Misc 2 - RIP and EIGRP

Question 1

What is RIP?

Answer 1

• Routing Information Protocol
• Distance vector IGP protocol (uses routing-by-rumor)
• Uses hop count as its metric. Doesn’t take into account link bandwidth
• Maximum hop count is 15. Anything more than this is considered unreachable.
• RIP and RIPv2 are used for IPv4, RIPng is used for IPv6
• By default, RIP will allow 4 paths to the same destination into the routing table and load balance between these

Question 2

What messages does RIP use?

Answer 2

• Request - To ask RIP enabled routers to send their routing table
• Response - To send the local router’s routing table to neighboring RIP enabled routers

Question 3

How often do RIP enabled routers share their routing table?

Answer 3

Every 30 seconds. This can cause congestion in networks with lots of RIP enabled routers.

Question 4

List drawbacks of RIP (v1).

Answer 4

• RIP only advertises classful addresses.
• It is also assumed that any address in the class A range (0.0.0.0 - 126.255.255.255) is a /8 and the same with class B and class C addresses. This is because RIP doesn’t include subnet masks in its advertisements.
• Doesn’t support CIDR or VLSM
• Messages are broadcast to 255.255.255.255 so all routers (even non RIP enabled ones)

Question 5

List benefits of RIPv2 over RIP (v1)

Answer 5

• Can support CIDR and VLSM so classless addresses can be advertised since RIPv2 includes subnet mask information in its advertisements
• Messages are sent to multicast 224.0.0.9 (only RIP enabled routers)

Question 6

What command do you use to enable RIPv2?

Answer 6

• From global config enter ‘router rip’
• It is best practice to then enter ‘version 2’ as this will ensure you are using RIPv2 instead of RIP
• It is also best to use ‘no auto-summary’ to stop RIPv2 from automatically advertising classful networks by default
• You then need to enter the ‘network <network>' command for each network that is configured on an interface that you want RIP to be enabled on. These will always be translated to classful networks (e.g. entering 'network 10.1.2.3' will automatically change to 'network 10.0.0.0').</network>
• RIP will then form adjacencies with other RIP enabled routers connected to these interfaces. It will advertise the prefixes configured on the interfaces. Not the prefixes entered in the ‘network’ command.

Question 7

True or False. RIP will only advertise the prefix of the network that you enter into the ‘network’ command.

Answer 7

False. It will advertise the prefix of the interface.

Question 8

True or False. The ‘network’ command used in RIP configuration tells the router what networks to advertise.

Answer 8

False. It tells the router which interfaces to enable RIP on.

Question 9

What does the ‘passive-interface’ command in RIP configuration do?

Answer 9

• Tells the router to stop sending RIP messages out of a specified interface
• This is done in RIP configuration mode and not interface configuration mode
• This will not stop the prefix configured on the passive interface from being advertised to other routers out of RIP enabled interfaces
• Same command can be used in OSPF to achieve the same outcome

Question 10

True or False. The ‘passive-interface’ command can be used in OSPF and EIGRP.

Answer 10

True. It is used for the same reason as when used in RIP.

Question 11

What does the ‘default-information originate’ command do?

Answer 11

• Allows a router to share a default route so that other RIP enabled routers can use this as their default route.
• Same command can be used in OSPF to achieve the same outcome

Question 12

What command can you use to find out what routing protocol(s) a router is using?

Answer 12

show ip protocols

Question 13

What is EIGRP?

Answer 13

• Enhanced Interior Gateway Routing Protocol (formerly Interior Gateway Routing protocol)
• Advanced distance vector routing protocol
• Used to be Cisco proprietary but can now be used in other vendor equipment
• Much faster than RIP in reacting to network changes
• Does not have a maximum hop count
• Sends messages to multicast 224.0.0.10 (only EIGRP enabled routers)
• The only IGP that performs unequal cost load balancing
• As with RIP, EIGRP will allow 4 paths to the same destination into the routing table and load balance between these
• Uses bandwidth of the slowest links + delay of all links to calculate its metric

Question 14

What commands do you use to enable EIGRP?

Answer 14

• From global configuration mode enter ‘router eigrp <AS>' The AS number can be any number but it must match between routers that you want to form adjacencies with.</AS>
• It is also best to use ‘no auto-summary’ to stop EIGRP from automatically advertising classful networks by default
• You then need to enter the ‘network <network>' command for each network that is configured on an interface that you want EIGRP to be enabled on. These will always be translated to classful networks unless you specify a wildcard mask (e.g. entering 'network 10.1.2.3' will automatically change to 'network 10.0.0.0').</network>
• EIGRP will then form adjacencies with other EIGRP enabled routers connected to these interfaces. It will advertise the prefixes configured on the interfaces. Not the prefixes entered in the ‘network’ command.

Question 15

What is the function of a wild card mask?

Answer 15

• Used in routing protocols like EIGRP and OSPF to match the network command configured to addresses on interfaces so that the routing protocol can be enabled.
• A 0 means that bit in the address must match the wildcard mask
• A 1 means that bit in the address doesn’t have to match the wildcard mask
• Example: A wildcard mask of 0.0.0.15 means that the first 28 bits of the address must match. Any interfaces with addresses on that do match will have EIGRP enabled.

Question 16

What command could you use to check routes that EIGRP has learned but not entered into its routing table?

Answer 16

show ip eigrp topology

Question 17

What is Feasible Distance?

Answer 17

The local router’s metric value to a destination

Question 18

What is Reported Distance/Advertised Distance

Answer 18

A neighbor’s metric value to a destination

Question 19

What are the two numbers next to each route when using the ‘show ip eigrp topology’ command?

Answer 19

• The number on the left is the Feasible Distance
• The number on the right is the Reported Distance. This is reported by the neighbor that the route goes via.

Question 20

What is a Successor?

Answer 20

The route with the lowest metric to the destination (the best route).

Question 21

What is a Feasible Successor?

Answer 21

An alternate route to a destination (not the best route). This route still meets the Feasability Condition.

Question 22

What is the Feasability Condition?

Answer 22

This states that a route is considered a Feasible Successor if its Reported Distance is lower than the Successor route’s Feasible Distance.
(Compare the Reported Distance of a Feasible Successor route with the Feasible Distance of the Successor route)

Question 23

Why is the Feasability Condition mechanism used?

Answer 23

Loop Prevention. If a route meets the Feasability Condition, it is confirmed that this route will not cause a loop.

Question 24

How would you enable Unequal Cost Multipath load balancing on an EIGRP enabled router?

Answer 24

• By using the ‘variance’ command in EIGRP config mode.
• This adjusts the EIGRP maximum metric variance
• The EIGRP maximum metric variance is a multiplier that is used to determine whether a Feasible Successor route can be used for routing, in turn enabling Unequal Cost Multipath.
• Whatever you set the number to is how many times the Feasible Successor’s Feasible Distance can be above the Successor’s Feasible Distance and be still added into the routing table for load balancing.
• A value of 2 means 2x whatever the Successor’s Feasible Distance is