through /24 is: 1,000 (Casablanca) + 20,000 (Teheran) + 20,000 (Quebec) + 1,000 (Yalta) = 42,000 We need this in 10-microsecond units: = (1,000/10)+(20,000/10) + (20,000/10) + (1,000/10) = , , or = (1, , ,000 +1,000) / 10 In either case the total sum is: Delay = 4,200 Which is the total of the DLY IGRP, the total Delays along the route! Calculating Delay

IGRP metric = Bandwidth + Delay IGRP metric = 19, ,200 = 23,731 IF we were using RIP, the RIP metric would be 3 hops. Slowest Bandwidth + Sum of Delays

Casablanca# show ip route Known via igrp 1, distance 100, metric 23,731 … , from on Ethernet 0 Route metric is 23,731 Total delay is 42,000 microseconds, minimum bandwidth is 512 Kbit... Not to be redundant, but if we were using RIP, the RIP metric would be 3 hops. show ip route

Ch.16/Mod. 7 – Distance Vector Routing Protocols Part 2 of 2: Distance Vector Routing and IGRP