default gateway FHRP

pinkiaiii

In a switched network, each client receives only one default gateway. There is no way to use a secondary gateway, even if a second path exists to carry packets off the local segment.

can someone shed light on sentence above,since other day in class i was told that is possible to set multiple gateways on host pc.yet according to netacad its not possible.and not sure how would one would do that,but wouldnt dhcp in place be able to provide default gateway if one router was taken out and another router with configuration in place would start receiving requests to be back and running,now not going here for scenario of large business that cant afford hours time out,but am i correct in theory that one could have backup router sitting and would need just to switch it on,configured previously thus in such way hosts would be able to get another or same default gateway out.

read more into the chapter and it mentions that is static Ips that suffer if router goes down,so wondering is there a way to make more then one DF gateway on servers or pcs or was i mislead.

Nans

Ok lets think you are speaking about this


Now if i understand you correct you say when the Router 1 fails the DHCP server is gonna send the address of router 2 to the hosts, which to my knowledge will not happen (anyone please correct me if i am wrong).

Let me explain this to my knowledge and I think this is what you are trying to ask.

In a switched network, each client receives only one default gateway. There is no way to use a secondary gateway, even if a second path exists to carry packets off the local segment.

This means even though there are two routers attached that switch the one configured on the DHCP server is sent to the hosts, there may be load balancing i guess but i don't know if DHCP server can do it. So in case of failure even though there is an alternative to sent packets the hosts still keep sending to the unreachable gateway and the switch as we know only switch doesn't inform the hosts about the alternative path.


Now the back up we are speaking comes by using the load balancing protocols like First Hop Redundancy protocol (FHRP), VRRP, GLBP. In this we configure the the routers in group and have a virtual address configured, and like OSPF on a broadcast link there will be elections about the active and standby which differs by the protocol you use any way. The DHCP will distribute this address as the gateway to the hosts. Now the routers will keep tracking others in the group and when the active goes down the standby will take over and do its job. The time we are speaking here is around 200 msec to 700 msec. So the hosts need not know anything other than the virtual address to which they keep sending the packets to and switch does the rest..

hope that cleared your doubt.

Priston

You can have a backup gateway using a lower route metric for the backup. All traffic will be sent to the first gateway unless it fails, then traffic will be sent to the backup.

ip route 0.0.0.0 0.0.0.0 10.0.10.1
ip route 0.0.0.0 0.0.0.0 10.0.20.1 50



You can also have 2 gateways (FHRP) where the 2 gateways share a virtual IP.
With this the host will only point to 1 IP since the IP is virtually shared using a FHRP.

pinkiaiii

thing that bothered me how that it was said its possible to have few default gateways,now ip route command seems sort of would be that way in theory,since tried to build lab on the fast but for some reason it still linked to first router even thou the two weren't connected directly,but once removed router packages got redirected to second one on PT.its not much of per exam question just thinking or real world scenarios,where say company would have one router and since its possible to do encapsulation thus many interfaces into one,wonder is there some sort of trick where if one of links on router went down other would kick in as in virtual router -and BTW it says this topic is outside ccna scope thus not really sure if its trickles from ccnp or general knowledge of whats possible.

james43026

pinkiaiii wrote: »

In a switched network, each client receives only one default gateway. There is no way to use a secondary gateway, even if a second path exists to carry packets off the local segment.

Simply not true. A windows PC can have multiple default gateways, albeit you would need to manually configure them, as I don't think DHCP can send more than one. But that is what we have FHRP's for, as there would be no second default gateway in the eyes of your end devices, even though there would be multiple paths off the LAN.

theodoxa

An FHRP uses a single Virtual IP Address for both physical gateways, so the client only receives a single default gateway. The FHRP takes care of determining which physical device the traffic should be sent through (HSRP, VRRP) or may even be able to load balance the traffic (GLBP).

As for the clients themselves, as indicated above it is possible to manually configure multiple gateways on a PC. Adding static routes to a PC can be useful when traffic to a specific network needs to go out a different gateway, but you can't change anything on the default gateway. Also, with ASA Firewalls, issues can arise from Asymmetric routing, which can be solved using a static route on the PC. You can even push an extra static route using DHCP option 33 or 121. The former only works with classful routes, but is supported on more devices whereas the latter supports classless routes, but may not be supported on some devices. Windows also has a RIPv1 Listener that you can install under Windows Features, but I have never been able to get it to work (learn routes).

james43026

theodoxa wrote: »

An FHRP uses a single Virtual IP Address for both physical gateways, so the client only receives a single default gateway. The FHRP takes care of determining which physical device the traffic should be sent through (HSRP, VRRP) or may even be able to load balance the traffic (GLBP).

As for the clients themselves, as indicated above it is possible to manually configure multiple gateways on a PC. Adding static routes to a PC can be useful when traffic to a specific network needs to go out a different gateway, but you can't change anything on the default gateway. Also, with ASA Firewalls, issues can arise from Asymmetric routing, which can be solved using a static route on the PC. You can even push an extra static route using DHCP option 33 or 121. The former only works with classful routes, but is supported on more devices whereas the latter supports classless routes, but may not be supported on some devices. Windows also has a RIPv1 Listener that you can install under Windows Features, but I have never been able to get it to work (learn routes).

I would also add to this, that HSRP can perform some load balancing as well, as it can have a different active router per VLAN. Allowing you to balance traffic across them.

pinkiaiii

thanks for replies guys.found short video on cbt nuggets that seems to explain it fast and in easy to implement way.will try to do it in lab if theres enough time since didnt give much tought to it,but seeing it work in progress seems quite efficient way and redundancy timeout is somewhat amazing.

Heres link for others:https://www.youtube.com/watch?v=Db0VfCnULxM&list=WL&index=85

OctalDump

Pretty sure this is just because that kind of set up is "out of scope" for CCNA.

DHCP only allows for one router address, Option 3. However, that address can be a virtual address shared by multiple devices.

You can specify multiple gateways (ie multiple routers), but at any moment only one will be the 'default' gateway. Some kinds of dynamic routing protocols can do load balancing, but even in this case a packet is still sent to one address or another.

But there's a whole lot more routing magic to learn which should shed some light on this for you.