MLS routing

[eng.abdoehab]

why multi layer Switches can't routing to the wan ? and what is the big difference between wan routing and switching ?..

 i also need to know is the wan cables must be serial ?
 

[Peter Paluch · Accepted solution]

Hello,

why multi layer Switches can't routing to the wan ?

They can and they do, but it depends on the requirements for the WAN connectivity whether a particular MLS would be suitable. Lower Cisco Catalyst MLS types do not, for example, support NAT. To my knowledge, NAT is available on Catalyst 6500 and 6800 series, and perhaps on 4500 series as well. Lower types, such as Catalyst 3560, 3650, 3750, 3850 - they do not support NAT, and without NAT, you cannot connect your privately addressed network to internet.

Encryption such as IPsec is also extremely uncommon on MLS platforms.

Furthermore, the amount of memory, Ternary Content Addressable Memory (TCAM) and CPU power is important if the MLS is required to run BGP and learn or advertise hundreds or thousands of networks. Simpler MLS platforms do not have enough RAM and TCAM space to hold so many routing table entries, and they would not be usable for this purpose.

In addition, MLS platforms have limited options of various interface types. MLS switching is so lightning fast because you do not need to convert between data link layer types and frame formats: It is Ethernet in, Ethernet out. WAN connectivity, on the other hand, was done using other technologies, such as HDLC, PPP, ISDN, Frame Relay, DOCSIS, cellular, DSL, etc. Enterprise-grade MLS switches do not have such interfaces and do not even allow such interfaces to be installed. To my best knowledge, only Catalyst 6500 had line cards in which you could also install some of these typical WAN interfaces (there may be some additional platforms - don't take this as an exhaustive list).

and what is the big difference between wan routing and switching ?..

In plain packet forwarding, there is absolutely no change. The difference lies in additional services you require on the WAN edge - NAT, firewalling, encryption, etc., that require very complex dedicated hardware to be performed at speeds comparable with plain packet forwarding.

i also need to know is the wan cables must be serial ?

No, they do not need to be serial. However, in the past, they were the typical choice because Ethernet was not yet so common as a WAN technology. Nowadays, these standalone WAN technologies are fading and Ethernet is taking over.

Best regards,
Peter

[Joseph W. Doherty · Accepted solution]

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Posting

As Peter has already described, forwarding packets is provided by both routing and (L3) switching, how packet forwarding is actually accomplished, though, differs between the two.

Traditional routing takes the destination IP and searches across its route table for the most specific route.  Since network addresses are variable length, such searching has been more complex and difficult to implement in hardware, compared to "switching".

Switching usually takes a fixed sized destination (e.g. MAC) and searches for an exact match in tables with the same fixed sized entries.  Such searching is less complex and easier to implement in hardware than routing.

The difference between switching lookups and routing lookups, is such, MPLS started as "tag" switching, i.e. a fixed size tag was attached to the packet and subsequent routers could do a switch lookup rather than a routing lookup.

Today, modern hardware can often route packets and fast as they might be switched, but such hardware is often more expensive (of course, as Peter also mentioned, "routers" often have additional features not provided on L3 switches).

[eng.abdoehab]

best answer ever, thanks Peter i appreciate