Reported Distance & Feasible Distance

Feasible Distance: The total path between a router and the next closest router + Sum of all cost.

Reported Distance is the distance (metric) towards a destination as advertised by an upstream neighbour.
Reported distance is the distance reported in the queries, the replies and the updates.
Reported Distance: The total path between a router and the next closest router

OSPF

• Dynamic Routing Protocol for IP Networks
• Link State Protocol
• Group of Interior Gateway Protocol
• Operates withing a single Autonomous System
• Conifgured with Wildcard Mask
• Process ID - Uses Dijkstra’s algorithm
• Network Types – point to point, and routers that are directly connected

RIPv2

• Distance Vector Protocol
• Supports classless routing protocol
• Authentication option available
• Uses multicast addresses in sending updates
• Sends subnet information in RIP header
• Supports authentication

Router Memory

RAM: The RAM stores the OS, running config file, IP routing table, Arp cache and packet buffer

ROM: Permanent storage – Scaled down version of IOS, Basic software

Flash Memory: Permanent storage of for OS, Cisco IOS

NVRAM: Used as permanent storage for startup config files.

3 Configuration Registry

0 * 2142 For password recovery.
0 * 2102 The default. Router looks for config file in NVRAM and for a valid IOS image.
0 * 2100 Router boots into ROM monitor mode.

4 types of Timers

Update timer: 90 seconds

Invalid timer: 270 seconds

Hold down timer: 180 seconds

Flush timer: 630 seconds

Feasible Distance & Successors

Feasible Distance: The total path between a router and the next closest router + Cost.
Successors is a neighboring router that has a least cost path to a destination.

5 types of LSP packets

1)hello (checks if neighbour is up)
2)Database Description (synchronize databases in the beginning)
3)Link State Request (Request Specific LSA)
4) Link State Update (LSA flooded)
5) Link State Acknowledgement (flooded LSA’s are exploited acknowledged reliable flooding)

Difference Between Reported Distance and Administrative Distance

Reported Distance is the distance (metric) towards a destination as advertised by an upstream neighbour. Reported distance is the distance reported in the queries, the replies and the updates.
Reported Distance → The total path between a router and the next closest router.

Admistrative Distance is a measure of the trustworthiness of the source of the routing information.

Roles Of A Router

DR – reduces the number of adjacencies required on a multi-access network, which in turn reduces the amongst of routing traffic and the size of the topological database.

BDR - A router that becomes the designated router if the designated router fails

DROther - The router with the highest priority becomes the designated router (DR). If the priorities are the same, then the router with the highest router ID becomes the DR. By default, priorities are set to 1. A router with a priority of 0 never becomes a DR or a backup designated router (BDR); it is always a DROTHER, meaning a router that is neither the DR or the BDR.

How is a router id assigned ?

The router-id is chosen based on the highest IP address present on the router. If a loopback interface is present, the loopback IP will be used. If more than one loopback interface is present, the highest loopback IP will be used.

What are Hello Packets used for ?

• To discover OSPF neighbours and establish neighbour adjacency.
• Advertise parameters on which two routers must agree to become neighbours.
• Elect DR and BDR on multi access networks

Difference between Metric and Administrative Distance ?

Administrative Distance determines what priority will be given to each Routing protocol redirected through a router.
Metric is a Calculation done by the Routing protocol to determine the best path through a network.

EIGRP Tables

Neighbours Table - shows all neighbours adjacent to router

Topology Table - shows all route entries learned by the router

Routing Table – shows the best route to get to other points on the network

OSPF Tables

Neighbours Table - shows all neighbours adjacent to router.

Topology Table - shows all route entries learned by the router.

Routing Table – shows the best route to get to other points on the network.

What makes EIGRP unique ?

• Uses Diffused Update algorithm (DUAL) for all route computation
• Uses multicast and unicast for sending packets
• Administrative Distance for Internal router is 90
• Administrative Distance for External router is 170

Link State vs Distance Vector Protocol

Link state transmits data based on the speed of the WAN link.
Distance Vector transmits data based on the cost of the link (usually based on hop count).

If start up config is not found in the RAM memory what does it do ?

It searches the NVRAM where all the it is permanently stored!

DR Selection Process

OSPF chooses the designated router (DR) on a LAN as the device that has the highest IP address. If routers are added or removed from the LAN, or if a router gains an interface with a higher address than that of the existing DR, the DR likely will change if the DR or backup designated router (BDR) fails.

Routing Table

A routing table contains the information necessary to forward a packet along the best path toward its destination. Each packet contains information about its origin and destination. When a packet is received, a network device examines the packet and matches it to the routing table entry providing the best match for its destination. The table then provides the device with instructions for sending the packet to the next hop on its route across the network.

A basic routing table includes the following information:

Destination: The IP address of the packet's final destination
Next hop: The IP address to which the packet is forwarded
Interface: The outgoing network interface the device should use when forwarding the packet to the next hop or final destination

Metric: Assigns a cost to each available route so that the most cost-effective path can be chosen

Routes: Includes directly-attached subnets, indirect subnets that are not attached to the device but can be accessed through one or more hops, and default routes to use for certain types of traffic or when information is lacking.