HSRP Interview
Questions and Answers

 

Configuration of HSRFP-

 

 

What is HSRP and what is it used for?

HSRP stands for Hot Standby Router
Protocol. It is a Cisco proprietary protocol that allows two or more routers to
work together to present the appearance of a single virtual router to hosts on
a LAN. This is done by sharing a virtual IP address and by maintaining
consistency of routing information. In the event of a failure of the active
router, a standby router takes over, ensuring that hosts on the LAN always have
a valid default gateway.

 

How does HSRP determine the active
router?

HSRP routers use hello messages to
communicate with each other and to determine the active router. The active
router is the router with the highest priority, and if multiple routers have
the same priority, the router with the highest IP address is chosen.

 

What is virtual mac address of HSRP?

The
virtual MAC address of an HSRP group is the MAC address that is associated with
the virtual IP address of the group. This virtual MAC address is used by hosts
on the LAN to communicate with the virtual router.

When
a host sends a packet to the virtual IP address, the packet is actually sent to
the virtual MAC address, which is then forwarded to the active HSRP router. The
active router then forwards the packet to its destination using its own MAC
address.

The
virtual MAC address for HSRP is in the format of 0000.0C07.ACxx , where xx is
the HSRP group number in hexadecimal format.

For
example, if the HSRP group number is 10, the virtual MAC address would be
0000.0C07.AC0A.

It’s
important to note that each HSRP group will have a unique virtual MAC address.

 

HSRP use TCP or UDP?

HSRP
(Hot Standby Router Protocol) uses UDP (User Datagram Protocol) as the
transport protocol for its communication. Specifically, HSRP uses UDP port 1985
to send and receive messages between routers in the same HSRP group.

UDP
is used because it is a connectionless, fast and lightweight protocol, which
makes it well suited for the type of control messaging used by HSRP. It is also
less reliable than TCP, which allows for faster failover in case of a problem
with the active router.

HSRP
routers use hello messages to communicate with each other and to determine the
active router. The active router is responsible for forwarding packets sent to
the virtual IP address, and for sending gratuitous ARP messages to update the
ARP caches of hosts on the LAN.

It’s
important to note that although HSRP uses UDP as its transport protocol, it is
not a routing protocol, it is a redundancy protocol. It’s used to provide
redundancy and failover capability in a LAN environment.

 

What is source and destination ip
address of HSRP hello Message?

In HSRP (Hot Standby Router Protocol),
the source IP address of a hello message is the IP address of the router
sending the message and the destination IP address is the multicast IP address
224.0.0.2.

When a router first joins an HSRP
group, it sends a hello message to the multicast IP address 224.0.0.2 on UDP
port 1985. This multicast IP address is used by all routers in the same HSRP
group to exchange hello messages.

The destination IP address of a hello
message is the multicast IP address 224.0.0.2, which is a reserved multicast
address that is used by HSRP routers.

The hello message contains information
about the HSRP group, including the virtual IP address, the virtual MAC
address, and the priority of the router. This information is used by other
routers in the same HSRP group to determine the active router, and to update
their own HSRP configuration.

It’s important to note that the
multicast IP address 224.0.0.2 is only used by HSRP routers and it doesn’t
reach any other device on the network.

 

Can HSRP be configure on multilayer
switch?

Yes, it is possible to configure HSRP
(Hot Standby Router Protocol) on a multilayer switch.

A multilayer switch is a device that
can perform both switching and routing functions. It is typically capable of
running routing protocols, as well as providing switching capabilities for
LANs.

 

HSRP default hold and hello timers?

In HSRP (Hot Standby Router Protocol),
the default hold time is 10 seconds and the default hello time is 3 seconds.

The hold time is the amount of time that a router will wait before
considering a neighbor router as down. It’s the time that a router will wait
after not receiving a hello message from its neighbor before considering it
down.

The hello time is the amount of time between hello messages that are
sent by the active router. Hello messages are used by HSRP routers to
communicate with each other and to determine the active router.

The default hold time of 10 seconds
and the default hello time of 3 seconds are recommended values for most
networks, but these values can be adjusted to meet the specific needs of a
network. For example, if your network is large and it takes longer for hello
messages to propagate, you might want to increase the hello time.

It’s important to note that the hold
time and the hello time values should be the same among all routers in an HSRP
group. If the values are different between routers, it may cause issues with
the HSRP group.

 

HSRP v1 vs v2?

HSRP version 1 is the original version of the protocol,
which was first introduced by Cisco. It uses multicast IP address 224.0.0.2 and
UDP port 1985 for communication between routers in the same HSRP group. HSRP
version 1 supports a maximum of 100 groups per interface and up to 1000 groups
per router.

HSRP version 2 is an extension of version 1 that was
introduced to address some limitations of version 1. It uses multicast IP
address 224.0.0.102 and UDP port 1985 for communication between routers in the
same HSRP group. HSRP version 2 supports a maximum of 100 groups per interface
and up to 1000 groups per router.

The main differences between HSRP version 1
and version 2 are:

HSRP version 2 uses a different
multicast IP address which is 224.0.0.102 compared to 224.0.0.2 in version 1

HSRP version 2 supports authentication
of hello messages, which provides added security by preventing unauthorized
routers from joining the HSRP group.

HSRP version 2 supports a different
type of authentication, called MD5 authentication, which is more secure than
the simple text authentication used in version 1.

It’s important to note that HSRP
version 2 is backwards compatible with version 1, which means that routers
running version 2 can communicate with routers running version 1.

It’s also recommended to use the
latest version of HSRP, as it provides more security and flexibility than the
older version.

Explain State in HSRP?

In HSRP (Hot Standby Router Protocol),
there are several states that a router can be in depending on its role and the
status of the group. The main states are:

Initial state: The router starts in this state when it is
first configured for HSRP. In this state, the router is not yet aware of any
other HSRP routers on the network, and it is not sending or receiving any HSRP
messages.

Listen state:
The router enters this state after it has sent an initial hello message. In
this state, the router is listening for hello messages from other routers in
the same HSRP group.

Speak state:
If the router receives a hello message from another router in the same HSRP
group, it enters this state. In this state, the router is sending and receiving
hello messages, and it is aware of the other routers in the group.

Standby state: In this state, the router has a lower
priority than the active router, and it is not forwarding any packets sent to
the virtual IP address. It is ready to take over as the active router in case
the active router fails.

Active state:
In this state, the router is the active router, and it is forwarding packets sent
to the virtual IP address. It also sends gratuitous ARP messages to update the
ARP caches of hosts on the LAN.

HSRP Disabled state: If the HSRP configuration is removed from a
router or if the router is rebooted, it goes to this state.

It’s important to note that these
states are not mutually exclusive. A router may transition between these states
depending on the status of the group and the role of the router.

 

 

 

If the active router goes down and
the standby router does not become active in HSRP? What can be the reason?

Priority Mismatch, VIP configuration
on the standby router, Reachability.

 

What is default timer in HSRP –

How to configure
HSRP –

Use below command to configure HSRP-
exp 10.10.10.3 and 10.10.10.2 is the ip address of both device and virtual ip –
10.10.10.1

 

We can use the
show standby command to see the status of our HSRP

R1#show standby
FastEthernet0/0 – Group 1
State is Active
2 state changes, last state change 00:23:53