05-31-2021 10:49 PM
Hi Folks,
Hope you are all safe and vaccinated, I have this question long time.
why we are preferring routers than a switch as L3 switches also do routing. What are the scenarios where we can use L3 switch instead of router with protocols like bgp, ospf and eigrp.
If we have small site and tight budget can we use a switch as router and what are the disadvantages over using it on Switches.
please explain based on protocols also
Thanks In advance..
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06-01-2021 02:05 AM - edited 06-01-2021 02:09 AM
Hello @prakashrajasekaran2807 ,
all low end switches are not able to perform NAT or to support a site to site IPSec VPN
This is the reason why when connecting a remote site we usually provide a router for the reasons above if the site is connected to the public internet.
A simple switch can act as an OSPF, EIGRP or BGP speaker.
If you have an MPLS L3 VPN service ( or a L2 service like EoMPLS or VPLS ) and access to the internet is performed at the central site a switch becomes a suitable choice for routing in the remote site as neither NAT or IPSEC VPN S2S are needed in a similar context.
Another aspect to be considered is that usually switches are very good in performance but only for the limited number of entries of their TCAM table.
Low end switches can handle hundreds or few thousands of IP prefixes.
Now a full BGP table is in the order of 880,000 routes and this makes almost all switches, unless last most powerful ones, not usable to receive a full table.
Routers are more scalable in terms of number of prefixes supported as it is enough to have enough RAM ( 8GB at least, 16 GB recommended) and a decent CPU to support a full table.
Hope to help
Giuseppe
06-01-2021 12:09 AM
? These protocols are routing-related hence will perform, at least , better on routing platforms.
M.
06-01-2021 02:05 AM - edited 06-01-2021 02:09 AM
Hello @prakashrajasekaran2807 ,
all low end switches are not able to perform NAT or to support a site to site IPSec VPN
This is the reason why when connecting a remote site we usually provide a router for the reasons above if the site is connected to the public internet.
A simple switch can act as an OSPF, EIGRP or BGP speaker.
If you have an MPLS L3 VPN service ( or a L2 service like EoMPLS or VPLS ) and access to the internet is performed at the central site a switch becomes a suitable choice for routing in the remote site as neither NAT or IPSEC VPN S2S are needed in a similar context.
Another aspect to be considered is that usually switches are very good in performance but only for the limited number of entries of their TCAM table.
Low end switches can handle hundreds or few thousands of IP prefixes.
Now a full BGP table is in the order of 880,000 routes and this makes almost all switches, unless last most powerful ones, not usable to receive a full table.
Routers are more scalable in terms of number of prefixes supported as it is enough to have enough RAM ( 8GB at least, 16 GB recommended) and a decent CPU to support a full table.
Hope to help
Giuseppe
06-01-2021 08:22 AM
Giuseppe nicely sums up the differences, when you might prefer a L3 switch over a router (generally the latter when you need features only found on them).
Another way to look at this, L3 switches (for many of the reasons noted by Giuseppe) are usually most suitable on LANs. Many of "newer" WAN technologies provide L2 connectivity, so often they too can be treated like LANs.
However, occasionally MetroEthernet like networks, sometimes have needs beyond what a typical LAN L3 switch offers. For such situations, there is a class of switches, MetroEthernet switches, that offer some additional features over LAN L3 switches, that address situations where a LAN L3 switch isn't quite "enough" while a router, especially a high performance model, is much, much more expensive.
That said, if you need to deal with full Internet BGP tables, most L3 switches cannot effectively deal with such large route tables. I.e. it's not something like BGP that's a problem, just capacity limits for number of routes that can be dealt with in hardware (also noted by Giuseppe).
One feature, not mentioned by Giuseppe, is L3 LAN switches generally have much less is the way of QoS features than a router. MetroE switches are sometimes better in supporting QoS features, but they too are weak in comparison to routers. This can become important because "WAN/MAN" links are often much, more expensive than LANs for "like" bandwidth and so congestion can be more likely with them.
06-01-2021 09:22 PM
Thanks. i have heard that most of this L3 switch with Dynamic routing protocols used in datacentre. what will be the usage over there?
Is it just route and expand there a network or something else.
06-01-2021 09:18 PM
Awsome Explanation.
06-01-2021 09:23 PM
Thanks. I have heard that most of this L3 switch with Dynamic routing protocols used in the datacentre. what will be the usage over there?
Is it just route and expand there a network or something else.
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