Re: Weired question - same DOT1Q tag on two interfaces on same

Hey there,

This depends on the underlying L2 infrastructure. If it's all the same
switching domain, then separating broadcast domains is *desirable*.
Not having it separate could introduce unusual issues and log messages
- depending on different factors (routing protocols in use, traffic
being carried, multicast, etc.).

Now, as far as R2 goes, depending on the hardware platform you are
using, you may or may not be able to use the same VLAN tag on
different interfaces.

If you are dealing with any decent router (ISR, 7200, 12k, GSR, etc.),
this is no issue, as VLAN space is local to the interface. In other
words, you can have traffic tagged as VLAN 10 on Fa0/0 and Fa0/1 and
it being totally separate as far as R2 is concerned. On the other
hand, if you are dealing with 6500/7600 swouters which have global
VLAN space, you may not have te same tag on two separate L3
subinterfaces... unless you are using "WAN" line cards.

Hth.

--
Marko Milivojevic - CCIE #18427
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On Wed, Apr 14, 2010 at 17:05, Radioactive Frog <pbhatkoti_at_gmail.com> wrote:
> Folks,
> Here is what I am trying to do.
>
> all routers are 2911 or 2811 with routed interface (no switching module
> ESW).
>
> R1------------R2----------R3----------R4
>
> Scenario1 - which I always implement:
> -----------------------------------------
> R1 - R2 = VLAN 10 - 10.10.10.1/30
> R2 - R3 = VLAN 20 - 20.20.20.1/30
> R3 - R4 = VLAN 30 - 30.30.30.1/30
>
> Number of VLANs consumed in above scenario:
> R1 = 1 i.e. fast0/0.10
> R2 = 2 i.e. fast0/0.10 B and fast0/1.20
> r3 = 2 i.e. fast 0/0.20 and fast0/1.30
> and so on......
>
>
> To reduce the number of VLAN per router and keep the routing intact can we
> do this?
>
> Scenario#2
> R1 - R2 = VLAN 10 - 10.10.10.1/30
> R2 - R3 = VLAN 20 - 20.20.20.1/30
> R3 - R4 = VLAN 30 - 30.30.30.1/30
>
> Number of VLANs consumed in above scenario:
> R1 = 1 i.e. fast0/0.10
> R2 = 1 i.e. fast0/0.10 B and fast0/1.10
> r3 = B 1 i.e. fast 0/0.10 and fast0/1.10
> and so on......
>
> Did you noticed in above case R2 and R3 are only using 2 VLAN ID.
>
> I'd be interested to hear from you guys if you see any issue in there i.e.
> using vlan id 10 on B R2/R3 both fe0/0 and 0/1 interfaces.
>
> Note; No briding, need routing and shoudl be able to use all IOS Feature.
>
>
> The idea is in multi tented environment, we should be able to use vlan ID 10
> for a CUSTOMER1 on all the routers. Having vlan10 on R1 and 20 on r2-r3 and
> so on is B a bit messy. This approach can also save number of vlan consumed
> and can tanent more customers.
>
> I went thr the Dot1q header but not B sure how router treat this on
> interfaces.
> http://www.cisco.com/en/US/docs/ios/12_3t/12_3t4/feature/guide/gtunvlan.html
>
> Anyone running this kind of single VLAN in a production environment?
>
> -frog
>
>
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