Hi Kuldeep,
you're welcome!
1) what do you want to explain through these lines
(Red highlighted), plz explain in simple way.
Does HSRP work on Serial port,
Can i replace s0/0 Wan port with FE port:
No, HSRP don't work on serial port simply because there are no hosts on this segment. You don't need to provide gateway redundancy to the other router, just hosts. Router does not need gateway , it uses routing table. Users - hosts do need gateway - and FHRP protocols are designed for them. So in my picture: HSRP works only at the green area - which is one IP network subnet 192.168.0.0/24.
HSRP will not work between different colour areas - simply because HSRP cannot function in different IP networks, it has to be one and the same!!!
Routers will keep an eye on the serial interface as
well (track interface command) and when it comes down,
the other Router will forward the traffic. This is handy
because otherwise the FHRP would work only when
the whole Router (or fa0/0) comes down. It would not
detect any failure, hosts would send packet to this Router
and Router, which serial interface is down, would drop the packets.
Once again, look a my diagram. I will provide basic HSRP configuration to tell you what I meant.
R1(config)#interface fastethernet0/0
R1(config-if)#ip address 192.168.0.2
R1(config-if)#standby 1 ip 192.168.0.1
R2(config)#interface fastethernet0/0
R2(config-if)#ip address 192.168.0.3
R2(config-if)#standby 1 ip 192.168.0.1In such case, Router2 will become active(forwarding) and Router1 will become standby router.
If Router2 fails for some reason(or fa0/0 fails) Router1 will not be able to contact it and will assume that Router2 is down.
Therefore Router1 will start to forward traffic instead of Router2. That is classic HSRP behaviour.
But what about this: Router2 is active router. Router1 is standby.
Router's 2 serial interface will come down. HSRP won't notice anything, all the traffic from hosts will be forwarded to Router2 although it's interface leading to internet (s0/0/0) is down! HSRP won't notice change because it communicates through LAN 192.168.0.0/24 and there is everything just fine.
In such case we can use the standby 1 track Serial0/0/0 on both routers. This command will keep an eye on the serial interface and in case of outage, the other router with functional link towards the internet will become the active router. That is what I meant with that red highlighted part. For more information please look here:
http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a0080094e8c.shtml
But it does not mean that HSRP then works on serial interface! Not at all! It just checks the interface for functionality. All of the HSRP messages are still exchanged only through the same IP network 192.168.0.0/24 in this case. HSRP cannot communicate through different IP networks, because its messages are not routed, they are only local to some LAN !
2) Can i use other method like Floating static route with
AD (administrative distance) at place of HSRP
in your attached diagram. If yes, then what is basic
functionality difference between them ?
Floating static route is a way to provide ROUTE BACKUPs, not First Hop redundancy to hosts! So these are two different things.
I see your confusion as you are placing these two completely different things side to side. Backing up routes or route redundancy does nothing to do with gateway redundancy for hosts!
If the route towards internet on Router1 or Router2 is lost or not present, HSRP will not find out!!! It only looks if the device in still there. In case of the track command that I have shown you - HSRP will look if the interface is up and running - but will not find out if the Router has no route to Internet!!!
3) @PETER stated :
HSRP is not to provide redundancy between routers -
HSRP provides redundancy of the default gateway for PC's ".
What does mean of this line ? But we are performing
redundancy with two router here ( like as your attached diagram) ?
I cannot explain in better than Peter. Trust me . He is the best amongst the best. I can try however write it in my own way.
Yes, there are two routers. But these routers are used as default gateways for users in this network. They are simply the first L3 device on the way out from the network (First hop). If any host in network 192.168.0.0/24 wants to communicate with Internet, all the packets are sent to one of these routers. But to which one? Only one of this router can be DEFAULT GATEWAY for host. If you look at the internet connection in your PC, there is IP address, IP mask and Gateway address. We are talking here about the last thing.
The idea behind having 2 routers in this topology is to provide default gateway redundancy to hosts. If one router fails, users will still be able to communicate with outside world thanks to HSRP! They won't be able to do it without HSRP simply because computer can use only ONE default gateway. But how do you tell the PC to change it if it fails? You simply cannot.
Remember that Router1 has IP address 192.168.0.2, Router2 has IP address 192.168.0.3
But PC can't have these 2 addresses configured as default gateway at one time!
HSRP will configure two routers with same VIRTUAL IP address 192.168.0.1 in this case.
PC will be configured with 192.168.0.1 as default gateway and that solves the problem!
HSRP will take care of which of the routers will forward the traffic for real.
Hope that helps a little.
Best regards,
Jan
