Link Aggregation Protocol (LaGP) is one of few modes for link aggregation. LAGP uses Active or Passive modes for each side of the link.

Port Aggregation Protocol (PaGP) uses dynamic desirable and dynamic auto in the styles of port negotiation.
Dynamic Desirable + Dynamic Desirable = PaGP formed.
Dynamic Desirable + Dynamic Auto = PaGP formed.
Dynamic Auto + Dynamic Auto = PaGP will not form.

On and off do not use a Link Aggregation Protocol, they are just ‘on’.

Spanning Tree Protocol:

Spanning Tree Protocol (STP) is used in switches (layer 2) to stop broadcasts from flowing over redundant links.

STP uses numerous steps to form a converged network.

1. Elect a root bridge.
2. Find the root ports.
3. Elect the designated ports.
4. Block everything else.

Elect a root bridge
A switch will win the root bridge election if;
1. The bridge ID is the lowest on the switch network.
– Switch priorities are set to 32768 by default on all switches.
2. If the priorities match, the switch with the lowest MAC address wins the bridge election.

Find the root ports
Root ports are ports that have the least cost to the root bridge.

Cost is determined by the speed of the link. For example, a 100Mbps link has the cost of 19.

Cost for links:
10Mbps – 100
100Mbps – 19
1Gbps – 4

Elect the designated ports

Designated ports (DP) are forwarding ports.

If two switches have the same cost to the root bridge, they need to find which will be the designated port, and which switch will have the blocking port.

1. The lowest cost to the root bridge
– If the costs tie;
2. The lowest switch MAC address.
– If the two switches have their ports aggregated;
– The lowest physical port is the DP, the higher is blocked.

Spanning Tree Timers

STP hello’s are sent every 2 seconds by default. The dead/hold down timer is 10x the hello timer (20 seconds by default).

Spanning Tree States

STP has a sequence of four states as to not create loops in the switch network whilst the network is converging.

The states are;

The switch clears the MAC address table of any learnt MAC addresses, and listens for any BPDU’s for a total of 15 seconds.

The switch actively learns the MAC addresses of any device for a total of 15 seconds by default.


The port is set to the forwarding state when the network has converged and no loops occur.


The blocking state occurs when the network has converged.

The STP topology will change if the root bridge disconnects, or another switch wins the root election by having a lower BID.
The ports will remain in their converged states until the Max Delay Timer (default 10x hello timer – 20 seconds) expires.
If a port was forwarding prior to the topology change, it can remain in a forwarding state or move directly to a blocking state if it receives a looped BPDU.
If the port was in a blocking state, it will cycle the Listening, Learning, Forwarding, Blocking states again.

First Hop Redundancy Protocols

There are many different First hop redundancy protocols.

Hot Standby Redundant Protocol

Hot Standby Redundant Protocol (HSRP) uses an Active/Passive stance, meaning that one router in the HSRP group will forward all network traffic, whereas the other will be in standby.


Configure HSRP

HSRP is configured under the interface for that network.
In my example below, I’ve got two routers directly connected with and for Router A and B respectively.

Router A:

interface FastEthernet0/0
ip address
duplex auto
speed auto
standby version 2
standby 1 ip
standby priority 110

Router B:

interface FastEthernet0/0
ip address
duplex auto
speed auto
standby version 2
standby 1 ip

After a no shutdown is enabled on both interfaces, the following happens on Router A.

%HSRP-6-STATECHANGE: FastEthernet0/0 Grp 1 state Speak -> Standby

%HSRP-6-STATECHANGE: FastEthernet0/0 Grp 1 state Standby -> Active

As you can see, router A is the active router in the HSRP group, with router B set as the standby router.

If no standby priority value is set, the router with the highest IP address will become the active HSRP router.

RouterA#show standby brief
P indicates configured to preempt.
Interface Grp Pri P State Active Standby Virtual IP
Fa0/0 1 100 Listen unknown

HSRP Timers

The default hello and dead timers for HSRP are 3 and 10 seconds.

HSRP Virtual Mac addresses


VRRP uses a lot of the same syntax from HSRP and is also set under the interfaces.

Instead of Active/Passive modes, VRRP uses Master and Backup.

If VRRP uses the same VRRP 1 ip as the physical IP address, the priority will be set to 255, and wins the Master election.

VRRP uses ‘track objects’ to notify the Backup router if something changes. One of the most common uses for Track objects is to monitor the line protocol of a WAN interface.
For example, if one router tracking the WAN interface line protocol goes down, and therefore loses connectivity to the Internet, it can switch to the Backup router for the WAN route.

Track objects are created from Global Config mode.
RouterA(config)# track 1 interface fastethernet 0/1 line-protocol

Gateway Load Balancing Protocol

Gateway load balancing protocol (GLBP) balances by HOST, not network.

There are two roles in GLBP. The Active Virtual Gateway (AVG) and the Active Virtual Forwarder (AVF).

The AVG hands out the virtual mac addresses for each AVF to clients on the network.

The configuration is similar to HSRP as well.

RouterA(config-if)#glbp 1 ip

The Hello and Dead timers are set to 3 and 10 respectively.

GLBP will assign the virtual mac addresses to clients in a round robin style.

Distance Vector and Link State Routing Protocols

Distance Vector (DV) concepts

Link State (LS) concepts

Link state routers hold each routers topology in a database. This means that every router in an LS routing protocol has every route within that router ID.

Distance Vector routing protocols use the Distance (hop count) and Vector (Out which interface) to build their routing protocol.

Maximum Distances

RIP has a maximum Hop Count as 16 hops.
EIGRP does not have a maximum Hop Count

Route Poisoning and Triggered Updates

If a router’s interface goes down, this causes a triggered update. Route poisoning advertises the just disconnected route as ‘unreachable’.
Route Poisoning sets a down interface to the Hop Count to 16 for RIP, as this is an invalid route, and therefore will remove it from the routing table from each RIP router.

Split Horizon

Split Horizon will not tell a neighbouring router something that a directly connected router told it.

If Split Horizon did not exist, routing loops would occur.

Issues occur when Frame Relay issues can occur with Split Horizon when using a Hub and Spoke topology.

If two spokes are on the same subnet, and connect to the hub, split horizon will not advertise one site.

Hold Down Timers

Hold down timers reduce flapping interfaces as they are awaiting the hold down timer before acting.

Enhanced Interior Gateway Routing Protocol (EIGRP)

EIGRP uses Bandwidth and delay by default.
Uses the multicast address of

Some uses for EIGRP are below:

1. Backup routes (Fast convergence)
2. Simple Configuration
3. Flexibility in Summarization
4. Unequal cost load-balancing
5. Combines the best of distance vector and link state.
– Uses a cost of hop + link bandwidth to determine the best route.

Backup routes.
Equal cost routes in EIGRP can exist, and a default of 4 will exist in the routing table.
To become considered a feasible successor, the Advertised Distance of a backup route must be LESS than the Feasible Distance of a successor route.
The Variance multiplier can be used to increase the width of a backup route.
For example, a FD of 500 with variance 2 set, will allow routes with a FD of 900 into the routing table.

EIGRP Tables and Terminology

1. Feasible Distance (FD)
– Calculates it’s FD to a remote network from itself.
2. Advertised Distance (AD)
– It’s AD is the distance from a neighbour.
3. Successor.
– The Successor is the primary route which made it to the routing table.
4. Feasible Successor.
– The feasible successor is the backup route sitting in the topology table waiting to be used in case the successor route fails.
5. Active Route
– If a successor route fails, and there is no feasible successor, the router will move to active mode which means it’s trying to find a new route.
6. Passive Route.
– This mode states the router is functioning correctly.

If there is only a Successor route in use, with no feasible successors, and the successor route goes down, EIGRP uses DUAL to actively look for a route to the down network.

EIGRP uses the multicast address of to transmit it’s hello and updates across.

The Hello packets form and maintain neighbour relationships.
Updates send their routing table updates to their neighbours.
If a route is lost, a query message is sent looking for the recently downed route. The router changes to Active during this phase.
A reply is a reply from the neighbouring routers in response to the Query message about the down route notifying the querying router if they have a route to that downed network or not.
Acknowledgement packets acknowledge Update, query and reply messages.

Configuring EIGRP

About The Author


Timothy started his networking career in 2014, working for one of the largest telecommunication operators in Australia. When he's not working, he's obsessing over German Shepherd Dogs.