Chapter 4 Distance Vector Routing Protocols CIS 82 Routing Protocols and Concepts Rick Graziani Cabrillo College Last Updated: 3/9/2009

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4 Topics Introduction to Distance Vector Routing Protocols  Distance Vector Technology  Routing Protocol Algorithms  Routing Protocol Characteristics Network Discovery  Cold Start  Initial Exchange of Routing Information  Exchange of Routing Information Routing Table Maintenance  Periodic Updates  Bounded Updates  Triggered Updates  Random Jitter Routing Loops  Defining a Routing Loop  Implications of Routing Loops  Count-to-Infinity Condition  Preventing Routing Loops by setting a Maximum Metric Value  Preventing Routing Loops with Hold-down Timers  Preventing Routing Loops with the Split Horizon Rule  Preventing Routing Loops with IP and TTL Distance Vector Routing Protocols Today  RIP  EIGRP

Introduction to Distance Vector Routing Protocols Distance Vector Technology Routing Protocol Algorithms Routing Protocol Characteristics

6 Introduction to Distance Vector Routing Protocols Understanding the operation of distance vector routing is critical to enabling, verifying, and troubleshooting these protocols.

7 Configuring and maintaining static routes for a large network would be overwhelming. What happens when that link goes down at 3:00 a.m.?

8 RIP: Routing Information Protocol originally specified in RFC IGRP: Interior Gateway Routing Protocol - Cisco proprietary EIGRP: Enhanced IGRP – Cisco proprietary IGRP and EIGRP: Cisco never submitted RFCs to IETF for these protocols. Why did Cisco develop IGRP/EIGRP?

9 Meaning of Distance Vector Distance vector (repeat)  Routes are advertised as vectors of distance and direction. Direction is simply the:  nexthop router or  exit interface. Routing protocol  Does not know the topology of an internetwork.  Only knows the routing information received from its neighbors.

10 Meaning of Distance Vector I can get to /24 in one hop out my S0/0/0. What is the Distance to /24?  1 hop What is the Direction:  S0/0/0 Does R1 have a topology map?  No, it only knows distance and direction!

11 Operation of Distance Vector Routing Protocols Periodic updates Some distance vector routing protocols periodically broadcast the entire routing table to each of its neighbors. (RIP and IGRP)  30 seconds for RIP Inefficient:  Bandwidth and CPU resources  Always sent, even no changes Timer Expires

12 Operation of Distance Vector Routing Protocols What are Neighbors? Neighbors are routers that:  Share a link  Use the same routing protocol. What are the only addresses routers know about before there is any routing knowledge?  Network addresses of its own interfaces  Network addresses of its neighbors. R1 is unaware of R3 and its networks Neighbor of R1

13 Operation of Distance Vector Routing Protocols Routing Protocols use  Broadcast updates ( )  Multicast updates Neighbor routers will process the updates. What will other devices on that link do if they receive a routing update but are not running that routing protocol including hosts?  They will process the update up to Layer 3 (Multicast update) or Layer 4 (Broadcast update) before discarding it. Timer Expires

14 Routing Protocol Algorithms The routing protocol algorithm used by a particular routing protocol is responsible for building and maintaining the router’s routing table.

15 Routing Protocol Algorithms The algorithm sends and receives updates. Update

16 Routing Protocol Algorithms The algorithm on each router:  Independently makes calculations updates its routing table Calculating best paths and installing new routes Update

17 Routing Protocol Algorithms The algorithm on each router:  Detect and react to topology changes. Detecting and reacting to topology change Update X

18 Routing Protocol Characteristics Time to convergence:  Faster the better. Scalability:  How large a network the routing protocol can handle. Classless (use of VLSM) or classful:  Support VLSM and CIDR Resource usage:  Routing protocol usage of RAM, CPU utilization, and link bandwidth utilization. Implementation and maintenance:  Level of knowledge that is required for a network administrator. More later

19 Advantages and Disadvantages of Distance Vector Routing Protocols Advantages:  Simplicity  Low resource requirements  Minimum link bandwidth Disadvantages:  Slow convergence  Limited scalability  Potential for routing loops (coming)

20 Comparing Routing Protocol Features Note: Some of this is relative such as Resource usage and Implementation and Maintenance.

Network Discovery Cold Start Initial Exchange of Routing Information Exchange of Routing Information

22 Cold Start Network discovery is part of the process of the routing protocol algorithm that enables routers to first learn about remote networks. First: Only knows directly connected networks.

23 Initial Exchange of Routing Information R1:  Sends an update about network out the Serial 0/0/0 interface with a metric of 1  Sends an update about network out the FastEthernet 0/0 interface with a metric of 1  Receives an update from R2 about network on Serial 0/0/0 with a metric of 1  Stores network in the routing table with a metric of 1 Update

24 Initial Exchange of Routing Information R2:  Sends an update about network out the Serial 0/0/0 interface with a metric of 1  Sends an update about network out the Serial 0/0/1 interface with a metric of 1 Update

25 Initial Exchange of Routing Information R3:  Sends an update about network out the Serial 0/0/1 interface with a metric of 1  Sends an update about network out the FastEthernet 0/0 interface with a metric of 1 Update

26 Initial Exchange of Routing Information Have we reached convergence?  No What needs to still be learned?  R1 does not have knowledge of  R3 does not have knowledge of

27 Next Exchange of Routing Information R1:  Sends out complete routing table. Does R2 learn anything new?  No Update Thanks, but nothing new

28 Next Exchange of Routing Information R2:  Sends out complete routing table. Does R1 Learn anything new?  Yes, Does R3 Learn anything new?  Yes, Update S0/0/1

29 Next Exchange of Routing Information R3:  Sends out complete routing table. Does R2 learn anything new?  No Update S0/0/1

30 Note on Split Horizon Distance vector routing protocols typically implement a technique known as split horizon.  Prevents information from being sent out the same interface from which it was received. More later Update X S0/0/1

31 Convergence The amount of time it takes for a network to converge is directly proportional to the size of that network. Routing protocols are compared based on how fast they can propagate this information—their speed to convergence

Routing Table Maintenance Periodic Updates Bounded Updates Triggered Updates Random Jitter

33 Periodic Updates Depending on the routing protocol, routers must maintain the routing tables so that they have the most current routing information. Some distance vector routing protocols use periodic updates.  RIP and IGRP Sent even when there is no new information. Periodic Update S0/0/1

34 Periodic Updates Routing update may contain a topology change. What might those changes be?:  Failure of a link  Introduction of a new link  Failure of a router  Change of link parameters Periodic Updates Periodic Update S0/0/1

35 RIP Timers IOS implements three additional timers for RIP. Update timer: 30 seconds. Invalid Timer: If an update has not been received in 180 seconds (the default), the route is marked as invalid by setting the metric to 16.  Route still is in routing table. Flush Timer: 240 seconds (default)  When the flush timer expires, the route is removed from the routing table. Hold-down Timer: 180 seconds (default)  Later in this chapter. Periodic Update No update for from R3 received, mark route as “possibly down”, but leave in routing table. Still no update for from R3 received. Remove this route from the routing table. S0/0/1

36 RIP timer values can be verified with two commands: show ip route and show ip protocols. R1# show ip route /16 is subnetted, 4 subnets C is directly connected, Serial0/0/0 R [120/1] via , 00:00:04, Serial0/0/0 C is directly connected, FastEthernet0/0 R [120/2] via , 00:00:04, Serial0/0/0 Elapsed time since the last update, expressed in seconds R1# show ip protocols Routing Protocol is “rip” Sending updates every 30 seconds, next due in 13 seconds Invalid after 180 seconds, hold down 180, flushed after 240 Routing Information Sources: Gateway Distance Last Update :00:27 RIP Timers

37 EIGRP does not send periodic updates. EIGRP sends bounded updates about a route when a path changes or the metric for that route changes. Note: More in Chapter 9 EIGRP. Bounded Updates

38 A triggered update is a routing table update that is sent immediately in response to a routing change. Triggered updates do not wait for update timers to expire. What is the advantage to a triggered update?  Speeds up convergence. Triggered Updates X Update timer not yet expired Triggered Update

39 Random Jitter To prevent the synchronization of updates between routers, Cisco IOS uses a random variable, called RIP_JITTER, which subtracts a variable amount of time to the update interval for each router in the network.  Ranges from 0 to 15 percent of the specified update interval.  25.5 to 30 seconds for the default 30-second interval. Collision ! We will randomize our updates between 25.5 and 30 seconds so collisions don’t happen.

Routing Loops Defining a Routing Loop Implications of Routing Loops Count-to-Infinity Condition Preventing Routing Loops by setting a Maximum Metric Value Preventing Routing Loops with Hold-down Timers Preventing Routing Loops with the Split Horizon Rule Preventing Routing Loops with IP and TTL

41 Defining a Routing Loop A routing loop is a condition in which a packet is continuously transmitted within a series of routers without ever reaching its intended destination network.  Can occur when two or more routers have inaccurate routing information to a destination network.  Issue with distance vector routing protocols but not link-state. The loop can be a result of:  Incorrectly configured static routes  Incorrectly configured route redistribution (CCNP-level courses)  Inaccurate routing because of slow convergence in a changing network

42 Implications of Routing Loops What might a some problems with a routing loop? A routing loop can create the following conditions:  Link bandwidth – looping traffic  Router’s CPU - Burdened with useless packet forwarding  Routing updates might get lost or not processed in time.  Packets might get lost in “black holes”. A routing loop can have a devastating effect on a network.

43 Implications of Routing Loops Assuming no split horizon, what if network goes down? Is there a potential for a problem here? Let’s see… Periodic Update hops thru me X This is great, I now have a route to again! S0/0/1 2 IP Packet: DA Loop until TTL is 0 S0/0/1

44 Implications of Routing Loops Mechanisms available to eliminate routing loops:  Defining a maximum metric to prevent count to infinity  Hold-down timers  Split horizon  Route poisoning or poison reverse  Triggered updates (covered previously) I mistakenly believe I have a route to S0/0/1

45 Count-to-Infinity Condition Count to infinity is a condition that exists when inaccurate routing updates increase the metric value to “infinity” for a network that is no longer reachable. Each protocol defines infinity at a different value. Periodic Update hops thru me X S0/0/1 2 Periodic Update hops thru me Periodic Update hops thru me 3 Periodic Update hops thru me 4 S0/0/1

46 Count-to-Infinity Condition This count continues indefinitely, each router thinking the other router has a route to To eventually stop the incrementing of the metric, “infinity” is defined by setting a maximum metric value. RIP defines infinity as 16 hops — an “unreachable” metric. When the routers “count to infinity,” they mark the route as unreachable. Periodic Update hops thru me X S0/0/1 10 Periodic Update hops thru me Periodic Update hops thru me Periodic Update hops thru me Periodic Update hops thru me 16 Periodic Update hops thru me 16 means “network unreachable” in RIP 16 S0/0/1

47 Preventing Routing Loops with Hold-Down Timers A routing loop could also be created by a periodic update that is sent by the routers during the instability. Hold-down timers:  Prevent routing loops from being created by these conditions.

48 Preventing Routing Loops with Hold-Down Timers Network attached to R3 goes down. R3 sends a triggered update. X Update timer not yet expired Triggered Update S0/0/1

49 Preventing Routing Loops with Hold-Down Timers R2 receives the update from R3 indicating that network is now no longer accessible. R2 marks the network as possibly down and starts the hold-down timer. X Possibly down - Start Hold-down Timer Triggered Update S0/0/1

50 Preventing Routing Loops with Hold-Down Timers If an update with a better metric for that network is received from any neighboring router during the hold-down period, R2 will reinstate the network and the hold-down timer will be removed. Note: In this example their can’t be a better metric than 1 hop. X Possibly down - Start Hold-down Timer Triggered Update Update with better metric S0/0/1

51 Preventing Routing Loops with Hold-Down Timers If an update from any other neighbor is received during the hold-down period with the same or worse metric for that network, that update is ignored. Thus, more time is allowed for the information about the change to be propagated. X Possibly down - Start Hold-down Timer Update with worse metric: hops Same or worse metric - Still possibly down - Keep Hold-down Timer going S0/0/1

52 Preventing Routing Loops with Hold-Down Timers R1 and R2 still forward packets to , even though it is marked as possibly down. This allows the router to overcome any issues associated with intermittent connectivity. If the destination network is truly unavailable and the packets are forwarded, black-hole routing is created and lasts until the hold-down timer expires. X Possibly down IP Packet: DA S0/0/1

53 Preventing Routing Loops with Hold-Down Timers When the hold-down timers expire on R1 and R2, is removed from the routing table. No traffic to will be routed – packets dropped by each router. X Possibly down Expires S0/0/1

54 Preventing Routing Loops with the Split Horizon Rule Split horizon rule says that a router should not advertise a network through the interface from which the update came.  Helps prevent routing loops caused by slow convergence. What network(s) will R1 NOT include in its routing updates to R2?  and What network(s) will R2 NOT include in its routing updates R1? R3?  R1: R3: What network(s) will R3 NOT include in its routing updates R2?  and S0/0/1

55 Preventing Routing Loops with the Split Horizon Rule 1. R3 advertises the network to R2. 2. R2 receives the information and updates its routing table. 3. R2 then advertises the network to R1 out S0/0/0.  R2 does not advertise to R3 out S0/0/1, because the route originated from that interface. 4. R1 receives the information and updates its routing table. 5. Because of split horizon, R1 also does not advertise the information about network back to R2. Periodic Update: X X S0/0/1

56 Preventing Routing Loops with the Split Horizon Rule What networks does R1 advertise to R2?  R1 advertises network to R2. What networks does R2 advertise to R1?  R2 advertises networks and to R1. What networks does R2 advertise to R3?  R2 advertises networks and to R3. What networks does R3 advertise to R2?  R3 advertises network to R2. Notice that each router increments the hop count before sending the update. Split horizon can be disabled by an administrator to achieve the proper routing under certain conditions. S0/0/1

57 Route Poisoning Route poisoning is used to mark the route as unreachable in a routing update that is sent to other routers.  Unreachable is interpreted as a metric that is set to the maximum.  For RIP, a poisoned route has a metric of 16. Route poisoning speeds the convergence process.

58 Split Horizon with Poison Reverse Split horizon with poison reverse The concept of split horizon with poison reverse is that explicitly telling a router to ignore a route is better than not telling it about the route in the first place. Periodic Update: = 16 S0/0/1

59 Preventing Routing Loops with IP and TTL The Time to Live (TTL) is an 8-bit field in the IP header that limits the number of hops a packet can traverse through the network before it is discarded. The TTL is decreased by 1 by every router on the route to its destination. If the TTL field reaches 0 before the packet arrives at its destination, the packet is discarded and the router sends an Internet Control Message Protocol (ICMP) error message back to the source of the IP packet.

60 Preventing Routing Loops with IP and TTL Situation where the routing tables do not have accurate information about the downed network. Even in the case of this routing loop, packets will not loop endlessly in the network. Eventually the TTL value will be decreased to 0 and the packet will be discarded by the router. Periodic Update hops thru me X S0/0/1 2 IP Packet: DA Loop until TTL is 0 S0/0/1

Distance Vector Routing Protocols Today RIP EIGRP

62 Distance Vector Routing Protocols Today Although link-state routing protocols have several advantages over distance vector routing protocols, distance vector routing protocols are still in use today. Link-state routing protocols will be discussed later.

63 RIP and EIGRP For distance vector routing protocols, there really are only two choices: RIP or EIGRP. The decision about which routing protocol to use in a given situation is influenced by a number of factors, including  Size of the network  Compatibility between models of routers  Administrative knowledge required

RIPv1: First Look

65 RIPv1: First Look Download Packet Tracer Topology: cis82-RIPv1-A-student.pkt

66 Specifying Networks Use the network command for each directly connected network. R1(config)# router rip R1(config-router)# network directly-connected-classful-network-address R1(config-router)#... R2(config)# router rip R2(config-router)# network directly-connected-classful-network-address R2(config-router)#... R3(config)# router rip R3(config-router)# network directly-connected-classful-network-address R3(config-router)#... Only directly connected classful network addresses!

67 Specifying Networks If you enter a subnet or host IP address, IOS automatically converts it to a classful network address. For example, if you enter the command network , the router will convert it to network R1(config)# router rip R1(config-router)# network R1(config-router)# network R2(config)# router rip R2(config-router)# network R2(config-router)# network R2(config-router)# network R3(config)# router rip R3(config-router)# network R3(config-router)# network Only directly connected classful network addresses! Verify with the commands: show ip route show ip protocols

68 Verifying RIP: show ip route Command R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, Gateway of last resort is not set R /24 [120/1] via , 00:00:02, Serial0/0/0 R /24 [120/2] via , 00:00:02, Serial0/0/0 C /24 is directly connected, FastEthernet0/0 C /24 is directly connected, Serial0/0/0 R /24 [120/1] via , 00:00:02, Serial0/0/0 An R in the output indicates RIP routes. Because this command displays the entire routing table, including directly connected and static routes, it is normally the first command used to check for convergence. Routes might not immediately appear when you execute the command because networks take some time to converge..

69 Verifying RIP: show ip route Command R2# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, Gateway of last resort is not set C /24 is directly connected, Serial0/0/1 R /24 [120/1] via , 00:00:12, Serial0/0/1 R /24 [120/1] via , 00:00:24, Serial0/0/0 C /24 is directly connected, Serial0/0/0 C /24 is directly connected, FastEthernet0/0

70 Verifying RIP: show ip route Command R3# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, Gateway of last resort is not set C /24 is directly connected, Serial0/0/1 C /24 is directly connected, FastEthernet0/0 R /24 [120/2] via , 00:00:08, Serial0/0/1 R /24 [120/1] via , 00:00:08, Serial0/0/1 R /24 [120/1] via , 00:00:08, Serial0/0/1

Chapter 4 Distance Vector Routing Protocols CIS 82 Routing Protocols and Concepts Rick Graziani Cabrillo College Last Updated: 3/9/2009