Cisco® CCNP Route Exam Notes : Describing Ripng

3. Layer3 Technologies

3.10 Describe RIPng

RIPng is an extension of RIP for support of IPv6.

The configuration of RIPng is requires at least two steps:

1. Enable RIPng using the global configuration command ipv6 router rip tag. The tag is used to differentiate between multiple RIP processes. It does not have to be the same on all routers.

2. Enable the routing protocol on the interface using the ipv6 rip tag enable. The tag has to match the one used in the ipv6 router rip tag command

The following lists the characteristics of the RIPng protocol:

1. Runs over User Datagram Protocol (UDP).

2. Uses the standard port number 521. Routers that use RIPng listen on the multicast address FF02::9 and send their update messages to this address.

3.11 EIGRP for IPv4

Some of the important terms used in Enhanced IGRP are

1. Successor: A route (or routes) selected as the primary route(s) used to transport packets to reach destination. Note that successor entries are kept in the routing table of the router.

2. Feasible successor: A route (or routes) selected as backup route(s) used to transport packets to reach destination. Note that feasible successor entries are kept in the topology table of a router. There can be up to 6 (six) feasible successors for IOS version 11.0 or later. The default is 4 feasible successors.

3. DUAL (Diffusing Update Algorithm): Enhanced IGRP uses DUAL algorithm to calculate the best route to a destination. Diffusing Update Algorithm used by EIGRP tracks all the routes advertised by neighbors and selects routes based on feasible successors. It inserts lowest cost paths into the routing table (these routes are known as primary routes or successor routes).

Please note that EIGRP does not summarize received routes. That is, if a network was not summarized at the major network boundary (this may happen, if you use "no auto-summary" command), then all the subnet routes will be carried into the routing tables of subsequent routers in the rest of the world.

Giving the following command starts EIGRP routing process:

Router(config)# router eigrp <Autonomous System Number>

The Autonomous System Number should be same the on all routers.

EIGRP uses multicasts to send queries to neighbor routers. EIGRP Hello packets are multicast to

Feature of EIGRP:

1. EIGRP has certain features that belong to link-state algorithms (like OSPF) than distance-vector algorithms.

2. Ex: EIGRP sends a partial routing table update, which includes just routes that have been changed, not the full routing table like distance-vector algorithms.

3. The feasible successor route will become the primary route when its advertised distance is higher than the feasible distance of the successor route. The feasible successor is kept in the topology table as a backup route and can be used in the event that the successor route goes down.

4. Support VLSM, route summarization, and routing update authentication". Therefore B is correct.

5. Unlike RIP and IGRP, EIGRP updates are not periodic. EIGRP updates are sent only when there is a topological change in the network.

6. In EIGRP, the router doing the summarization will build a route to null0 for the summarized address. This ensures that the packets that are not destined for any network are routed to null and thus dropped.

7. EIGRP provides the option of disabling route summarization. The command no auto-summary can be used for this purpose. This option is not available in RIP and IGRP.

8. You can summarize routes in EIGRP at any arbitrary bit boundary.

9. EIGRP uses a distributed algorithm called DUAL when a route fails and has no feasible successor to discover a replacement for a failed route. When a new route is found, DUAL adds it to the routing table.

IGRP (as well as EIGRP) uses the following routing as metrics:

1. Delay: Calculated by adding up the delay along the path to the next router.

2. Reliability: This is representative of how many errors are occurring on the interface. The best reliability value is 255. A value of 128 represents only 50% reliability.

3. Load: Load metric also has a range from 1 to 255. If a serial link is being operated at 50% capacity, the load value is 255X0.5 or 12.5. Lower load value is better.

4. MTU: Stands for Maximum Transmit Unit size, in bytes. Ethernet and serial interface has a default MTU of 1500. Larger MTU size means that the link is more efficient.

5. Bandwidth: The bandwidth is specified in Kbps. Larger the bandwidth, better the link. This is represents the maximum throughput of a link.

6. MTU (Maximum Transmission Unit): This is the maximum message length that is acceptable to all links on the path. The larger MTU means faster transmission of packets.

7. Reliability: This is a measurement of reliability of a network link. It is assigned by the administrator or can be calculated by using protocol statistics.

8. Delay: This is affected by the band width and queuing delay.

9. Load: Load is based among many things, CPU usage, packets processed per sec.

The following are some of the important characteristics of an autonomous system:

1. An autonomous system consist of routers, that present a consistent view of the routing to the external world.

2. Exterior routing protocols are used for communication between autonomous systems

3. Interior routing protocols are used within a single autonomous system

4. An autonomous system can run both interior and exterior protocol simultaneously. However, Interior protocols such as RIP, IGRP are used for communication within the autonomous system, and exterior routing protocols such as BGP are used for communication between autonomous systems.

EIGRP uses multicasts to send queries to neighbor routers.

The following are main features of route summarization in EIGRP:

1.By default, EIGRP summarizes routes at the major network boundaries (classful boundaries).

2.To enable summarization at any level other than major network boundary, you need to disable auto summarization using the command: "No auto-summary"

3.The following command enables summarization at an arbitrary network boundary:

Ip summary-address <as-number> <address-mask>

4.Note that you need to specify the IP address and routing mask of the summary route. No need to specify the metrics.

To turn off automatic summarization, use the command : router(config-router)#no auto-summary

Please note that EIGRP automatically summarizes routes at classful boundary (i.e. the network boundary), unless otherwise specified.

The command : "ipx router eigrp 10" specifies that eigrp is used for routing protocol, and 10 is the autonomous system number.

The command : "network 20" assigns EIGRP for IPX updates to network 20.

EIGRP maintains a number of timers and variables containing time intervals. These include an update timer, an invalid timer, a hold-time period, and a flush timer. The update timer specifies how frequently routing update messages should be sent. The EIGRP default for this variable is 90 seconds. 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. The EIGRP default for this variable is three times the update period. The hold-time variable specifies the holddown period. The EIGRP default for this variable is three times the update timer period plus 10 seconds. Finally, the flush timer indicates how much time should pass before a route should be flushed from the routing table. The EIGRP default is seven times the routing update period.

The correct command to disable auto-summary in EIGRP environment Is : no auto-summary

The above command will turn off route summarization in EIGRP network.

show ip eigrp topology: To display entries in the Enhanced Interior Gateway Routing Protocol (EIGRP) topology table, use the show ip eigrp topology command in EXEC mode.

The following is the sample output of "show ip eigrp topology" command

show ip eigrp topology command output

P: Passive: means the router is not looking for the route actively, thus it means it is in good situation. The status of "Active" means some instability in network.

FD: Feasible Distance: metric to a destination

2172416 / 28160: In the output 2172416 is the feasible distance and 28160 is the advertised distance.

Advertised distance is the distance from your neighbor to destination.

show ip eigrp neighbors: To display the neighbors discovered by Enhanced Interior Gateway Routing Protocol (EIGRP), use the show ip eigrp neighbors command in EXEC mode. It shows when neighbors become active and inactive. The neighbor parameters displayed include Address, Interface, Holdtime, Uptime, Q, Seq Num, SRTT, and RTO.

The following is sample output of the "show ip eigrp neighbors" command

show ip eigrp neighbors command output

The fields in the neighbor table are as under

  • H: Handle: Order in which neighbor adjacency is formed. The first router will have '0' the following one will have '1' and so on.
  • Address: IP address of the neighbor
  • Interface: Interface of the neighbor connected
  • Hold Time: Timer how long to hold a neighbor if a hello is not received. By default it is 15 seconds.
  • Uptime: Since when the neighbor is up
  • SRTT: Smooth Round Trip Time: Time taken for a packet to reach the neighbor and get an acknowledgment back. This time is in milliseconds.
  • RTO(Retransmission Timeout): Time taken to wait before router retransmits a packet to the neighbor
  • Q Cnt: Queue Count: Number of packets that are waiting to be transmitted (Update, Reply, Query). Any number greater than 0, signifies some congestion in the network.
  • Seq Number: Sequence Number: It is the sequence number of the last packet received from neighbor.

show ip route eigrp: Displays the EIGRP routes installed in the route table. Displays the current EIGRP entries in the routing table.

By giving the command "show ip route eigrp", we can see the routes found by eigrp. A route discovered by EIGRP is denoted by letter "D" before start of the entry. Cisco chose letter D for EIGRP, because letter E was already taken by Exterior Gateway Protocol (EGP)

A typical output from a show ip route command is as shown below:

R1#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set is subnetted, 5 subnets
C is directly connected, Loopback3
D [90/156160] via, 00:14:46, FastEthernet0/0
D [90/156160] via, 00:14:55, FastEthernet0/0
C is directly connected, FastEthernet0/0
D [90/2172416] via, 00:14:46, FastEthernet0/0
[90/2172416] via, 00:14:46, FastEthernet0/0 is subnetted, 2 subnets
C is directly connected, Loopback15
C is directly connected, Loopback11

Show ip eigrp interface: Use the "show ip eigrp interfaces" command to determine on which interfaces EIGRP is active, and to find out information about EIGRP relating to those interfaces. The details shown include interfaces on which EIGRP is configured, number of directly connected EIGRP neighbors on each interface, Mean SRTT, etc.

The following is sample output of the "show ip eigrp interfaces" command

Show IP route command output

The significant field are described below

  • Peers : Number of EIGRP neighbors connected on this interface.
  • Xmit Queue Un/Reliable : Number of packets remaining in the Unreliable and Reliable transmit queues.
  • Mean SRTT : Mean smooth round-trip time interval, in milliseconds.
  • Pacing Time Un/Reliable : Pacing time used to determine when reliable and unreliable EIGRP packets should be sent out of the interface.
  • Multicast Flow Timer : Maximum number of seconds the router sends multicast EIGRP packets.
  • Pending Routes : Number of routes in the packets in the transmit queue waiting to be sent.

show ip eigrp traffic: This command can be used to learn the number of EIGRP packets sent and received.

The following is sample output of the "show ip eigrp traffic" command

Show ip eigrp interface command output

The significant fields in the above display are described as below

  • Hellos sent/received : Number of hello packets sent and received.
  • Updates sent/received : Number of update packets sent and received.
  • Queries sent/received : Number of query packets sent and received.
  • Replies sent/received : Number of reply packets sent and received.
  • Acks sent/received : Number of acknowledgement packets sent and received.
  • SIA-Queries sent/received : Number of stuck in active query packets sent and received.
  • SIA-Replies sent/received : Number of stuck in active reply packets sent and received.
  • Hello Process ID : Hello process identifier.
  • PDM Process ID : Protocol-dependent module IOS process identifier.
  • Socket Queue : The IP to EIGRP Hello Process socket queue counters.
  • Input queue : The EIGRP Hello Process to EIGRP PDM socket queue counters.

The neighbor table in EIGRP include the following key elements:

1. Neighbor address: This is the network layer address of the neighbor router.

2. Queue: This represents the number of packets waiting in queue to be sent.

3. Smooth Round Trip Time (SRTT): This represents the average time it takes to send and receive packets from a neighbor. This timer is used to determine the retransmit interval (RTO).

4. Hold Time: This is the period of time that a router will wait for a response from a neighbor before considering the link unavailable.

Neighbor table: The neighbor table stores information about neighboring EIGRP routers:

  • Network address (IP)
  • Connected interface
  • Holdtime - how long the router will wait to receive another HELLO before dropping the neighbor; default = 3 * hello timer
  • Uptime - how long the neighborship has been established
  • Sequence numbers
  • Retransmission Timeout (RTO) - how long the router will wait for an ack before retransmitting the packet; calculated by SRTT
  • Smooth Round Trip Time (SRTT) - time it takes for an ack to be received once a packet has been transmitted
  • Queue count - number of packets waiting in queue; a high count indicates line congestion

Topology table: Topology Table: Confusingly named, this table does not store an overview of the complete network topology; rather, it effectively contains only the aggregation of the routing tables gathered from all directly connected neighbors. This table contains a list of destination networks in the EIGRP-routed network together with their respective metrics. Also for every destination, a successor and a feasible successor are identified and stored in the table if they exist. Every destination in the topology table can be marked either as "Passive", which is the state when the routing has stabilized and the router knows the route to the destination, or "Active" when the topology has changed and the router is in the process of (actively) updating its route to that destination.

Routing table: Stores the actual routes to all destinations; the routing table is populated from the topology table with every destination network that has its successor and optionally feasible successor identified (if unequal-cost load-balancing is enabled using the variance command). The successors and feasible successors serve as the next hop routers for these destinations.

Successor: A successor for a particular destination is a next hop router that satisfies these two conditions: The successor route provides the least distance to that destination, and guaranteed not to be a part of some routing loop The successor route is installed in the Routing table.

Feasible successor: The feasible successor effectively provides a backup route in the case that existing successors die. Also, when performing unequal-cost load-balancing (balancing the network traffic in inverse proportion to the cost of the routes), the feasible successors are used as next hops in the routing table for the load-balanced destination.

By default, the total count of successors and feasible successors for a destination stored in the routing table is limited to four. This limit can be changed in the range from 1 to 6. In more recent versions of Cisco IOS (e.g. 12.4), this range is between 1 and 16

EIGRP will use six different packet types when communicating with its neighboring EIGRP routers

Hello Packets - EIGRP sends Hello packets once it has been enabled on a router for a particular network. These messages are used to identify neighbors and once identified, serve or function as a keepalive mechanism between neighbors. EIGRP Hello packets are sent to the link local Multicast group address Hello packets sent by EIGRP do not require an Acknowledgment to be sent confirming that they were received. Because they require no explicit acknowledgment, Hello packets are classified as unreliable EIGRP packets. EIGRP Hello packets have an OPCode of 5.

Acknowledgement Packets - An EIGRP Acknowledgment (ACK) packet is simply an EIGRP Hello packet that contains no data. Acknowledgement packets are used by EIGRP to confirm reliable delivery of EIGRP packets. ACKs are always sent to a Unicast address, which is the source address of the sender of the reliable packet, and not to the EIGRP Multicast group address. In addition, Acknowledgement packets will always contain a non-zero acknowledgment number. The ACK uses the same OPCode as the Hello Packet because it is essentially just a Hello that contains no information. The OPCode is 5.

Update Packets - EIGRP Update packets are used to convey reachability of destinations. Update packets contain EIGRP routing updates. When a new neighbor is discovered, Update packets are sent via Unicast to the neighbor which can build up its EIGRP Topology Table. It is important to know that Update packets are always transmitted reliably and always require explicit acknowledgement. Update packets are assigned an OPCode of 1.

Query Packets - EIGRP Query packets are Multicast and are used to reliably request routing information. EIGRP Query packets are sent to neighbors when a route is not available and the router needs to ask about the status of the route for fast convergence. If the router that sends out a Query does not receive a response from any of its neighbors, it resends the Query as a Unicast packet to the non-responsive neighbor(s). If no response is received in 16 attempts, the EIGRP neighbor relationship is reset. EIGRP Query packets are assigned an OPCode of

Reply Packets - EIGRP Reply packets are sent in response to Query packets. The Reply packets are used to reliably respond to a Query packet. Reply packets are Unicast to the originator of the Query. The EIGRP Reply packets are assigned an OPCode of 4.

Request Packets - Request packets are used to get specific information from one or more neighbors and are used in route server applications. These packet types can be sent either via Multicast or Unicast, but are always transmitted unreliably.

Previous   Contents   Next

CCNP Route Cram Notes Contents
certexams ad

simulationexams ad