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Metric or Value

The best path is selected by a routing protocol based on the value or metric it uses to determine the distance to reach a network. A metric is the quantitative value used to measure the distance to a given network. The best path to a network is the path with the lowest metric.

Routing over PPP:

When the interface is a point-to-point (P2P) serial connection, the router encapsulates the IPv4 packet into the proper data link frame format used by the exit interface (HDLC, PPP, etc.). Because there are no MAC addresses on serial interfaces, so router sets the data link destination address to an equivalent of a broadcast.

Administrative Distance and  Equal cost load balancing

Cisco IOS uses what is known as the administrative distance (AD) to determine the route to install into the IP routing table. The AD represents the “trustworthiness” of the route; the lower the AD, the more trustworthy the route source.

Route Source Default Distance Values
Connected interface 0
Static route 1
Enhanced Interior Gateway Routing Protocol (EIGRP) summary route 5
External Border Gateway Protocol (BGP) 20
Internal EIGRP 90
IGRP 100
OSPF 110
Intermediate System-to-Intermediate System (IS-IS) 115
Routing Information Protocol (RIP) 120
Exterior Gateway Protocol (EGP) 140
On Demand Routing (ODR) 160
External EIGRP 170
Internal BGP 200

When a router has two or more paths to a destination with equal cost metrics, then the router forwards the packets using both paths equally.If configured correctly, load balancing can increase the effectiveness and performance of the network. Equal cost load balancing can be configured to use both dynamic routing protocols and static routes.

Note: Only EIGRP supports unequal cost load balancing.

Routing table

Entries in the routing table can be added as:

  • Local Route interfaces(L-The interfaces belongs to the router) – Added when an interface is configured and active. This entry is only displayed in IOS 15 or newer for IPv4 routes and all IOS releases for IPv6 routes.
  • Directly connected interfaces (C-the subnet connected to the router interface)– Added to the routing table when an interface is configured and active.
  • Static routes – manually configured .
  • Dynamic routing protocol – Added when routing protocols that dynamically learn about the network, such as EIGRP or OSPF, are implemented and networks are identifie.