Questions IP addressing and switching feature ( Juniper switch and Cisco switch )

[bensonlei]

Hi, guy,

Sorry for my poor knowledge. A LAN network infrastructure:

A Juniper EX4550 (Layer 3 device) connects to some user switches ( C2960x ).

Configuration in EX4550:

set interfaces vlan unit 10 family inet no-redirects
set interfaces vlan unit 10 family inet address 172.3.11.1/24
set interfaces vlan unit 10 family inet address 172.3.12.1/24
set interfaces vlan unit 10 family inet address 172.3.13.1/24
set interfaces vlan unit 10 family inet address 172.3.14.1/24
set interfaces vlan unit 10 family inet address 172.3.11.3/24 primary

Interfaces configuration in User switches (C2960x ):

 interface Gi1/0/4

  switchport mode access

  switchport access vlan 10

TCP/IP setting in user workstation connects to the user switch :

IP:     172.3.13.100

Mas:  255.255.255.0

GW:  172.3.11.3

My Question:

1. Any idea TCP/IP setting in the user workstation works ( the default gateway is 172.3.11.3 )??

2. One VLAN in user switch can include subnets 11, 12, 13, 14 and the default gateway is only 172.3.11.3 (255.255.255.0 ) ?

3. This configuration will consume CPU power (overhead ) to route between the local default gateway (172.3.11.1, 172.3.12.1, 172.3.13.1,172.3.14.1) to 172.3.11.3,  instead of simple configuration in DHCP server ?

4. One hop count or two hop counts in Juniper switch as item 3 ?

Thanks

[chrihussey · Accepted solution]

Hello,

1. Any idea TCP/IP setting in the user workstation works ( the default gateway is 172.3.11.3 )??

ANS. The IP address of a workstation will determine the default gateway. So if 172.3.11.x, then 172.3.11.3. If 172.3.12.x then 172.3.12.1, if 172.3.13.x then 172.3.13.1, etc. A host's default gateway should be on the same subnet.

2. One VLAN in user switch can include subnets 11, 12, 13, 14 and the default gateway is only 172.3.11.3 (255.255.255.0 ) ?

ANS. Although probably not the best way to do things, you can assign multiple networks to a VLAN.

3. This configuration will consume CPU power (overhead ) to route between the local default gateway (172.3.11.1, 172.3.12.1, 172.3.13.1,172.3.14.1) to 172.3.11.3,  instead of simple configuration in DHCP server ?

ANS. Again not the best way to do things for communication between hosts in VLAN 10. It does require the VLAN interface to constantly redirect traffic. If there isn't much communication between devices in VLAN 10, the CPU hit will probably be negligible.

4. One hop count or two hop counts in Juniper switch as item 3 ?

ANS. Good question, my guess would be one hop, but not sure.

Hope this helps

[ITexpert]

Hello chrihussey,

Can you explain. how multiple networks work in one vlan.  How switch will determine the packet path from one subnet to another subnet ?

If it works then why we need intervlan routing.  I never seen multiple subnets in single vlan, In which scenario it will be best use.

Thanks in Advance

[chrihussey]

With multiple networks in a single VLAN, the switch will make the decision on how to forward a packet based on the mac address in the L2 header. So if a host in one subnet wants to reach a host in another subnet on the same VLAN, the host will probably send the packet to its default gateway mac. In some instances, since they share the same broadcast domain, I suppose a host could learn the mac of a device on another subnet and communicate directly, but that would not be the norm.

This configuration would not be considered inter VLAN routing as there is only one VLAN involved. It is more redirection than anything else. If each subnet was in its own VLAN, with each its own L3 VLAN interface, then inter VLAN routing would be necessary for communications.

[ITexpert]

Hello Chrihussey,  

Thanks alot,

If both works then why everybody always use intervlan routing instead of this scenario.

[chrihussey]

Because in this scenario, there are 4 different networks, up to 1000 hosts, all sharing the same broadcast domain. So as each host/device sends a broadcast, it is seen by all, even those not in the same network. This makes the environment very chatty and potentially "noisey" as hosts are constantly having to listen and discard traffic that is irrelevant.

By assigning each network to its own VLAN, this limits the broadcast domains and makes for a quieter network, allows for greater control and security if needed. It is generally the preferred method.

Regards

[bensonlei]

Hi, guys,

You guys' discussions are very appreciated and enhance my knowledge.

More questions:

1. each host finds the its default gateway ( like 172.3.13.1/24 ) firstly, and then forward to the big default gateway (172.3.11.3/24) - two processes ?, or just only one default gateway ?

2. in item1, in layer 2 concern, each host holds only MAC address of 172.3.11.3/24 and so only one broadcast domain ?

3. the mechanism between a host ( like 172.3.13.10/24 ) to 172.3.11.3/24 is switching, not routing ? ( I mean the forwarding path 172.3.13.10 ---> 172.3.13.1 ---> 172.3.11.3, due to the host mask is 255.255.255.0 )

Again, sorry for my too questions:

1. Due to my network is using this configuration (ex-people's masterpiece), it is only Juniper feature or it can also be configured in Cisco Layer 3 device ( please be noted that these subnets are not continuous ) ?

2. or for my own good and future support, better to follow the normal network theory ( one Class C  with one default gateway; like 172.3.13.0/24, GW = 172.3.13.x, and one VLAN ) ?

Many thanks in advance.

Thanks

[chrihussey · Accepted solution]

Hello,

1. each host finds the its default gateway ( like 172.3.13.1/24 ) firstly, and then forward to the big default gateway (172.3.11.3/24) - two processes ?, or just only one default gateway ?

ANS. A host will find the default gateway associated with its subnet, if the next hop is a network on the same interface if will simply be redirected, one process. If to a network that is not on the same interface it would be forwarded to the next hop / interface. So to forwarding to the next hop is one process, but then it becomes the next hop interface to decide how to forward the packet.

2. in item1, in layer 2 concern, each host holds only MAC address of 172.3.11.3/24 and so only one broadcast domain ?

ANS. As stated earlier, the default gateway for a network should be an IP on that network. The fact that multiple networks are assigned to a single interface / broadcast domain only means all will see everyone else's MAC. However, with few exceptions, hosts will need to go to their respective gateways to get to anything off their subnet.

3. the mechanism between a host ( like 172.3.13.10/24 ) to 172.3.11.3/24 is switching, not routing ? ( I mean the forwarding path 172.3.13.10 ---> 172.3.13.1 ---> 172.3.11.3, due to the host mask is 255.255.255.0 )

ANS. It's routing for the most part. Since the 172.3.13.1 and 172.3.11.3 share the same interface it's all in and out the same interface. Your still routing between subnets.

1. Due to my network is using this configuration (ex-people's masterpiece), it is only Juniper feature or it can also be configured in Cisco Layer 3 device ( please be noted that these subnets are not continuous ) ?

ANS. It can be done on Cisco switches, you just define any additional subnets as "secondary".

!
interface GigabitEthernet0/2
 ip address 172.18.26.1 255.255.255.0 secondary
 ip address 172.18.24.1 255.255.254.0

!

2. or for my own good and future support, better to follow the normal network theory ( one Class C  with one default gateway; like 172.3.13.0/24, GW = 172.3.13.x, and one VLAN ) ?

ANS. Yes that generally is the way to do things. There are always exceptions, but a good rule to follow.

Hope this helps