cisco 7 hsrp

8/3/2019 Cisco 7 Hsrp

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Cisco 7 HSRP lecture


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How does a workstation get adefault gateway (IP address of

router)? 1. DHCP gives the workstation the default

gateway

2. IRDP (ICMP Router Discovery Protocol)extension to ICM that allows an end-station toautomatically discover a default gateway. RPs(Route Processors) periodically generate specialmulticast packets that announce the routersexistence to the clients every 5 to 10 minutes.Multicast packet has the RPs address and a life-time value. Could take up to 30 minutes.


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3. Proxy ARPPC dynamically discoversdefault IP address and MAC of the default

gateway. When default gateway fails, traffic

is dropped. After a lengthy period of time,PC will re-perform the Proxy ARP, but in

most situations, PC will continue using

same failed default gateway.


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What happens to the

workstation when router fails? PC cant communicate with other networks


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Solution is HSRP (Hot Standby

Routing Protocol)

Cisco-proprietaryprotocol

Provides Layer 3redundancy

Transparent to end stations

RP (Route Processor) monitors the status ofother RPs and provides a quick failoverwhen primary default gateway fails.


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HSRP


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HSRP


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HSRP Group A group of 2 or more RPs that represent a

single default gateway. It has a virtual IPaddress and a virtual MAC address. If the

primary RP fails, another RP takes over.

One RP can be the backup for multipleprimary default gateways

Only one RP forwards data for a LAN.


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HSRP Group Group has the following type of RPs:

Virtual RP Active RP

Standby RP

Other RPs

Virtual RP

Provides asingle RP that is available to end stations.

Not a real RPthe IP and MAC addresses are notphysically assigned to any one interface on any of theRPs in the broadcast domain


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HSRP Group Active RP

Responsible for forwarding all traffic destinedfor the Virtual RPs MAC address.

Elected in an election processRP with highest

priority is active. If priorities are same, highestIP address wins. Default priority is 100.

Only one active RP per

network/subnetwork/VLAN


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HSRP Group Standby RP

Elected in an election process Keeps tabs on Active RP by looking for HSRP

multicast messages (HSRP hellos). Hellos are sent by

active RP every 3 seconds. If standby doesnt hear any

hellos for 10 seconds, it promotes itself and becomesthe active RP.

Sends out its own hellos every 3 seconds so that if it

fails, one of the other possible HSRP RPs in the

standby group will become the standby.

Only one standby RP per network/subnetwork/VLAN


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HSRP Group


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HSRP Group Other HSRP RPs

Listen for hellos from standby and active RPs. If any end-station uses a REAL MAC address of one of

the RPs in the broadcast domain, that specific RP(whether active, standby or other RP) will process andforward the frame.

Each standby group must have a unique virtual IPaddress and a virtual MAC address.

These addresses are unique across different VLANs.

MAC address is 000.0c07.acxx (000.0c is Ciscosvendor code; 07.ac is HSRPs well-known address;xx is the HSRP group number.

End stations perform an ARP request with thevirtual IP address and get the virtual MAC addressof the default gateway RP.


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HSRP Group


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Configuration for HSRP Only one command is needed to enable HSRP

standbyon an interface or subinterface. (config)#interface xxxxx (config-if)#standbygroup_no. ipip_address

note that group_no.is optional. If you dont put it in, itdefaults to 0. It is required if you have multiple standbygroups. IP address is the virtual IP address.

To ensure the end-stations do not discover a RPsreal MAC address on the interface, HSRP disablesICMP redirects.


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Configuration for HSRP

A ti & St db El ti d


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Active & Standby Election and

Priorities

The priority number on an interfaceinfluences whether or not a RP becomes theactive or standby router.

Default priority is 100

Highest priority becomes active router

Can assign a number for 0 to 255

Interface command: standby group_no.prioritypriority_no.



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Priorities



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Priorities


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PREEMPTION Allows an RP to preempt the current standby or

active RP when the RP comes on line.

Interface command: standbygroup_no. prioritypriority_no.preempt [delay time] where time ishow long the RP waits to begin the preemptionprocess. Default is0 secs. Can use

0 to 3,600seconds (one hour).


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Interface Tracking

Allows a RP to track specific interfaces and itdecrements its priority value if the interface hasproblems.

Interface command: standby group_no.track

interface_type interface_no. [decrement_value]where the interface_type and interface_no. is theinterface you want the RP to track. The optionaldecrement_value is a priority number and the

default is 10. This means whatever its priorityvalue is currently will be reduced by 10.


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Interface Tracking


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Verifying HSRP

Global configuration command: show

standby

Brief description: show standby interfacebrief

Debug: debug standby


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Verifying HSRP


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Verifying HSRP


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HSRP Config


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HSRP Config

Router-A is initially the activerouter, with a priority of 150, whileRouter-B is the standby router, witha priority of 100. However, because

we are tracking interfaceFastEthernet 0/1, when it goesdown you can see that the priorityof Router-A goes down to 95. Atthat point, Router-B becomes the

active router and Router-Abecomes the standby router.


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HSRP HINT

Enable PORTFAST when using HSRP.

Why?

cisco 7 hsrp

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