Tutorial 7

Question 1

Look at the code snippets below:

Router A

interface fastethernet0/1
ip address 10.1.0.1 255.255.0.0
standby 1 ip 10.1.0.254
standby 1 priority 120 preempt

Router B

interface fastethernet0/1
ip address 10.1.0.2 255.255.0.0
standby 2 ip 10.1.0.254
standby 2 priority 90 preempt

How would these routers function as a standby group?

Answer 1

this is a misconfiguration.

Router A is running HSRP group 1 with VIP 10.1.0.254 and prio 120.
Router B is running HSRP group 2 with the same VIP 10.1.0.254 and prio 90.

What you should do instead
To have a proper active/standby pair for a single default gateway, use the same HSRP group number and VIP on both:

On both routers
interface fa0/1
standby 1 ip 10.1.0.254

Then set priorities/preempt as desired

Rtr A:
standby 1 priority 120 preempt

Rtr B:
standby 1 priority 90 preempt

If you intended load sharing, use two different groups with two different VIPs, and split host defaults (e.g., odd VLANs use .253 via group 1 active on A; even VLANs use .254 via group 2 active on B).

Question 2

How would you implement HSRP load balancing for a large group of hosts residing in two different VLANs?

Answer 2

HSRP load balancing for a large group of hosts across two VLANs can be achieved by configuring multiple HSRP groups and assigning different routers as the active gateway for each VLAN.

For example, Router A can be configured as the active router for VLAN 10 and standby for VLAN 20, while Router B is the active router for VLAN 20 and standby for VLAN 10. Each VLAN uses its own virtual IP address, which acts as the default gateway for hosts in that VLAN.

This design provides load sharing—traffic from VLAN 10 passes through Router A, and traffic from VLAN 20 passes through Router B—while still maintaining redundancy. If either router fails, the standby router automatically takes over the virtual IP for that VLAN, ensuring continuous connectivity.
In larger networks, further load balancing can be achieved by using multiple HSRP groups within each VLAN and distributing hosts between the groups (for example, through different default gateways assigned by DHCP).

Question 3

Explain how priority numbers are used by routers participating in a HSRP group. In your answer explain how this behaviour is affected by the use of the pre-empt command.

Answer 3

In an HSRP (Hot Standby Router Protocol) group, priority numbers are used to determine which router becomes the active router. The router with the highest priority value is elected as the active router, while the others remain in the standby state.

By default, the priority value is 100, but it can be manually configured between 0 and 255. A higher number indicates a higher priority for becoming the active router.

Normally, once a router becomes active, it remains active even if another router with a higher priority later joins the group.

However, when the preempt command is configured, it changes this behaviour. With preemption enabled, if a router with a higher priority comes online or recovers from a failure, it will take over the active role from the current router.

This ensures that the router with the highest configured priority is always the active router whenever it is available, maintaining optimal routing performance and control.

Question 4

How does interface tracking help with HSRP scenarios?

Answer 4

Interface tracking in HSRP is used to prevent sub-optimal routing paths, improve failover efficiency and reliability. It allows a router to monitor the status of a specific interface (for example, a WAN or uplink connection).

If the tracked interface goes down, the router’s HSRP priority is automatically reduced by a configured value. This often causes another router—with a higher effective priority—to become the active router.
By doing this, HSRP ensures that traffic is always forwarded through the router that still has a fully operational path to the network, avoiding situations where packets would travel through a router whose main link is unavailable.

In short, interface tracking helps HSRP maintain optimal paths and ensures faster, intelligent failover during link failures.

Question 5

What is the functional difference between GLBP and HSRP?

Answer 5

The main functional difference between GLBP (Gateway Load Balancing Protocol) and HSRP (Hot Standby Router Protocol) lies in how they handle load sharing and gateway redundancy.

• HSRP provides gateway redundancy only. It allows multiple routers to form a standby group where one router is active (forwarding traffic) and the others are standby (ready to take over if the active router fails). This means only one router actively forwards traffic at a time.
• GLBP, on the other hand, provides both redundancy and load balancing. Multiple routers can actively forward traffic simultaneously, each acting as an Active Virtual Forwarder (AVF) under a single virtual IP address. The Active Virtual Gateway (AVG) assigns different virtual MAC addresses to hosts, allowing traffic to be distributed across multiple routers.

In summary, HSRP = redundancy only, while GLBP = redundancy + load balancing.