HSRP Operations
This area deals with router alternation in an HSRP standby accumulation and discusses the concept
of the alive router, which is amenable for the forwarding of packets. Additionally discussed is the
ARP process, as able-bodied as the analysis of an HSRP bulletin and the altered states of HSRP.
Active Router
One router in anniversary accumulation is adopted to be the alive router. The acclamation action occurs through
the sending and accepting of accost messages. The accost bulletin contains a antecedence akin for the
sending router. The router with the accomplished standby antecedence in the accumulation becomes the active
router. The alive router assiduously the packets beatific to the basic router.
If the antecedence akin is the aforementioned for anniversary router in the group, the aboriginal router to appear up and obtain
the basic router IP abode becomes the alive router. If the priorities are equal, the accomplished IP
address wins.
Although assorted routers can abide in an HSRP group, alone the alive router assiduously the
packets beatific to the basic router.
Within the standby group, one router is adopted to be the alive router. The router with the
highest standby antecedence in the accumulation becomes the alive router. The absence antecedence for an
HSRP router is 100. This advantage is user-configurable.
The alive router responds to cartage for the basic router. If an end base sends a packet to the
virtual router MAC address, the alive router receives and processes that packet. If an end
station sends an ARP appeal with the basic router IP address, the alive router replies with
the basic router MAC address.
Locating the Basic Router MAC Address
The ARP action makes an affiliation amid Layer 3 arrangement addresses and Layer 2
hardware addresses. An archetype would be an IP abode and accouterments Ethernet address. Each
router maintains a table of bound addresses. The router checks this ARP accumulation before
attempting to acquaintance a accessory to actuate if the abode has already been resolved.
The IP abode and agnate MAC abode of the basic router is maintained in the ARP
table of anniversary router in an HSRP standby group.
Example 9-1 shows the achievement from the appearance ip arp command (entered in advantaged EXEC
mode), which displays the ARP accumulation on a router.
314 Affiliate 9: Overview of Hot Standby Router Protocol
Note the afterward in Archetype 9-1:
• The basic router IP abode is 172.16.10.110.
• The basic router MAC abode is the acclaimed MAC abode 0000.0c07.ac and the
HSRP accumulation identifier is 0a.
• The interface is VLAN10 on an RSM, which is the VLAN actuality routed.
Active and Standby Router Behavior
As mentioned beforehand in this chapter, anniversary HSRP accumulation contains the afterward entities:
• An alive router
• A standby router
• A basic router
The action of the alive router is to advanced packets beatific to the basic router. Another router
in the accumulation is adopted as the standby router. The alive router assumes and maintains its active
role through the manual of accost messages.
The action of the standby router is to adviser the operational cachet of the HSRP accumulation and
quickly accept packet-forwarding albatross if the alive router becomes inoperable. The
standby router additionally transmits accost letters to acquaint all added routers in the accumulation of the
standby router’s role and status.
The action of the basic router is to present a consistently accessible router to the end user.
An HSRP standby accumulation may accommodate added routers. These routers adviser the accost messages
but do not acknowledge to ARP requests. These routers do advanced any packets addressed to the
routers’ IP addresses, but do not advanced packets for the basic router.
When the alive router fails, the added HSRP routers stop accepting accost letters (they must
miss three hellos for this to happen), and the standby router assumes the role of the alive router.
Because the new alive router assumes both the IP and MAC addresses of the basic router, the
end stations see no disruption in service. The end-user stations abide to accelerate packets to the
virtual router MAC address, and the new alive router delivers the packets to the destination.
Example 9-1 appearance ip arp Command Achievement Displays ARP Accumulation on a Router
RSM_Dallas_1># appearance ip arp
Protocol Abode Age (min) Accouterments Addr Blazon Interface
Internet 172.16.10.82 - 0010.f6b3.d000 ARPA Vlan10
Internet 172.16.10.169 - 0010.0b79.5800 ARPA Vlan10
Internet 172.16.10.110 0000.0c07.ac0a ARPA Vlan10
HSRP Operations 315
In the accident that both the alive and standby routers fail, all routers in the accumulation argue for the
active and standby router roles, and the accomplished antecedence router will become the alive router.
Anatomy of an HSRP Message
All routers in a standby accumulation accelerate or accept HSRP messages. These letters are acclimated to
determine and advance the router roles aural the group. HSRP letters are encapsulated in
the abstracts allocation of User Datagram Agreement (UDP) packets and use anchorage cardinal 1985. These
packets are addressed to an “all router” multicast abode with a Time to Live (TTL) of one (1).
Figure 9-5 shows the accepted architecture for an HSRP message.
Figure 9-5 HSRP Bulletin Format
The HSRP bulletin contains the afterward information:
• The Adaptation acreage indicates the adaptation of the HSRP.
• The Op Code describes the blazon of bulletin independent in this packet.
• Accost letters are beatific to announce that a router is alive and able of acceptable either
the alive or standby router.
• Accumulation letters are beatific back a router wants to become the alive router.
• Reserved letters are beatific back a router no best wants to be the alive router.
• Internally, anniversary router in the standby accumulation accouterments a accompaniment machine. The Accompaniment field
describes the accepted accompaniment of the router sending the message.
• The Hellotime acreage is alone allusive in accost messages. This acreage contains the
approximate aeon amid the accost letters that the router sends. The time is accustomed in
seconds.
• The Holdtime acreage is alone allusive in accost messages. This acreage contains the amount
of time that the accepted accost bulletin should be valid. The time is accustomed in seconds.
Version Op Code Accompaniment Hellotime
Holdtime Antecedence Accumulation Reserved
Authentication Data
Authentication Data
Virtual IP Address
1 Octet 1 Octet 1 Octet 1 Octet
Anatomy of an HSRP Message
316 Affiliate 9: Overview of Hot Standby Router Protocol
• The Antecedence acreage is acclimated to accept the alive and standby routers. Back comparing
priorities of two altered routers, the router with the numerically college antecedence wins. In
the case of routers with according priority, the router with the college IP abode wins.
• The Accumulation acreage identifies the standby group. Values in the ambit of 0 and 255 are valid.
• The Authentication Abstracts acreage contains a clear-text, eight-character reused password.
• The Basic IP Abode acreage contains the IP abode of the basic router acclimated by this
group.
Only the alive and the standby routers accelerate alternate HSRP letters afterwards the agreement has
completed the acclamation process.
HSRP States
HSRP defines six states in which an HSRP configured router may exist. Back a router exists in
one of these states, the router performs the all-important accomplishments appropriate in that state. The HSRP
states are as follows:
• Antecedent state—All routers activate in the antecedent state. This starting accompaniment indicates that HSRP
is not running. This accompaniment is entered via a agreement change or back an interface first
comes up.
• Apprentice state—In the apprentice state, the router is still cat-and-mouse to apprehend from the alive router.
The router has not yet apparent a accost bulletin from the alive router, nor abstruse the IP
address of the basic router.
• Accept state—In the accept state, the router knows the basic IP address, but is neither the
active router nor the standby router. The router listens for accost letters from those
routers.
• Allege state—In the allege state, the router sends alternate accost letters and is actively
participating in the acclamation of the alive and/or standby router. A router cannot access the
speak accompaniment unless the router has the IP abode of the basic router.
• Standby state—In the standby state, the router is a applicant to become the abutting active
router and sends alternate accost messages. There charge be at atomic one standby router in the
HSRP group.
• Alive state—In the alive state, the router is currently forwarding packets that are sent
to the basic MAC abode of the group. The alive router sends alternate accost messages.
At atomic one alive router charge be in the HSRP group.
Not all HSRP routers will alteration through all states. For example, a router that is not in the
standby or alive router will not access the standby or alive states.