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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 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 Cisco Default Administrative Distances Route Source Connected interface Administrative Distance 0 Static Route 1 EIGRP summary route 5 External BGP 20 EIGRP 90 IGRP 100 OSPF 110 IS-IS 115 RIP 120 EGP 140 External EIGRP 170 Internal BGP 200 Unknown 255 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 Update timer • • The update timer specifies how frequently routing update messages should be sent. The IGRP default for this variable is 90 seconds. IGRP Timers Invalid timer • 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.” IGRP Timers Holddown timer • 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. IGRP Timers Flush timer • • • • • • 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. 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 Router(config-router)#router igrp 100 Router(config-router)#timers basic update invalid holddown flush [sleeptime] Router(config-router)# no timers basic 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. 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 • • 172.16.0.0 192.168.1.0 172.16.0.0 192.168.1.0 Enable IGRP Suggestion: Remove RIP configuration from routers even though the administrative distance will prefer RIP Verifying IGRP Verifying IGRP Verifying IGRP Verifying IGRP Verifying IGRP Troubleshooting IGRP Troubleshooting IGRP Troubleshooting IGRP Two Types of Autonomous Systems: Process Domains and Routing Domains Process Domain Process Domain Router Router Router BGP Router Router Router Router Router Router Router IGRP 30 Router IGRP 40 AS 90 Routing Domain AS 10 Routing Domain Summary But there is still more! IGRP Metric Information Metric Calculation Router> show interfaces s1/0 Serial1/0 is up, line protocol is up delay bandwidth Hardware is QUICC Serial Description: Out to VERIO Internet address is 207.21.113.186/30 MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 246/255 Encapsulation PPP, loopback not set Keepalive set (10 sec) <output omitted> reliability load Displaying Interface Values Router> show interface s0/0 Serial0/0 is up, line protocol is up Hardware is QUICC Serial Bandwidth Delay Description: Out to VERIO Internet address is 207.21.113.186/30 MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 246/255 Encapsulation PPP, loopback not set Keepalive set (10 sec) <output omitted> Reliability Load Routing Table Metric • Default: Slowest of bandwidth plus the sum of the delays of all outgoing interfaces from “this router” to the destination network. Metric Calculation 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 IGRP Metrics Values displayed in show interface commands and sent in routing updates. Media 100M ATM Fast Ethernet FDDI HSSI 16M Token Ring Ethernet T1 (Serial Default) 512K DS0 56K Bandwidth K= kilobits 100,000K 100,000K 100,000 45,045K 16,000K 10,000K 1,544K 512K 64K 56K BWIGRP 10,000,000/Bandwidth 100 100 100 222 625 1,000 6,476 19,531 156,250 178,571 DLYIGRP Delay 100 S 100 S 100 S 20,000 S 630 S 1,000 S 20,000 S 20,000 S 20,000 S 20,000 S Delay/10 10 10 10 2,000 63 100 2,000 2,000 2,000 2,000 BWIGRP and DLYIGRP display values as sent in IGRP updates and used in calculating the IGRP metric. Metric Calculation Router> show interfaces s1/0 Serial1/0 is up, line protocol is up delay bandwidth Hardware is QUICC Serial Description: Out to VERIO Internet address is 207.21.113.186/30 MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 246/255 Encapsulation PPP, loopback not set Keepalive set (10 sec) <output omitted> reliability load From Casablanca to 172.20.40.0/24 Using the Calculated Values 1,000 100 1,000 100 172.20.4.0/24 1,000 100 6,476 2,000 6,476 2,000 172.25.1.0/24 Casablanca Bandwidth (lowest or minimum) slowest bandwidth: Quebec = 19,531 Delay (sum of outgoing interfaces) = 100+2,000+2,000+100 = 4,200 Teheran Legend: Bandwidth Delay 172.20.2.0/24 Quebec 19,531 2,000 172.20.20.0/24 19,531 2,000 172.20.40.0/24 Yalta Metric = 19,531 + 4,200 = 23,731 1,000 100 From Casablanca to 172.20.40.0/24 Using BWIGRP and DLYIGRP to calculate the IGRP metric: The slowest bandwidth has the highest BWIGRP value. IGRP metric = highest BWIGRP + total of the DLYIGRP = 19,531 + (100 + 2,000 + 2,000 + 100) = 19,531 + 4,200 = 23,731 Calculating the IGRP Metric Using the Raw Values: Bandwidth and Delay From Casablanca to 172.20.40.0/24 Using the Raw Values 10,000K 1,000uS 10,000K 1,000uS 172.20.4.0/24 10,000K 1,000uS 1,544K 20,000uS 1,544K 20,000uS 172.25.1.0/24 Casablanca Bandwidth (lowest or minimum) lowest bandwidth = 512 = 10,000,000/512 = 19,531 Delay (sum of outgoing interfaces) = (1,000/10) + (20,000/10) + (20,000/10) + (1,000/10) = 42,000/10 = 4,200 Metric = 19,531 + 4,200 = 23,731 Teheran Legend: Bandwidth Delay 172.20.2.0/24 Quebec 512K 20,000uS 172.20.20.0/24 512K 20,000uS 172.20.40.0/24 Yalta 10,000K 1,000uS Calculating Bandwidth So how is Bandwidth, BWIGRP, calculated? • The bandwidth metric is calculated by taking 107 (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 107 (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 BWIGRP along the route Calculating Delay So how is Delay, DLYIGRP, calculated? • Delay is the total sum of delays on the outgoing interfaces, in 10microsecond units • The sum of the delays on each of the outgoing interfaces between Casablanca and Yalta, from 172.20.1.0/24 through 172.20.40.0/24 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) = 100 + 2,000 + 2,000 + 100 or = (1,000 + 20,000 + 20,000 +1,000) / 10 In either case the total sum is: Delay = 4,200 Which is the total of the DLYIGRP, the total Delays along the route! Slowest Bandwidth + Sum of Delays IGRP metric = Bandwidth + Delay IGRP metric = 19,531 + 4,200 = 23,731 IF we were using RIP, the RIP metric would be 3 hops. show ip route 172.20.40.0 Casablanca# show ip route 172.20.40.0 Known via igrp 1, distance 100, metric 23,731 … 172.20.1.2, from 172.20.1.2 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. Ch.16/Mod. 7 – Distance Vector Routing Protocols Part 2 of 2: Distance Vector Routing and IGRP