Routing Protocols

Routing Protocol Foundations

Static routes are entered manually. Dynamic protocols discover networks, exchange reachability, calculate paths, and reconverge after change.

Manual

Static route

Predictable and low-overhead, but the administrator owns updates and recovery.

Learned

Dynamic route

Protocols share reachability and recalculate when topology changes.

Best match

Prefix length

The longest matching destination prefix is considered before source trust or metric.

Stable state

Convergence

Routers agree on current reachable paths after updates and calculations complete.

Selection orderLongest prefix → lowest administrative distance → best protocol metric → optional equal-cost multipath.

Administrative Distance Explorer

AD compares different route sources advertising the same prefix. Select a source to inspect its default value.

BoundaryAD 255 means unusable. AD is local to the router; it is not advertised as an end-to-end score.

Routing Algorithm Families

RIP

Distance-vector

Learns direction and distance from neighbors; RIP uses hop count.

OSPF ~ IS-IS

Link-state

Floods link information, builds a topology database, and runs SPF.

EIGRP

Advanced distance-vector

Uses DUAL, neighbor relationships, and prequalified backup paths.

BGP

Path-vector

Uses path attributes and routing policy between autonomous systems.

RIP Essentials

Metric

Hop count

15 is the maximum usable distance; 16 means unreachable.

Updates

Every 30 seconds

Periodic full-table advertisements make RIP simple but less scalable.

Transport

UDP 520

RIPv2 multicasts updates to 224.0.0.9 and has AD 120.

Feature	RIPv1	RIPv2
Addressing model	Classful	Classless
Updates	Broadcast	224.0.0.9 multicast
Subnet masks	Not included	Included
VLSM / discontiguous	Unsupported	Supported
Authentication	No	Supported

EIGRP: DUAL and Three Tables

Installed best

Successor

The best loop-free path placed in the routing table.

Ready backup

Feasible successor

A prequalified loop-free backup path that satisfies the feasibility condition.

Local total

Feasible distance

The current router’s best total metric to the destination.

Neighbor claim

Reported distance

The neighbor’s advertised metric from itself to the destination.

Know the factsDUAL ~ bandwidth + delay ~ route code D ~ AD 90 internal / 170 external ~ 224.0.0.10 for IPv4 ~ FF02::A for IPv6.

BGP: Policy Between Autonomous Systems

Across AS boundaries

eBGP

Exchanges routes between different autonomous systems; default AD 20.

Inside one AS

iBGP

Distributes externally learned BGP reachability internally; default AD 200.

Loop visibility

AS path

Lists traversed autonomous systems and supports policy and loop prevention.

Policy tools

Attributes

Higher local preference is preferred; lower MED is commonly preferred; prefix filters control exchange.

Core purposeBGP is the primary inter-AS protocol of the Internet. It selects policy-compliant paths, not merely the lowest technical cost.

Link-State Learning Sequence

Advance through the common link-state process used to build a consistent topology view.

Routers begin by discovering neighbors.

OSPF Areas and Router Roles

Backbone

Area 0

All other OSPF areas must connect to the backbone directly or logically.

Inside one area

Internal router

All OSPF interfaces belong to the same non-backbone or backbone area.

Area boundary

ABR

Connects Area 0 to another area and maintains information for each.

External boundary

ASBR

Redistributes routes from another routing domain into OSPF.

OSPF profileOpen standard ~ link-state ~ Dijkstra SPF ~ cost metric ~ classless ~ triggered LSAs ~ OSPFv2 for IPv4 and OSPFv3 for IPv6.

OSPF versus IS-IS

Dimension	OSPF	IS-IS
Family	Link-state IGP	Link-state IGP
Hierarchy	Area 0 backbone plus other areas	Level 1 inside areas; Level 2 between areas
Control plane	Runs directly over IP	Uses CLNS for control traffic
Common association	Enterprise and service-provider networks	Large provider cores
Area 0	Required backbone	No Area 0 concept

First-Hop Redundancy

Open standard

VRRP

Uses master and backup roles to protect a virtual default gateway.

Cisco

HSRP

Uses active and standby roles with a shared virtual IP and MAC.

Cisco + balancing

GLBP

Can use multiple active forwarders to provide redundancy and load distribution.

Clients send to the virtual IP; the active gateway forwards.

Subinterfaces and Router-on-a-Stick

One physical port

802.1Q trunk

A trunk carries frames for several VLANs between the switch and router.

Logical gateways

G0/0.10 ~ G0/0.20

Each subinterface uses VLAN encapsulation and an IP address serving as that VLAN’s gateway.

Consistency

Native VLAN

The native VLAN must match on both ends or untagged traffic can be misdirected.

Exam phrase“Several VLAN gateways over one physical router interface” points to subinterfaces and router-on-a-stick.

IPv6 Router Discovery and NDP

IPv6 value	Meaning
FE80::/10	Link-local unicast
FF02::1	All nodes on the link
FF02::2	All routers on the link
FF02::1:FF00:0/104	Solicited-node multicast range
::1	Loopback
::	Unspecified address

IPv6 Transition Explorer

IPv6 Routing and PowerShell 7

Protocol	Transport / multicast	Association
RIPng	UDP 521 ~ FF02::9	RIP for IPv6
EIGRPv6	FF02::A	DUAL for IPv6
OSPFv3	FF02::5 / FF02::6	All OSPF routers / designated routers

Get-NetRoute -AddressFamily IPv4IPv4 routes

Get-NetRoute -AddressFamily IPv6IPv6 routes

Get-NetRoute | Sort-Object AddressFamily,DestinationPrefixSorted route view

Get-NetRoute -DestinationPrefix 0.0.0.0/0IPv4 default

Get-NetRoute -DestinationPrefix ::/0IPv6 default

Get-NetNeighborARP and NDP neighbors

Get-NetIPConfigurationInterface configuration

Test-NetConnection 1.1.1.1 -TraceRouteTrace routed path

netsh interface ipv6 show neighborsIPv6 neighbor cache

netsh interface ipv6 show routeIPv6 routing table

Exam Check

Fifteen review questions drawn from the Chapter 10 study manual.

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