The “router” command in global configuration mode is an important command for start configuration of any dynamic routing protocol. The topology displayed in Figure 1 is used to configure and demonstrate the EIGRP routing protocol. Figure 2 (marked with a red triangle) illustrates the “router ?” with question marks to demonstrate the list of dynamic routing protocol supported by the current IOS.
To configure the routing protocol EIGRP, enter the command as shown below in the global configuration mode.
Router(config)# router eigrp <autonomous-system>
Theautonomous-systemargument is a 16-bit value between the number 1 and 65,535 as shown in figure 2 marked with a yellow triangle. All routers within the EIGRP routing domain must use the same autonomous system number. The configuration of the autonomous system and enable EIGRP routing protocol for the topology shown in figure 1 is the following:
Router R1
R1>enable
R1#configure terminal
R1(config)#router eigrp 1
R1(config-router)#
Router R2
R2>enable
R2#configure terminal
R2(config)#router eigrp 1
R2(config-router)#
Router R3
R3>enable
R3#configure terminal
R3(config)#router eigrp 1
R3(config-router)#
Notice that the prompt has been changed from a global configuration mode prompt to router configuration mode. We have configured the autonomous system number same on all router because it is must be same on all routers to establish neighbour adjacencies. EIGRP can support multiple instances of the routing protocol, though multiple routing protocol implementation is not usually needed or recommended.
Therouter eigrp<autonomous-system> command is not enough for starting the EIGRP process. The command only enables the EIGRP and provides access to configure the EIGRP settings.
We can remove the EIGRP routing process from a device, using the“no router eigrp<autonomous-system>” command in global configuration mode. The command will stop the EIGRP process and removes all existing EIGRP router configurations.
Autonomous system numbers (ASNs) is a unique identifier that is globally available and allows its autonomous system to exchange routing information with other systems. An Internet Assigned Numbers Authority (IANA) globally assigned autonomous system is a group of networks under the administrative control of a single body that presents a common routing policy to the Internet. The guidelines for the creation, selection, and registration of an autonomous system are described in RFC 1930.
Global autonomous system numbers are assigned by IANA, the similar authority that assigns IP address space. The local regional Internet registry (RIR) can assign an autonomous system number to an entity from its block of assigned autonomous system numbers.
Usually, Internet Service Providers (ISPs) use autonomous System Numbers to control routing within their networks and to exchange routing information with other Internet Service Providers (ISPs). They use the exterior gateway routing protocol, Border Gateway Protocol (BGP), to propagate routing information. BGP is the only routing protocol that uses an actual autonomous system number in its configuration.
Autonomous System Numbers Format
The Autonomous System has two different formats to represent: 2-byte and 4-byte format. A 2-byte format provides 65,536 Autonomous System number ranges from 0 to 65535. The Internet Assigned Numbers Authority (IANA) reserved 1,023 numbers (64512 to 65534) for private use.
A 4-byte Autonomous System format provides for 4,294,967,296 ASNs ranges from 0 to 4294967295. The Internet Assigned Numbers Authority (IANA) reserved a block of 94,967,295 ASNs ranges from 4200000000 to 4294967294 for private use.
Autonomous System Numbers in EIGRP
EIGRP uses the “router eigrp <autonomous-system>” command in global configuration mode to enable the EIGRP process. The autonomous system number uses in the EIGRP configuration are not related to the Internet Assigned Numbers Authority (IANA).
The autonomous system number uses for EIGRP configuration are only important to the EIGRP routing domain. It helps routers keep track of multiple, running instances of EIGRP. The autonomous system number is required because it is possible to have more than one instance of EIGRP running on a network. Each instance of EIGRP can be configured to support and exchange routing updates for different networks.
EIGRP uses 5 different types of packet in communication with its neighbours. EIGRP packets are sent with reliable or unreliable delivery and can be sent as a unicast, multicast, or sometimes both. EIGRP packet types are also called EIGRP packet format or EIGRP messages. The details of the EIGRP packets are the following:
EIGRP Hello Packets
EIGRP uses Hello packets to discover EIGRP-enabled neighbours on directly connected links. The router uses hello packets to perform EIGRP neighbour adjacencies, also known as neighbour relationships. The hello packet is sent as IPv4 and IPv6 multicasts. It is sent using RTP unreliable delivery, which does not require an acknowledgment packet.
The IPv4 address 224.0.0.10 is reserved for IPv4 hello packet multicasting, and the IPv6 address FF02::A is reserved for IPv6 hello packet multicasting. On most networks, EIGRP Hello packets are sent as multicast packets every five seconds. But in some networks, for example non-broadcast multi-access network (NBMA), X.25, Frame Relay, and Asynchronous Transfer Mode (ATM) interfaces with access links of T1 (1.544 Mb/s) or slower, the timing of Hello packets are 60 seconds due to slow speed.
Hello, packets are also used to maintain previously established adjacencies. An EIGRP-enabled router assumes that as long as it receives Hello packets from a neighbour, the neighbour and its routes remain workable. EIGRP uses a hold timer to determine the maximum time the router should wait to receive the next Hello before declaring that neighbour unreachable.
The default hold time is three times the Hello interval. The Hello interval for Ethernet and T1 lines with higher speed is 5 seconds, and for slower speed networks like non-Broadcast Multi-access networks, it is 60 seconds. If the hold time expires, EIGRP declares the route as down, and DUAL searches for a new path by sending out queries.
EIGRP Update Packets
EIGRP Update Packets are used to send routing information. The update Messages are sent to both Unicast and Multicast. Update packets are sent only when needed; the updates contain only the routing information needed and are sent only to those routers requiring it. EIGRP is a distance vector routing protocol like RIP, but it does not send periodic updates like RIP, and the route entries do not expire. The EIGRP sends updates only when changes occur in the network. The update occurs when a new network becomes available, an existing network becomes unavailable, or a change occurs in the routing metric for an existing network.
EIGRP also uses the terms partial and bounded for its updates. The term partial is used for routing updates, which include information about route changes, and the term bounded refers to the broadcast of partial updates that are sent only to those routers that the changes affect. The partial and bounded update minimizes the bandwidth required to send EIGRP updates.
EIGRP propagates routing information to EIGRP neighbours’ uses reliable delivery, which means the sending router must require an acknowledgement. An updating packet is sent to a new neighbor as unicast. If this update is related to any route change, it is sent as Multicast.
EIGRP Acknowledgment Packets
EIGRP Acknowledgment Packets are used as feedback to the Update, Query, or Reply packets as a feedback mechanism. It is not used for Hello Packets and Acknowledgment Packets. It is an empty hello message without any data, and they are sent as unreliable unicast.
EIGRP Query Packets
EIGRP Query Packets ask for and request any routing update. If a successor fails with Query messages, a backup route is asked. EIGRP queries use reliable delivery as a unicast or multicast message. When a router loses connectivity, it sends queries to all EIGRP neighbors, searching for any possible routes to the LAN. Due to reliable delivery, the receiving router must return an EIGRP acknowledgment. The acknowledgment informs the sender about the query reception.
EIGRP Reply Packets
EIGRP Reply Packets are used as a response to the Query Packets, including the alternate routes to the requested destination. They are sent with reliable delivery as unicast messages.
EIGRP uses a Reliable Transport Protocol (RTP) to deliver and receive EIGRP packets instead of TCP and UDP. Reliability is a key feature of EIGRP, and it is designed to enable fast update delivery and data reception tracking.EIGRP was designed as a network-layer independent routing protocol. This allows EIGRP to be used for protocols other than those from the TCP/IP protocol suite, such as IPX and AppleTalk. The figure conceptually shows how RTP operates.
Although RTP was developed for reliability, it includes reliable and unreliable delivery of EIGRP packets, similar to TCP and UDP. Reliable RTP requires an acknowledgment to be returned by the receiver to the sender, just like TCP. An unreliable RTP packet does not require an acknowledgment, just like UDP. The example is an EIGRP update packet, which requires an acknowledgment, but the Hello packet is sent over RTP without acknowledgment. RTP can send EIGRP packets as unicast or multicast. The Multicast EIGRP packets for IPv4 use the reserved IPv4 multicast address 224.0.0.10, and the Multicast EIGRP packets for IPv6 are sent to the reserved IPv6 multicast address FF02::A.
EIGRP uses a protocol-dependent module (PDM) for routing with several different protocols including IPv4 and IPv6. EIGRP also used PDMs to route Novell’s IPX and Apple Computer’s AppleTalk network-layer protocols. The protocol-dependent module (PDM) is also responsible for carrying information from the routing table to the topology table.
protocol-dependent module (PDM) is also responsible for network layer protocol-specific tasks. For example, the EIGRP module that is responsible for sending and receiving EIGRP packets encapsulated in IPv4 is also responsible for analyzing EIGRP packets and informing DUAL about the new information that is received. DUAL makes routing decisions on the received information and stored the result in the IPv4 routing table.
The responsibilities of the protocol-dependent module (PDM) are:-
Maintaining the neighbour table including topology tables of EIGRP enabled routers belonging to the protocol suite
Building a protocol-specific packet and translate the packet for Diffusing Update Algorithm (DUAL)
Interfacing Diffusing Update Algorithm (DUAL) to the protocol-specific routing table
Determine the metric and transient this information to Diffusing Update Algorithm (DUAL)
Implementing filtering and accessing lists
Carry out redistribution functions to and from other routing protocols
Redistributing routes that are learned by other routing protocols
Send and receive EIGRP packets that allow IP data.
When a router discovers a new neighbour, it saves the address and receiving interface as an entry in the neighbour table. One neighbour table is maintaining for each protocol-dependent module (PDM), such as a neighbour table for IPv4 and neighbour table for IPv6.
EIGRP protocol also maintains a topology table both for IPv4 and IPv6. The topology table consists of all destinations that are advertised by neighbouring routers and also has a separate topology table for each protocol-dependent module (PDM).
Enhanced Interior Gateway Routing Protocol (EIGRP) was introduced as a distance-vector routing protocol in 1992. It was originally designed to work as a Cisco proprietary protocol on Cisco devices only. In 2013, Enhanced Interior Gateway Routing Protocol became a multi-vendor routing protocol.
Enhanced Interior Gateway Routing Protocol is an advanced version of IGRP that lets routers exchange information more efficiently than then previous network protocols. As the name suggested, Enhanced Interior Gateway Routing Protocol is an enhancement of IGRP(Interior Gateway Routing Protocol). IGRP is obsolete since IOS 12.3 release. It was a classful, distance vector routing protocol.
Enhanced Interior Gateway Routing Protocol is a distance vector routing protocol with features of link-state routing protocols. It is suitable for many different topologies and media. In a well-designed network, Enhanced Interior Gateway Routing Protocol can scale to include multiple topologies and can provide extremely quick convergence times with minimal network traffic.
Enhanced Interior Gateway Routing Protocol is also known as the hybrid routing protocol in some older documentation. But this term is false because Enhanced Interior Gateway Routing Protocol is not a hybrid between distance vector and link-state routing protocols. Enhanced Interior Gateway Routing Protocol is only a distance vector routing protocol.
Routers using either Enhanced Interior Gateway Routing Protocol and IGRP can interoperate because the metric used with one protocol can be translated into the metrics of the other protocol. Enhanced Interior Gateway Routing Protocol can be used not only for Internet Protocol (IP) networks but also for AppleTalk and Novell NetWare networks.
Features of EIGRP
Enhanced Interior Gateway Routing Protocol is a routing protocol which includes features of both link-state and distance vector routing protocols. However, the key principles of Enhanced Interior Gateway Routing Protocol are still based on the key distance vector routing protocol, because it gets information from directly connected neighbours. EIGRP is an advanced version of the distance vector routing protocol because it includes features not found in any other distance vector routing protocols.
Diffusing Update Algorithm (DUAL)
The diffusing update algorithm (DUAL) is the algorithm used by EIGRP routing protocol to make sure that a given route is recalculated globally whenever it might cause a routing loop. It is guarantees loop-free and backup paths throughout the routing domain. EIGRP store all available backup routes using DUAL and then adapt the route when needed.
Establishing Neighbor Adjacencies
EIGRP establishes Neighbor adjacencies with directly connected routers that are also enabled EIGRP to track the status of these directly connected routers.
Reliable Transport Protocol
EIGRP depends upon proprietary protocol Reliable Transport Protocol (RTP) to provide delivery of EIGRP packets to neighbours. RTP and the tracking of neighbour adjacencies set the stage for DUAL.
Partial and Bounded Updates
Unlike RIP, EIGRP not sending any periodic updates and route entries because it does not age out. It uses “partial and bounded” terms for updates. The term partial use for update only includes information about the route changes, for example, a new link or a link becoming unavailable. The term bounded means propagation of partial updates that are sent only to those routers that the changes affect. Partial and bonded update minimizes the bandwidth for EIGRP updates.
Equal and Unequal Cost Load Balancing
EIGRP supports both equal and unequal cost load balancing. Equal and Unequal cost load balancing allows better traffic flow in the network. It is possible due to changing the value of variance. The default, the variance is 1, therefore, supports equal-cost load balancing but if we want to use unequal cost load balancing then we can change the value of variance according to the amount of traffic we want to split across different paths.
The majority of the command for multiarea OSPF verification is the same as the command we already used in the verification of single area OSPF. The command, “show ip ospf neighbor”, “show ip ospf” and show ip ospf interface are the same command. The following command specifically includes the information of multiarea OSPF.
show ip protocols
show ip ospf interface brief
show ip route ospf
show ip ospf database
The command can also be used for OSPFv3, simply substitute ip with ipv6. In this article, we will use the same topology of OSPF, which we have used in the previous article. Figure 1 illustrates the OSPF topology.
Verify General Multiarea OSPF Settings
We can verify the general multiarea OSPF setting using the “show ip protocols” command. The output of the command displays the configured routing protocols on a router including router ID, several areas in the router, and networks included within the routing protocol configuration. Figure 2 illustrates the OSPF settings of R2. Notice that the command shows that there are two areas. The Routing for Networks segment identifies the networks and their particular areas.
We can also use the “show ip ospf interface brief” command to show brief OSPF-related information of OSPF-enabled interfaces. This command provides helpful information, such as the OSPF process ID, the area that the interfaces are in, and the cost of the interface.
Verify the OSPF Routes
The “show ip route” is the most common command to verify a multiarea OSPF configuration. We can add the “ospf” parameter to display only OSPF-related information.
Figure 3 displays the routing table of R2. See the O IAentries in the routing table, which illustrates networks learned from other areas. O specificallyrepresents OSPF routes, andIArepresents interarea routes, meaning that the route is originated from another area.
To examine the contents of the LSDB we can use “show ip ospf database” command. Several options are available with this command. Figure 4 illustrates the content of the LSDB of R2.
Notice R2 has database entries for area 0 and area 10, so ABRs must maintain a separate LSDB for each area to which they belong. In the output, Router Link States identifies the number of routes for each area. The Summary Net Link States for identifies networks learned from other areas and which neighbour advertised the network for both areas.
Verify Multiarea OSPFv3
We can verify the OSPFv3 with similar verification commands with only substitute ip with ipv6. We can use all the commands discussed for OSPFv2 verification.
Interarea route summarization in the OSPF must be manually configured on ABRs because OSPF does not support auto-summarization. We can perform internal routes summarization only on ABRs. When route summarization is configured on ABRs, a single type 3 LSA (summary LSA) describing the summary route are forwarded to the backbone area.
Multiple routes inside the area are summarized by the single LSA. A summarized route is generated if at least one subnet within the area comes down in the summary-address range. The summarized route metric is the same as the lowest cost of all subnets in the summary-address range. The ABR can only sum up routes that are within the areas connected to the ABR.
Figure 1 illustrates a multiarea OSPF topology. Figure 2 illustrates the routing tables of R3 before the route summarization is configured on R2.
Calculating the Summary Route
Summarizing different adjacent subnetworks into a single address and mask can calculate in three steps. The steps are the following:-
List all networks in binary format. For example, the two networks of area 10, 192.168.0.0/24 and 192.168.1.0/24 are listed in binary format in figure 3 (1).
Count the matching bits from left to right. The matching bits determine the mask for the summary route. As masked, the first 23 far left bits match. This results in the prefix/23 or subnet mask255.255.254.0. Figure 3(2) illustrates the counting of matching bits.
Copy the matching bits from left to right and then add zero bits to remaining positions to determine the summarized network address as shown in figure 3(3). In this example, the matching bits with zeros at the result is a network address of 192.168.0.0/23. This summary-address summarizes two networks: 192.168.0.024, and 192.168.1.0/24. In the example the two networks matched out of three networks.
Configuring Interarea Route Summarization
We can manually configure the route summarization on ABRs. The R2 is ABR for area 10 and area 0. In the example, R1 is configured to summarize the internal area 10 routes.
To configure route summarization manually on ABR, use the area <area-id> range <address> <mask> command on router configuration mode. This command instructs the ABR to summarize routes for a specific area before injecting them into a different area, via the backbone as type 3 summary LSAs. In our example the configuration on R2 is:
R2(config)#router ospf 10
R2(config-router)# area 10 range 192.168.0.0 255.255.254.0
R2(config-router)# do wr
In OSPFv3, the command is the same except for the IPv6 network address. The command syntax for OSPFv3 is area area-id range prefix/prefix-length.
Figure 4 displays the updated R3 routing table. You can examine that there is only one interarea route entry now available in the routing table for both routes. Though this example only reduced the routing table by one entry, summarization could be implemented, to sum up a lot of networks to reduce the size of routing tables extremely.
Route summarization helps convert multiple routes into a single route, which reduces routing tables. After converting a large routing table into a small routing table, it is propagated into the backbone area. We have already discussed the type of LSAs in the previous article.
The LSA types 1 and 2 are generated inside each area, translated into type 3 LSAs, and sent to other areas. Suppose area 1 had 20 networks to advertise, and 20 types of 3 LSAs would be forwarded into the backbone. In summary, the ABR combines the 20 networks into one of two advertisements.
OSPF doesn’t support automatic summarization, and we cannot summarize routes on every router in an EIGRP network. OSFP can summarize routes only on ABRs and ASBRs routers. Route summarization helps minimize OSPF traffic and reduce route computation.
In Figure 1, R2 combines all network advertisements into one summary LSA. Instead of forwarding individual LSAs for each route in Area 10, R2 forwards a summary LSA to Area 0. In this case, the R3 forwarded the summary LSA to all respected routers in area 20 to R4.
Route summarization increases the network’s stability because it reduces gratuitous LSA flooding. It also reduces the extra overhead on the bandwidth, CPU, memory resources, and routing table process. With route summarization, every specific-link LSA is not propagated further into the OSPF backbone, causing unnecessary network traffic and router overhead.
Figure 2 illustrates that if the network link on R1 fails. R1 sends an LSA to R2 (ABR). However, R2 (ABR) does not propagate the update to the backbone because it configures a summary route. Specific-link LSA flooding outside the area does not occur.
Interarea and External Route-Summarization
As I already discussed earlier in this lesson, summarization in OSPF can only be configured on ABRs or ASBRs. The ABR and ASBR routers advertise only a summary route. ABR routers summarize type 3 LSAs, and ASBR routers summarize type 5 LSAs. By default, the type 3 and type 5 LSAs do not contain summarized routes because, by default, summary LSAs are not summarized. Summarization can be configured as follows:
Inter-area route summarization
OSPF interarea route summarization enables an ABR to summarize neighboring networks into a single network and advertise the network to other areas. The summarization does not apply to external routes joining the OSPF via redistribution. For better route summarization, it is important to plan network addresses closely so that these addresses can be summarized into the fewest summary addresses.
External route summarization
This is the OSPF route summarization of the injected route via redistribution. It is important to ensure the continuity of the external address ranges being summarized. The ASBRs can summarize the external routes. We can configure the external route summarization on ASBRs using the summary-address address mask router configuration mode command.
OSPF can be implemented as single-area OSPF or multiarea OSPF. The basic steps for a single area or multiarea OSPF implementation are the following:-
Gather the network requirements and parameters –The first step for implementing multiarea OSPF is the collecting network requirement and important parameters. The requirement includes the number of hosts and required network devices, the IP addressing plan, the routing domain size including, the size of the routing tables. It is also important to consider the risk of topology changes, and other network characteristics.
OSPF parameters –The first step is the base of the second step, the network administrator must find out if single-area or multiarea OSPF is the ideal implementation. If it is multiarea OSPF, there are several considerations to determine the OSPF parameters including IP addressing plan, OSPF areas, and Network topology.
Configure multiarea OSPF – Configure the multiarea OSPF implementation based on the parameters.
Verifies the Multiarea OSPF –Verify the multiarea OSPF implementation based on the parameters.
Configuring Multiarea OSPFv2
Figure 1 displays the reference multiarea OSPF topology. In this example:
R1 is an internal router for area 10 because all of its interfaces are in area 1.
R2 is an ABR because it has interfaces in area 10 and also an interface in area 0.
R3 is also an ABR because it has interfaces in area 20 and an interface in area 0.
R4 is an internal router for area 20 because all of its interfaces are in area 20.
There are no unique commands for multiarea OSPF configuration. A router simply becomes an Area Border Router (ABR) when it has two “network” statements in different areas.
R1 is the member of area 10, and we have assigned the router ID 1.1.1.1 to R1. The configuration on R1 is the following:
R1(config)#router ospf 10
R1(config-router)router-id 1.1.1.1
R1(config-router)network 192.168.0.0 0.0.0.255 area 10
R1(config-router)network 192.168.2.0 0.0.0.255 area 10
R1(config-router)network 10.10.10.8 0.0.0.3 area 10
R1(config-router) do wr
Both network 192.168.1.0/24 and 10.10.10.8/30 are configured in the same area because this is an internal router for area 10. The autonomous number for the OSPF is 10. Now let’s configure the R2, which is the ABR for Area 10 and Area 0 (backbone area). The configuration of R2 is the following:
R2(config)#router ospf 10
R2(config-router)router-id 2.1.1.1
R2(config-router)network 10.10.10.0 0.0.0.3 area 0
R2(config-router)network 10.10.10.8 0.0.0.3 area 10
R2(config-router) do wr
The above configuration enables OSPF in the two different areas. The GigabitEthernet 0/2 interface is configured as part of OSPF area 10 and GigabitEthernet 0/0 as part of OSPF area 0. Because R2 has interfaces connected to two different areas, so it is an ABR between area 10 and area 0. Now I am going to configure R3, which is also an ABR. The configuration is:
R3(config)#router ospf 10
R3(config-router)router-id 3.1.1.1
R3(config-router)network 10.10.10.0 0.0.0.3 area 0
R3(config-router)network 172.16.0.0 0.0.0.3 area 20
R3(config-router) do wr
You can see the configuration of R3, GigbitEthernet0/0 is the part of area 0 and GigabitEthernet 0/2 is the member of area 20. The R2 is a member of both area 0 and area 20, therefore it is an ABR for both areas. R4 is the internal router of area 20, the router-id of the R4 is 4.1.1.1. the configuration is simple, just like R1:
R4(config)#router ospf 10
R4(config-router)router-id 4.1.1.1
R4(config-router)network 192.168.2.0 0.0.0.255 area 20
R4(config-router)network 192.168.3.0 0.0.0.255 area 20
R4(config-router)network 172.16.0.0 0.0.0.3 area 20
R4(config-router) do wr
Notice that we have used the wildcard mask in the network statement. It is the inverse of subnet masks.
Configuring Multiarea OSPFv3
The OSPFv3 is not more different than the OSPFv2, we are using the same topology for multiarea OSPFv3 configuration. The figure-2 illustrates the topology with the IP addressing scheme. A router simply becomes an ABR when it has two interfaces in different areas just like OSPFv2 and there are no special commands required.
Now lest come first of all configure the R1, which is the member of area 10 because all interfaces are connected to area 10. We should assign the same router IDs as OSPFv2.
