Routing

The Administrative Distance of a Static Route by default is 1, but it adjustable from 1 to 255. Adjusting the administrative distance of a static route is a common method of providing back-up connectivity for prefixes learned by dynamic routing protocols, by specifying the administrative distance of the backup static route higher than the dynamic…

There are three main types of static routes that can be configured: Directly attached static routes Recursive static routes Fully specified static routes Directly Attached Static Routes Point-to-point serial interfaces do not need to maintain an adjacency table and do not use address resolution protocol, when used in a static route the configuration can directly…

By default on Cisco devices, routing protocols will only install the route into the routing table with the lowest metric. There is a feature within EIGRP that allows multiple routes to be installed with different metric values. This will allow for unequal cost load balancing across multiple paths. Traffic will be transmitted out over the…

When a routing protocol identifies multiple paths as a best path to a destination and the router supports multiple path entries. The router can install an additional router up a maximum number of paths per destination. This is known as equal-cost multipathing or ECMP, providing load sharing across all links. RIP, EIGRP, OSPF, and IS-IS…

Metric calculations can vary between different routing protocols. The general common concept across them is that internally learned routes are preferred than externally (such as through distribution) and the lowest metric results in a preferred route chosen.

When each routing protocol on a router receives information, it will attempt to install any routing paths into the routing table. If the route already exists in the routing table, the administrative distance is compared and the routing protocol with the lower administrative distance is inserted into the routing protocol. Routing Protocol Default Administrative Distance…

When a packet needs to be forwarded, the chosen route is dependant on that routes prefix length. A /30 (255.255.255.252) would be preferred over a /24 (255.255.255.0) due to a more specific match. This forward decision is part of the Forwarding Information Base (FIB) rather than Routing Information Base (RIB).

When a packet arrives on the router, the router identifies the destination path that packet should traverse by evaluating the prefix length as programmed in the Forwarding Information Base (FIB). The Forwarding Information Base is programmed the routing table, also known in this context as the Routing Information Base (RIB). The Routing Information Base contains…

Path Vector Algorithms, such as BGP, are similar to a distance vector protocol. The major difference between the two is that instead of looking at just distance to determine a loop free path to the destination, various other attributes are considered instead, in the case of BGP: Autonomous system path (AS_Path) Multi exit discriminator (MED)…