Broadcast in HSRP Topology

[amolwaghmare]

Hi ,

I just wanted to know how will the broadcast behave in case of hsrp topolgy,I have attached the diagram and shown with arrows that broadcast will flow !

Any suggestions to avoid this?

[ankbhasi]

Hi Amol,

Your arrows indicate correct behavior. Other than broadcast traffic for respective vlans , hsrp hello packets will also be treated as broadcast on infra switches and will follow the same path.

Regards,

Ankur

[Jon Marshall]

Hi

Not sure what your question is ?

If the vlans exist on both distribution switches and the distribution switches are connected via a L3 link as they are in your setup then HSRP traffic will have to flow via your layer 2 access-layer links.

If you don't want this to happen you need to make your link between ypour 2 distribution switches a L2 trunk link that carries all the vlans.

Jon

[amolwaghmare]

Hi jon,

I think that Multicast packet can go through L3 link also, only the point is that the multicast packet cannot cross more than 1 hop of L3 device.

Hi ankur,

If i am correct then the broadcast can bring the network down !

What is the use of HSRP ? how can i avoid it?

[ankbhasi]

Hi Amol,

If you are talking about HSRP hello packets yes they will be broadcast but not in that extent that it may bring the network down. It is very normal behavior. HSRP is used as gateway redudancy protocol and good to have in your network. As far as you mentioned HSRP hello packets will always take the path via your access switch and not via the L3 link between your distribution switches.

Multicast packet can definetely go through L3 links if they have available routes in multicast routing table but HSRP hello packets have TTL 1 and will always stay in their subnet.

HTH

Ankur

*Pls rate all helpfull post

[amolwaghmare]

Hi Ankur,

TTL 1 means that it can go to 1 hop of L3 device means to directly connected devices?

Also i was just guessing what will be effect of unicast flooding in my network by any pc ?

Any PC with viruses and trojans can bring my network down ?

[s.arunkumar]

hi,

hsrp hello packs are multicast to des 224.0.0.2 on port 1985,and the ttl is 1.

then if the L3 link can carry the multicast and switch is directly connected ,then i think for hsrp hello,the packet will take L3 link,and for all other broadcast it takes the path as in the diagram.

Unicast flooding,will be also forwarded via all ports,hence should take the path as in diagram for a vlan..

[Jon Marshall]

Hi

As Ankur has said, HSRP multicast packets stay with the local subnet because they have a ttl of 1.

So from your diagram you have a layer 3 link between your 2 distro switches. Taking one of the vlans as an example - vlan 20.

Vlan 20 is active on the left hand switch (SW1) in your diagram. So the standby interface will be on the right hand switch (SW2).

If the link between your 2 switches was a layer 2 trunk then the HSRP packets would be forwarded over that trunk because remember HSRP stays within the subnet.

But the link is a L3 link. So vlan 20 interface on SW1 issues an HSRP packet with a TTL of 1. It cannot be sent across the L3 link because it would have to be routed and and soon as it is routed the TTL is decremented down to 0 and so the packet is dropped.

The HSRP packets will however go across the L2 uplinks down to the access-layer and back up to SW2.

Jon

[s.arunkumar]

hi jon,

Thanks for clearing that and Sorry if i am asking any blunder,but just to clear my doubt.

Is this because the virtual address subnet and the L3 link address subnet are diffrent?

[amolwaghmare]

Hi Guys,

Sorry Jon but i completely disagree with you.By googling i found this from searchnetworking.techtarget.com:-

Using the multicast IP protocol, the TTL value indicates the scope or range in which a packet may be forwarded. By convention:

0 is restricted to the same host

1 is restricted to the same subnet

32 is restricted to the same site

64 is restricted to the same region

128 is restricted to the same continent

255 is unrestricted

So i think the packet will go through the L3 link only , it will not cross the next L3 hop as when the packet reaches to next hop its TTL will be decremented to 0 from 1 and when the L3Switch detects TTL 0 packet it drops it!

Correct me if wrong!

Thanks

**************************************

RATE USEFUL POST

**************************************

[shrikar.dange]

hi,

In my opinion the router/L3 switch decrements the TTl before forwarding it to the next hop.Correct me if i am wrong, so it will not go through the L3 link.

regards,

shri :)

[amolwaghmare]

i think when L3switch sends multicast hello packets on L3 link it will send with TTL value 1 and the next L3switch will decrement it!

[ankbhasi]

Hi Amol,

Please do not take me wrong but this is very basic that TTL 1 is restricted to same subnet and it will not cross its subnet. Now you layer 3 virtual interface where you can configured HSRP wll generate HSRP hello packets with TTL as 1 and your L3 link which connects 2 distribution switches is different subnet then how will your HSRP HELLO PACKET cross its subnet?

Also HSRP hello packet is reserve address and as per standards ip multicast routing table never holds any reserve addresses in its routing table then how will it know it has to take l3 link?

It will remain in its own subnet and will follow the layer 2 path from your access switch to second distribution switch.

HTH

Ankur

[amolwaghmare]

Thanks Ankur,

I forgot that HSRP is created on vlan and the message will flow through the vlan only and not through another subnet.

But here comes the question what is the use of ttl in hsrp then?

Is that if multicast packet ecounters L3 switch in same vlan then the message will not be forwarded to next devices?

!!

[ankbhasi]

Hi Amol,

TTL is a fild in IP header so cannot be ignored and the reason it is set 1 is because HSRP messages i.e hello packets are to communicate between 2 nodes in same subnet and tell one node who is active and who is standby. It is not designed to cross the layer 3 link and travel on different subnet.

The reason we made it to 1 is because there is no use of HSRP packet to leave its subnet and move around into different subnet.

Hope I am able to write my thaughts well explained?

Regards,

Ankur