interVLAN routing n00b question

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Jul 4th, 2009
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Hello experts,

im looking foward to route VLANs on my gateway instead of my switching infrastructure (image upgrade required), i'd be routing 2 data vlans and 1 voice VLAN, is there any performance implications by configuring VLAN routing on my gateway? (voice gateway 3845)


Thanks!!

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Giuseppe Larosa Sat, 07/04/2009 - 08:26
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Hello Itzcoatl,


>> is there any performance implications by configuring VLAN routing on my gateway? (voice gateway 3845)


Yes, there can be performance issues:

even the C3845 has GE ports is not expected to handle full GE traffic.


However, you are describing a branch office with only two data vlans and 1 voice vlan.


Another point is that the router is already configured as a voice gateway with interfaces to PSTN or SIP provider and probably SRST and if using MGCP MGCP fallback.

Or even it can be running CUCM Express.


if you don't run CUCME on the router I think you can do it if the traffic volume on the vlans is not huge.


In old times with flow based MLS a router could assist a lan switch with some additional feature cards in performing multilayer switching.

With CEF based multilayer switching that runs on modern IOS based switches this option is not possible anymore.


Hope to help

Giuseppe


Joseph W. Doherty Sat, 07/04/2009 - 09:49
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The 3845 is rated at 500 Kpps, which is good for about 100 Mbps (duplex), minimum sized packets, and no services. If all your routing is to/from a typical WAN link (DS-3 or slower), there shouldn't be much in the way of performance issue since the WAN bandwidth will be the bottleneck. However, if you expect to be doing LAN routing, for instance between your two data VLANs, the 3845 can be a bottleneck. If this is the case, you might want to consider a small L3 switch, even the 8 or 12 port 3560s will offer much more performance. (If you're doing both LAN and WAN, use the L3 switch for LAN routing and the 3845 for WAN routing.)

Joseph W. Doherty Sat, 07/04/2009 - 10:22
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". . . instead of my switching infrastructure (image upgrade required), . . ."


BTW, what's your switching infrastructure (now) that would require a upgrade? Many of Ciscos L3 switches support static and RIP routing in their base images. For a small branch, although OSPF and EIGRP are nice, you might do fine w/o them.

itzcoatl.m Sun, 07/05/2009 - 08:42
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thank you all,, your answers have been very helpful, we are using 3750 switches.

Joseph W. Doherty Sun, 07/05/2009 - 18:00
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". . . we are using 3750 switches."


Ah, then LAN routing best done on them. Again, if your routing topology is small, statics, RIP and/or RIPv2 routing might work for you; precluding the need to upgrade your 3750 image(s). This often is especially true with the 3750s since they stack which can often remove the need for dual routers in a redundant topology.

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