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Any Questions?. Chapter 8 Routing Protocol Theory. Dynamic Routing Protocol Overview Distance Vector Routing Protocol Features Link-State Routing Protocol Features. Do I know this?. Go through the Quiz- 5 minutes.

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Any questions l.jpg

Any Questions?


Chapter 8 routing protocol theory l.jpg

Chapter 8 Routing Protocol Theory

  • Dynamic Routing Protocol Overview

  • Distance Vector Routing Protocol Features

  • Link-State Routing Protocol Features


Do i know this l.jpg

Do I know this?

Go through the Quiz-

5 minutes


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1. Which of the following routing protocols are considered to use distance vector logic?

a. RIP-1

b. RIP-2

c. EIGRP

d. OSPF

e. BGP

f. Integrated IS-IS


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1. Which of the following routing protocols are considered to use distance vector logic?

a. RIP-1

b. RIP-2

c. EIGRP

d. OSPF

e. BGP

f. Integrated IS-IS

Answer: A&B


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2. Which of the following routing protocols are considered to use link-state logic?

a. RIP-1

b. RIP-2

c. EIGRP

d. OSPF

e. BGP

f. Integrated IS-IS


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2. Which of the following routing protocols are considered to use link-state logic?

a. RIP-1

b. RIP-2

c. EIGRP

d. OSPF

e. BGP

f. Integrated IS-IS

Answer D&F


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3. Which of the following routing protocols use a metric that is, by default, at least

partially affected by link bandwidth?

a. RIP-1

b. RIP-2

c. EIGRP

d. OSPF

e. BGP


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3. Which of the following routing protocols use a metric that is, by default, at least

partially affected by link bandwidth?

a. RIP-1

b. RIP-2

c. EIGRP

d. OSPF

e. BGP

Answer: C&D


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4. Which of the following interior routing protocols support VLSM?

a. RIP-1

b. RIP-2

c. EIGRP

d. OSPF

e. Integrated IS-IS


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4. Which of the following interior routing protocols support VLSM?

a. RIP-1

b. RIP-2

c. EIGRP

d. OSPF

e. Integrated IS-IS

Answer: B, C, D&E


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5. Which of the following situations would cause a router using RIP-2 to remove all the routes learned from a particular neighboring router?

a. RIP keepalive failure

b. No longer receiving updates from that neighbor

c. Updates received 5 or more seconds after the last update was sent to that neighbor

d. Updates from that neighbor have the global “route bad” flag


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5. Which of the following situations would cause a router using RIP-2 to remove all the routes learned from a particular neighboring router?

a. RIP keepalive failure

b. No longer receiving updates from that neighbor

c. Updates received 5 or more seconds after the last update was sent to that neighbor

d. Updates from that neighbor have the global “route bad” flag

Answer: B


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6. Which of the following distance vector features prevents routing loops by causing the routing protocol to advertise only a subset of known routes, as opposed to the full routing table, under normal stable conditions?

a. Counting to infinity

b. Poison reverse

c. Holddown

d. Split horizon

e. Route poisoning


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6. Which of the following distance vector features prevents routing loops by causing the routing protocol to advertise only a subset of known routes, as opposed to the full routing table, under normal stable conditions?

a. Counting to infinity

b. Poison reverse

c. Holddown

d. Split horizon

e. Route poisoning

Answer: D


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7. Which of the following distance vector features prevents routing loops by advertising an infinite metric route when a route fails?

a. Holddown

b. Full updates

c. Split horizon

d. Route poisoning


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7. Which of the following distance vector features prevents routing loops by advertising an infinite metric route when a route fails?

a. Holddown

b. Full updates

c. Split horizon

d. Route poisoning

Answer: D


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8. A router that is using a distance vector protocol just received a routing update that lists a route as having an infinite metric. The previous routing update from that neighbor listed a valid metric. Which of the following is not a normal reaction to this scenario?

a. Immediately send a partial update that includes a poison route for the failed route

b. Put the route into holddown state

c. Suspend split horizon for that route and send a poison reverse route

d. Send a full update listing a poison route for the failed route


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8. A router that is using a distance vector protocol just received a routing update that lists a route as having an infinite metric. The previous routing update from that neighbor listed a valid metric. Which of the following is not a normal reaction to this scenario?

a. Immediately send a partial update that includes a poison route for the failed route

b. Put the route into holddown state

c. Suspend split horizon for that route and send a poison reverse route

d. Send a full update listing a poison route for the failed route

Answer: A


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9. An internetwork is using a link-state routing protocol. The routers have flooded all LSAs, and the network is stable. Which of the following describes what the routers will do to reflood the LSAs?

a. Each router refloods each LSA using a periodic timer that has a time similar to distance vector update timers.

b. Each router refloods each LSA using a periodic timer that is much longer than distance vector update timers.

c. The routers never reflood the LSAs as long as the LSAs do not change.

d. The routers reflood all LSAs whenever one LSA changes.


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9. An internetwork is using a link-state routing protocol. The routers have flooded all LSAs, and the network is stable. Which of the following describes what the routers will do to reflood the LSAs?

a. Each router refloods each LSA using a periodic timer that has a time similar to distance vector update timers.

b. Each router refloods each LSA using a periodic timer that is much longer than distance vector update timers.

c. The routers never reflood the LSAs as long as the LSAs do not change.

d. The routers reflood all LSAs whenever one LSA changes.

Answer: B


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10. Which of the following is true about how a router using a link-state routing protocol chooses the best route to reach a subnet?

a. The router finds the best route in the link-state database.

b. The router calculates the best route by running the SPF algorithm against the information in the link-state database.

c. The router compares the metrics listed for that subnet in the updates received from each neighbor and picks the best (lowest) metric route.


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10. Which of the following is true about how a router using a link-state routing protocol chooses the best route to reach a subnet?

a. The router finds the best route in the link-state database.

b. The router calculates the best route by running the SPF algorithm against the information in the link-state database.

c. The router compares the metrics listed for that subnet in the updates received from each neighbor and picks the best (lowest) metric route.

Answer: B


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Any Questions?


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Dynamic Routing Protocols

  • Routing protocol: A set of messages, rules, and algorithms used by routers for the overall purpose of learning routes. This process includes the exchange and analysis of routing information. Each router chooses the best route to each subnet (path selection) and finally places those best routes in its IP routing table. Examples include RIP, EIGRP, OSPF, and BGP.

  • Routed protocol and routable protocol: Both terms refer to a protocol that defines a packet structure and logical addressing, allowing routers to forward or route the packets. Routers forward, or route, packets defined by routed and routable protocols. Examples include IP and IPX (a part of the Novell NetWare protocol model).

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Routing Protocol Functions

  • Routers need routing protocols to fill the routing table with destinations

    • If no matching destination for a packet, router will drop it

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Routing Protocols

1. Learn routing information about IP subnets from other neighboring routers.

2. Advertise routing information about IP subnets to other neighboring routers.

3. If more than one possible route exists to reach one subnet, pick the best route based on a metric.

4. If the network topology changes—for example, a link fails—react by advertising that some routes have failed, and pick a new currently best route. (This process is called convergence.)

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Routing Functions in action

  • Function 1

    • Both R1 and R3 learn about a route to subnet 172.16.3.0/24 from R2 (function 1).

  • Function 2

    • After R3 learns about the route to 172.16.3.0/24 from R2, R3 advertises that route to R1. R1 must make a decision about the two routes it learned about for reaching subnet 172.16.3.0/24: one with metric 1 from R2, and one with metric 2 from R3.

  • Function 3

    • R1 chooses the lower metric route through R2

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Interior vs. exterior

  • IGP: A routing protocol that was designed and intended for use inside a single autonomous system (AS)

  • EGP: A routing protocol that was designed and intended for use between different autonomous systems

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Autonomous Systems

  • An AS is an internetwork under the administrative control of a single organization.

    • An internetwork created and paid for by a single company is probably a single AS,

    • An internetwork created by a single school system is probably a single AS.

    • Large divisions of a state or national government,

    • Each ISP is also typically a single different AS.

  • Some routing protocols work best inside a single AS

    • These routing protocols are called IGPs.

  • Routing protocols designed to exchange routes between routers in different autonomous systems are called EGPs.

    • Currently, only one legitimate EGP exists: the Border Gateway Protocol [BGP]

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EGP, IGP and AS

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Any Questions?


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IGP Algorithms

  • Distance vector (sometimes called Bellman-Ford after its creators)

  • Link-state

  • Balanced hybrid (sometimes called enhanced distance vector)

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Brief History

  • Distance Vectors first

    • RIP then Cisco’s IGRP

  • Links States solved some of the Distance Vector Problems

    • Slow convergence

    • Routing loops

  • Link States require more planning

    • OSPF

    • IS-IS

  • Hybrid

    • EIGRP

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Routing Metrics

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Metrics Compared

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Metrics Compared

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IGP Comparisons

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IGP Features

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Any Questions?


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Administrative Distance

  • Measures the quality of the routing information

    • The “better” the information, the lower the AD

    • If more than one protocol is in use, the AD determines which routes will be used

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AD

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ADs

  • Admin distance can also be controlled manually

    • ip route 10.1.3.0 255.255.255.0 10.1.130.253 210

    • The 210 is the Admin Distance

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Any Questions?


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Distance Vector Protocols

  • Distance Vector describes what the router knows about the route to the destination

    • Distance-How many hops

    • Vector-Which direction to go

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Distance Vector Protocols

  • Send full routing table periodically

    • Sends the destinations it knows

    • Sends the metric

      • Figure 8-6-R2 tells R1 that 172.30.21.0 is 1 hop

  • Routers that receive updates will add routes to their tables

    • The next hop will be the interface through which they received the update

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Distance Vector Functioning

  • Periodic: The hourglass icons represent the fact that the updates repeat on a regular cycle. RIP uses a 30-second update interval by default.

  • Full updates: The routers send full updates every time instead of just sending new or changed routing information.

  • Full updates limited by split-horizon rules: The routing protocol omits some routes from the periodic full updates because of split-horizon rules. Split horizon is a loopavoidance feature that is covered in the next few pages.

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Routing Loops

  • Since we are routing based on other routers info, and we send our full table, sometimes we get loops

  • How do we prevent loops

    • Route Poisoning

    • Split Horizon

    • Poison Reverse and Triggered Updates

    • Hop Count Limit

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Route Poisoning

  • If a route fails, loops can happen until everyone knows

  • Route poisoning marks route as unreachable

    • Sets hop count to infinity

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Route Poisoning

1. R2’s Fa0/1 interface fails.

2. R2 removes its connected route for 172.30.22.0/24 from its routing table.

3. R2 advertises 172.30.22.0 with an infinite metric, which for RIP is metric 16.

4. R1 keeps the route in its routing table, with an infinite metric, until it removes the route from the routing table.

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Counting To infinity

  • Timing is an issue

    • Since updates are periodic, sometimes it can take a while for all routers to converge

  • One router may send a route poison which will remove the route, but it will hear about the route from it’s neighbor

    • Causes a loop

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Count to infinity

1. R2’s Fa0/1 interface fails, so R2 removes its connected route for 172.30.22.0/24 from its routing table.

2. R2 sends a poisoned route advertisement (metric 16 for RIP) to R1, but at about the same time, R1’s periodic update timer expires, so R1 sends its regular update, including an advertisement about 172.30.22.0, metric 2.

3. R2 hears about the metric 2 route to reach 172.30.22.0 from R1. Because R2 no longer has a route for subnet 172.30.22.0, it adds the two-hop route to its routing table, nexthop router R1.

4. At about the same time as Step 3, R1 receives the update from R2, telling R1 that its former route to 172.30.22.0, through R2, has failed. As a result, R1 changes its routing table to list a metric of 16 for the route to 172.30.22.0.

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Split Horizon-Fixing Count to infinity

  • In routing updates sent out interface X, do not include routing information about routes that refer to interface X as the outgoing interface.

    • Don’t tell the person who told you the rumor the same rumor

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Split Horizon in action

1. R1 sends its normal periodic full update, which, because of split-horizon rules, includes only one route.

2. R2 sends its normal periodic full update, which, because of split-horizon rules, includes only two routes.

3. R2’s Fa0/1 interface fails.

4. R2 removes its connected route for 172.30.22.0/24 from its routing table.

5. R2 advertises 172.30.22.0 with an infinite metric, which for RIP is metric 16.

6. R1 temporarily keeps the route for 172.30.22.0 in its routing table, later removing the route from the routing table.

7. In its next regular update, R1, because of split horizon, still does not advertise the route for 172.30.22.0.

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Poison Reverse

  • Distance vector protocols can attack the counting-to-infinity problem by ensuring that every router learns that the route has failed, through every means possible, as quickly as possible. The next two loop-prevention features do just that and are defined as follows:

  • Triggered update: When a route fails, do not wait for the next periodic update. Instead, send an immediate triggered update listing the poisoned route.

  • Poison reverse: When learning of a failed route, suspend split-horizon rules for that route, and advertise a poisoned route.

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Poison Reverse and Triggered Update

1. R2’s Fa0/1 interface fails.

2. R2 immediately sends a triggered partial update with only the changed information— in this case, a poison route for 172.30.22.0.

3. R1 responds by changing its routing table and sending back an immediate (triggered) partial update, listing only 172.30.22.0 with an infinite metric (metric 16). This is a poison reverse route.

4. On R2’s next regular periodic update, R2 advertises all the typical routes, including the poison route for 172.30.22.0, for a time.

5. On R1’s next regular periodic update, R1 advertises all the typical routes, plus the poison reverse route for 172.30.22.0, for a time.

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Problems with Timing-Redundant Paths

  • If an interface fails, even with poison reverses and split horzon timing issues may lead to bad routes being sent.

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Any Questions?


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Holddown Timers

  • After hearing a poisoned route, start a holddown timer for that one route. Until the timer expires, do not believe any other routing information about the failed route, because believing that information may cause a routing loop. However, information learned from the neighbor that originally advertised the working route can be believed before the holddown timer expires.

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Holddown in action

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Holddown Timers

1. R2’s Fa0/1 interface fails.

2. R2 immediately sends a triggered partial update, poisoning the route for 172.30.22.0. R2 sends the updates out all still-working interfaces.

3. R3 receives R2’s triggered update that poisons the route for 172.30.22.0, so R3 updates its routing table to list metric 16 for this route. R3 also puts the route for 172.30.22.0 in holddown and starts the holddown timer for the route (the default is 180 seconds with RIP).

4. Before the update described in Step 2 arrives at R1, R1 sends its normal periodic update to R3, listing 172.30.22.0, metric 2, as normal. (Note that Figure 8-14 omits some details in R1’s periodic update to reduce clutter.)

5. R1 receives R2’s triggered update (described in Step 2) that poisons the route for 172.30.22.0, so R1 updates its routing table to list metric 16 for this route.

6. R3 receives the update from R1 (described in Step 4), listing a metric 2 route for 172.30.22.0. Because R3 has placed this route in a holddown state, and this new metric 2 route was learned from a different router (R1) than the original route (R2), R3 ignores the new routing information.

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DV summary

  • During periods of stability, routers send periodic full routing updates based on a short update timer (the RIP default is 30 seconds). The updates list all known routes except the routes omitted because of split-horizon rules.

  • When changes occur that cause a route to fail, the router that notices the failure reacts by immediately sending triggered partial updates, listing only the newly poisoned(failed) routes, with an infinite metric.

  • Other routers that hear the poisoned route also send triggered partial updates, poisoning the failed route.

  • Routers suspend split-horizon rules for the failed route by sending a poison reverse route back toward the router from which the poisoned route was learned.

  • All routers place the route in holddown state and start a holddown timer for that route after learning that the route has failed. Each router ignores all new information about this route until the holddown timer expires, unless that information comes from the same router that originally advertised the good route to that subnet.

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Any Questions?


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Link State Protocols

  • Routers need to FLOOD details to other routers to synchronize Link State Dabase (LSDB)

  • OSPF uses Links State Advertisements (LSA) to update routers

    • Router LSA: Includes a number to identify the router (router ID), the router’s interface IP addresses and masks, the state (up or down) of each interface, and the cost (metric) associated with the interface.

    • Link LSA: Identifies each link (subnet) and the routers that are attached to that link. It also identifies the link’s state (up or down).

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LSA Flooding

  • R8 Sends LSA

  • Other routers will forward R8 LSA

    • They will first ask other routers if they have already received the LSA

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LSA

  • Contain the details about physical interfaces (Links)

  • Also containing the status of the link

    • Is it up or down

  • After initial LSA flood, LSA only reflood if:

    • There is a change to the network

    • After LSA timer goes (30 minutes)

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Dijkstra Shortest Path First

  • Algorithm Used to find best route to destination networks

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SPF

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Link State Convergence

  • When something Changes

    1. R5 and R6 flood LSAs that state that their interfaces are now in a “down” state. (In a network of this size, the flooding typically takes maybe a second or two.)

    2. All routers run the SPF algorithm again to see if any routes have changed. (This process may take another second in a network this size.)

    3. All routers replace routes, as needed, based on the results of SPF. (This takes practically no additional time after SPF has completed.) For example, R1 changes its route for subnet X (172.16.3.0/24) to use R2 as the next-hop router.

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Summary

  • All routers learn the same detailed information about all routers and subnets in the internetwork.

  • The individual pieces of topology information are called LSAs. All LSAs are stored in RAM in a data structure called the link-state database (LSDB).

  • Routers flood LSAs when 1) they are created, 2) on a regular (but long) time interval if the LSAs do not change over time, and 3) immediately when an LSA changes.

  • The LSDB does not contain routes, but it does contain information that can be processed by the Dijkstra SPF algorithm to find a router’s best route to reach each subnet.

  • Each router runs the SPF algorithm, with the LSDB as input, resulting in the best (lowest-cost/lowest-metric) routes being added to the IP routing table.

  • Link-state protocols converge quickly by immediately reflooding changed LSAs and rerunning the SPF algorithm on each router.

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Any Questions?


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