Skip to main content

EIGRP Interview Questions - Part 1

Something about EIGRP..

  • Enhanced Interior Gateway Routing protocol
  • It is an enhanced distance vector routing protocol based on Diffused Update Algorithm (DUAL) to calculate shortest path.
  • Classless routing protocol
  • Support for VLSM, route-summarization
  • Supports unequal cost load balancing, incremental updates

What parameters should match for EIGRP formation?

  • Primary address should be used for forming EIGRP neighborship
  • Autonomous System number
  • K-values
  • In case a static neighborship is configured, it must be done on both the sides

What is the multicast address of EIGRP?

What are the packet types used by EIGRP?

  • Hello
  • Acknowledgement
  • Update
  • Query
  • Reply

What are the basic components of EIGRP?

  • Diffused Update Algorithm - It is used for selecting the lowest cost loop-free path for a given destination
  • Reliable Transport Protocol - RTP is used in EIGRP for detecting packet loss and to ensure ordered delivery of packets
  • Protocol Independent Module - Support for IP, IPX, AppleTalk, IPv6
  • Neighbor Discovery (ND) and Recovery Module - Hello packets are used for ND and Recovery

What is RTP?

  • RTP - Reliable Transport Protocol
  • EIGRP uses RTP to deliver packets between neighbors in an ordered and reliable manner. In case of packet loss, RTP ensures re-transmission of the packet to ensure information transfer consistency.

Which EIGRP packets are RTP enabled?

  • Update Packet
  • Query Packet
  • Reply Packet

Do duplicate Router-IDs in EIGRP prevent neighborship? If yes, under what conditions?

Duplicate Router-IDs do not prevent routers from forming neighborships. The only instance when the value of EIGRP router ID is considered is during route redistribution in EIGRP. Manually EIGRP router id can be configured using the command:
eigrp router id x.x.x.x

What is unequal cost load balancing in EIGRP?

Like other protocols, EIGRP supports load-balancing the routers with equal metrics. However, in addition, EIGRP also supports unequal cost load balancing thereby utilizing 'slightly' less preferred links. (The degree of "slightly" depends on the "variance" command). Variance (an integer between 1 and 128, both inclusive) is used for Unequal cost load balancing.
The router multiplies the variance by the successor route's FD (metric of the best route to reach that subnet). Any feasible successor route whose metric is less than or equal to the product of the variance by the successor's FD is considered to be "worthy" of being placed in the routing table (along with the best route).

router eigrp 100
variance 2

What is a split horizon?

Loop prevention concept used in both EIGRP and RIP, split horizon ensures a route learned on one interface is not advertised back out of that same interface.

What is null zero route?

Another loop prevention mechanism stored in the routing table (in case of summarization) to terminate or flush (black hole) unwanted packets that do not match any other route (don't consider default route)

What is Active state and Passive state?

In EIGRP, "Active" is bad!
In a normal stable network, all EIGRP routes should exist in Passive state with the router having a successor for every EIGRP route. However, in case of a route being unreachable due to link failure or any other reason, it transitions to "Active" state.
Routes where the successor route fails and no feasible successor route exists move to Active state forcing the EIGRP to send out query packets and to reconverge.


Popular posts from this blog

Checkpoint - Exporting Objects in CSV format

Be it a Network Operations Manager, Security Architect or a Security Auditor, the people up the hierarchy always harangue the Security Engineers to compile the list of firewall objects or rules or policies or the traffic statistics and so on.. This can turn out to be quite hectic especially if there are no built in features to systematically provide the output in a "layman-readable" format. Come, Checkpoint's "Object Explorer..."  which not only provides the output in the "layman-readable" format, but also provides in-built filtering mechanisms, thereby ensuring that the Security Engineer doesn't have to rely on Google for building his scarce Microsoft Excel data filtering skills. The following screenshots will show how easy it is, with Checkpoint R80.10 to generate the firewall configuration inventory. On the SmartConsole Unified Portal, navigate to Menu >> Open Object Explorer... Select the Categories you wish to see in your output: Click o

Tejas Jain - GCP Constraints & Random Facts

1.  Google Cloud Interconnect Security Cloud Interconnect does not encrypt the connection between your on-premises network and Google's network. Cloud VPN cannot be used with Dedicated Interconnect For additional security, use application-level encryption or your own VPN 2. While using Cloud CDN, the default time-to-live (TTL) for content caching is 3600 seconds = 60 mins 3. Cloud NAT sends only the translation logs and error logs to Cloud Logging service. 4. GCP Dedicated Interconnect - On Premises network device requirements:     10-Gbps circuits, single mode fiber or 100-Gbps circuits, single mode fiber     IPv4 link local addressing     LACP, even if you are using single circuit     EBGP-4 with multi-hop     802.1Q VLANs 5. While using Cloud VPN, the recommended MTU to be configured on the peer VPN  gateway = 1460 bytes 6. Each instance must have at least one network interface. The maximum number of network instances per instance is 8, depending on the instance's machine

MITRE ATT&CK - Kerberos Vulnerabilities and Security

From the previous post, the summary of Kerberos authentication process is as below: For the initial authentication, the user’s client machine sends a request to the KDC  Authentication Service (AS) . The request includes details like the user’s username, and the date and time. All information except the username is encrypted using the hash of the user’s password. The KDC AS uses the username to look up its copy of the user’s password hash and uses it to decrypt the rest of the request. If the decryption is successful, that means the client used the correct password hash and the user has successfully authenticated. Once the user is authenticated, the KDC AS sends the user’s client a  ticket granting ticket   (TGT) . The TGT includes a unique session key and a timestamp that specifies how long that session is valid (normally 8 or 10 hours). Importantly, before sending the TGT, the KDC encrypts it using the password hash for a special account, the  KRBTGT account.  That password hash is s