Question 1. Does Eigrp Support Secondary Addresses?
EIGRP does support secondary address. Since EIGRP sources data packets from the primary addresses, Cisco recommends that we configure all routers on a particular subnet with primary addresses that belong to the similar subnet. Routers don’t form EIGRP neighbors over secondary network. For which, if all of the primary IP addresses of routers don’t agree, problems arises with neighbor adjacency.
Question 2. What Does The Eigrp Stuck In Active Message Mean?
When EIGRP returns a stuck in active message, this means that it has not received a reply from a query. It sends a query when a route is lost and another feasible route does not exist inside the topology table. The SIA is caused by 2 sequential events:
- The reported route by the SIA has gone away.
- An EIGRP neighbor (or neighbors) have not replied to the query for that route.
When the SIA occurs, the router clears the neighbor that did not reply for the query. When it happens, determine which neighbor has been cleared. We should Keep in mind that this router can be many hops away.
Question 3. What Does The Neighbor Statement In The Eigrp Configuration Section Do?
The neighbor command is used in EIGRP in order to define a neighboring router with which to exchange information of routing. Because of the current behavior of the command, EIGRP exchanges information of routing with the neighbors in the form of unicast packets whenever the neighbor command is configured for interface. EIGRP holds on processing all multicast packets that come inbound on that interface. EIGRP also mutes sending multicast packets on that interface.
The ideal behavior of this command is for EIGRP to start sending EIGRP packets as unicast packets to the specified neighbor, but don’t stop sending and receiving multicast packets on that interface. As the command does not behave as intended, the neighbor command must used carefully.
Question 4. Why Does The Eigrp Passive-interface Command Remove All Neighbors For An Interface?
The passive interface command disables the transmission and receipt of EIGRP hello packets on interface. Not similarly IGRP or RIP, EIGRP sends hello packets in order to form and sustain neighbor adjacency. Not having a neighbor adjacency, EIGRP doesn’t exchange routes with a neighbor. For which the passive interface command prevents the exchange of routes on the interface. Although EIGRP does not receive or send routing updates on an interface configured with the passive interface command, it includes the address of the interface in routing updates sent out of other non-passive interfaces.
Question 5. What Is The Difference In Metric Calculation Between Eigrp And Igrp?
The EIGRP metric is obtained when you multiply the IGRP metric of 256. The IGRP uses only 24 bits in its update packet for the metric field, but 32 bit is used by EIGRP in its update packet for the metric field. E.g the IGRP metric to a destination network is 8586, but the EIGRP metric will be 8586 x 256 = 2,198,016. Integer division is used when we divide 10^7 by lowest BW, so the calculation involves division of integer, which may leads to a variation from manual calculation.
Question 6. How Does Eigrp Behave Over A Gre Tunnel Compared To A Directly Connected Network?
EIGRP will use the same administrative distance and metric calculation to the GRE tunnel. The calculation of cost is based on delay and bandwidth. The bandwidth and delay of the GRE tunnel must be taken from the tunnel interface configured on the router. The tunnel will be treated like a directly connected network.
If there are 2 paths to reach a n/w either through a tunnel interface or VLAN interface, EIGRP prefers the Virtual-Access Interface VLAN interface because the VLAN interface has greater bandwidth than the tunnel interface. To influence the routing through the tunnel interface, It increases the bandwidth parameter of the tunnel interface, or increases the delay parameter of the VLAN interface.
Question 7. Can An Ospf Default Be Originated Into The System Based On External Information On A Router That Does Not Itself Have A Default?
OSPF generates a default only if it is configured using the command default-information originate and if there is a default network in the box from a different process. In OSPF the default route is 0.0.0.0. If we want an OSPF enabled router to generate a default route even if it does not have a default route, use the command default-information originate always.
Question 8. How Does Cisco Accommodate Ospf Routing On Partial-mesh Frame Relay Networks?
We can configure OSPF to understand whether it should attempt to use multicast facilities on a multi-access interface. If multicast is available, OSPF usually uses it for its normal multicasts
Cisco IOS Software Release 10.0 includes a feature called sub-interfaces. We can use sub interfaces with Frame Relay to tie together a set of virtual circuits to form a virtual interface, it acts as a single IP subnet. All systems inside the sub-net should be fully meshed.
Question 9. Which Address-wild-mask Pair Should I Use For Assigning An Unnumbered Interface To An Area?
When we configured an unnumbered interface, it references an another interface on the router. When we enable OSPF on the unnumbered interface, we should use the address-wild-mask pair of interfaces to which the unnumbered interface is pointing.
Question 10. What Is The Link-state Re-transmit Interval, And What Is The Command To Set It?
OSPF sends acknowledgment of every newly received link-state advertisement. It does this by sending LSA packets. After that LSAs are re-transmitted until they are acknowledged. The link state when re-transmit the interval it defines the time between the re-transmissions. We can use the command IP OSPF re-transmit interval to set the re-transmit interval. 5 seconds is the default value.
Question 11. Does Eigrp Require An Ip Default-network Command To Propagate A Default Route?
Although EIGRP can propagate a default route using the default network method, it is not required. EIGRP redistributes default routes directly.
Question 12. Should I Always Use The Eigrp Log-neighbor-changes Command When I Configure Eigrp?
Yes, this command makes it easy to determine why an EIGRP neighbor was reset. This reduces troubleshooting time.
Question 13. What Debugging Capabilities Does Eigrp Have?
There are protocol-independent and -dependent debug commands. There is also a suite of show commands that display neighbor table status, topology table status, and EIGRP traffic statistics. Some of these commands are:
- show ip eigrp neighbors
- show ip eigrp interfaces
- show ip eigrp topology
- show ip eigrp traffic
Question 14. What Does The Word Serno Mean On The End Of An Eigrp Topology Entry When You Issue The Show Ip Eigrp Topology Command?
show ip eigrp topology
P 172.22.71.208/29, 2 successors, FD is 46163456
via 172.30.1.42 (46163456/45651456), Serial0.2, serno 7539273
via 172.30.2.49 (46163456/45651456), Serial2.6, serno 7539266
Serno stands for serial number. When DRDBs are threaded to be sent, they are assigned a serial number. If you display the topology table at the time an entry is threaded, it shows you the serial number associated with the DRDB.
Threading is the technique used inside the router to queue items up for transmission to neighbors. The updates are not created until it is time for them to go out the interface. Before that, a linked list of pointers to items to send is created (for example, the thread).
These sernos are local to the router and are not passed with the routing update.
Question 15. What Percent Of Bandwidth And Processor Resources Does Eigrp Use?
EIGRP version 1 introduced a feature that prevents any single EIGRP process from using more than fifty percent of the configured bandwidth on any link during periods of network convergence. Each AS or protocol (for instance, IP, IPX, or Appletalk) serviced by EIGRP is a separate process. You can use the ip bandwidth-percent eigrp interface configuration command in order to properly configure the bandwidth percentage on each WAN interface. Refer to the EIGRP White Paper for more information on how this feature works.
In addition, the implementation of partial and incremental updates means that EIGRP sends routing information only when a topology change occurs. This feature significantly reduces bandwidth use.
The feasible successor feature of EIGRP reduces the amount of processor resources used by an autonomous system (AS). It requires only the routers affected by a topology change to perform route re-computation. The route re-computation only occurs for routes that were affected, which reduces search time in complex data structures.
Question 16. Does Eigrp Support Aggregation And Variable Length Subnet Masks?
Yes, EIGRP supports aggregation and variable length subnet masks (VLSM). Unlike Open Shortest Path First (OSPF), EIGRP allows summarization and aggregation at any point in the network. EIGRP supports aggregation to any bit. This allows properly designed EIGRP networks to scale exceptionally well without the use of areas. EIGRP also supports automatic summarization of network addresses at major network borders.
Question 17. Does Eigrp Support Areas?
No, a single EIGRP process is analogous to an area of a link-state protocol. However, within the process, information can be filtered and aggregated at any interface boundary. In order to bound the propagation of routing information, you can use summarization to create a hierarchy.
Question 18. Can I Configure More Than One Eigrp Autonomous System On The Same Router?
Yes, you can configure more than one EIGRP autonomous system on the same router. This is typically done at a redistribution point where two EIGRP autonomous systems are interconnected. Individual router interfaces should only be included within a single EIGRP autonomous system.
Cisco does not recommend running multiple EIGRP autonomous systems on the same set of interfaces on the router. If multiple EIGRP autonomous systems are used with multiple points of mutual redistribution, it can cause discrepancies in the EIGRP topology table if correct filtering is not performed at the redistribution points. If possible, Cisco recommends you configure only one EIGRP autonomous system in any single autonomous system. You can also use another protocol, such as Border Gateway Protocol (BGP), in order to connect the two EIGRP autonomous systems.
Question 19. If There Are Two Eigrp Processes That Run And Two Equal Paths Are Learned, One By Each Eigrp Process, Do Both Routes Get Installed?
No, only one route is installed. The router installs the route that was learned through the EIGRP process with the lower Autonomous System (AS) number. In Cisco IOS Software Releases earlier than 12.2(7)T, the router installed the path with the latest timestamp received from either of the EIGRP processes. The change in behavior is tracked by Cisco bug ID CSCdm47037.
Question 20. Why Are Routes Received From One Neighbor On A Point-to-multipoint Interface That Runs Eigrp Not Propagated To Another Neighbor On The Same Point-to-multipoint Interface?
The split horizon rule prohibits a router from advertising a route through an interface that the router itself uses to reach the destination. In order to disable the split horizon behavior, use the no ip split-horizon eigrp as-number interface command. Some important points to remember about EIGRP split horizon are:
- Split horizon behavior is turned on by default.
- When you change the EIGRP split horizon setting on an interface, it resets all adjacencies with EIGRP neighbors reachable over that interface.
- Split horizon should only be disabled on a hub site in a hub-and-spoke network.
- Disabling split horizon on the spokes radically increases EIGRP memory consumption on the hub router, as well as the amount of traffic generated on the spoke routers.
- The EIGRP split horizon behavior is not controlled or influenced by the ip split-horizon command.
Question 21. When I Configure Eigrp, How Can I Configure A Network Statement With A Mask?
The optional network-mask argument was first added to the network statement in Cisco IOS Software Release 12.0(4)T. The mask argument can be configured in any format (such as in a network mask or in wild card bits). For example, you can use network 10.10.10.0 255.255.255.252 or network 10.10.10.0 0.0.0.3.
Question 22. I Have Two Routes: 172.16.1.0/24 And 172.16.1.0/28. How Can I Deny 172.16.1.0/28 While I Allow 172.16.1.0/24 In Eigrp?
In order to do this you need to use a prefix-list, as shown here:
router eigrp 100
distribute-list prefix test in
no eigrp log-neighbor-changes
ip prefix-list test seq 5 permit 172.16.1.0/24
This allows only the 172.16.1.0/24 prefix and therefore denies 172.16.1.0/28.
Note: The use of ACL and distribute-list under EIGRP does not work in this case. This is because ACLs do not check the mask, they just check the network portion. Since the network portion is the same, when you allow 172.16.1.0/24, you also allow 172.16.1.0/28.
Question 23. I Have A Router That Runs Cisco Express Forwarding (cef) And Eigrp. Who Does Load-balancing When There Are Multiple Links To A Destination?
The way in which CEF works is that CEF does the switching of the packet based on the routing table which is populated by the routing protocols such as EIGRP. In short, CEF does the load-balancing once the routing protocol table is calculated.
Question 24. How Can I Use Only One Path When A Router Has Two Equal Cost Paths?
Configure the bandwidth value on the interfaces to default, and increase the delay on the backup interface so that the router does not see two equal cost paths.
Question 25. What Is The Eigrp Stub Routing Feature?
The Stub routing feature is used to conserve bandwidth by summarizing and filtering routes. Only specified routes are propagated from the remote (Stub) router to the distribution router because of the Stub routing feature. For more information about the Stub routing feature, refer to EIGRP Stub Routing. The EIGRP stub feature can be configured on the switch with the eigrp stub [receive-only] [leak-map name] [connected] [static] [summary] [redistributed] command. This feature can be removed with the no eigrp stub command. When you remove the eigrp stub command from the switch, the switch that runs the IP Base image throws this error:
EIGRP is restricted to stub configurations only
Question 26. How Can I Send A Default Route To The Stub Router From The Hub?
Do this under the outbound interface on the hub router with the ip summary-address eigrp X 0.0.0.0 0.0.0.0 command. This command suppresses all the more specific routes and only sends the summary route. In the case of the 0.0.0.0 0.0.0.0, it means it suppresses everything, and the only route that is in the outbound update is 0.0.0.0/0. One drawback to this method is that EIGRP installs a 0.0.0.0/0 route to Null0 is the local routing table with an admin distance of 5.
Question 27. What Are Different Route Types In Eigrp?
There are three different types of routes in EIGRP:
- Internal Route—Routes that are originated within the Autonomous System (AS).
- Summary Route—Routes that are summarized in the router (for example, internal paths that have been summarized).
- External Route—Routes that are redistributed to EIGRP.
Question 28. How Do You Redistribute An Ipv6 Default Route In Eigrp?
For redistributing an IPv6 default route in EIGRP, a sample configuration is shown here:
ipv6 prefix-list DEFAULT-ONLY-V6 seq 10 permit ::/0
route-map DEFAULT_2EIGRP-V6 permit 10
match ipv6 address prefix-list DEFAULT-ONLY-V6
router eigrp Starz_EIGRP
address-family ipv6 unicast
redistribute static route-map DEFAULT_2EIGRP-V6
Question 29. What Is An Offset-list, And How Is It Useful?
The offset-list is an feature used to modify the composite metrics in EIGRP. The value configured in the offset-list command is added to the delay value calculated by the router for the route matched by an access-list. An offset-list is the preferred method to influence a particular path that is advertised and/or chosen.
Question 30. How Can I Tag External Routes In Eigrp?
You can tag routes that EIGRP has learned from another routing protocol using a 32 bit tag value. Starting with ddts CSCdw22585, internal routes can also be tagged. However, the tag value cannot exceed 255 due to packet limitations for internal routes.
Question 31. What Are The Primary Functions Of The Pdm?
EIGRP supports 3 protocol suites: IP, IPv6, and IPX. Each of them has its own PDM. These are the primary functions of PDM:
- Maintaining the neighbor and topology tables of EIGRP routers that belong to that protocol suite
- Building and translating protocol specific packets for DUAL
- Interfacing DUAL to the protocol specific routing table
- Computing the metric and passing this information to DUAL; DUAL handles only the picking of the feasible successors (FSs)
- Implement filtering and access lists.
- Perform redistribution functions to/from other routing protocols.
Question 32. What Are The Various Load-balancing Options Available In Eigrp?
The offset-list can be used to modify the metrics of routes that EIGRP learns through a particular interface, or PBR can be used.
Question 33. What Does The %dual-5-nbrchange: Ip-eigrp(0) 100: Neighbor 10.254.0.3 (tunnel0) Is Down: Holding Time Expired Error Message Mean?
This message indicates that the router has not heard any EIGRP packets from the neighbor within the hold-time limit. Because this is a packet-loss issue, check for a Layer 2 problem.
Question 34. From The 16:29:14.262 Poison Squashed: 10.x.x.x/24 Reverse Message, What Does Poison Squashed Mean?
The router threads a topology table entry as a poison in reply to an update received (the router sets up for poison reverse). While the router is building the packet that contains the poison reverse, the router realizes that it does not need to send it. For example, if the router receives a query for the route from the neighbor, it is currently threaded to poison. Thus, it sends the poison squashed message.
Question 35. Is It Normal That Eigrp Takes Over 30 Seconds To Converge?
EIGRP taking longer to converge under heavy CPU usage is a normal behavior. EIGRP convergence is faster when you lower the hold time. The lowest values for hello and hold time are 1 second and 3 seconds respectively. For example:
Router(Config)# interface Fa0/0
!— (Under an interface directly connected to EIGRP peers.)
Router(Config-if)#ip hello-interval eigrp 1
Router(Config-if)#ip hold-time eigrp 3
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