The second thing the book covered was the inside guts of a switch. How it does exactly what it does. Which for me was just really cool. It covers the different ways Cisco has figured out how to make things work and work fast. Interesting that the CAM, TCAM, and FIB tables look up's all happen at the same time and all in hardware. Bravo.
If you're a little confused on what CAM, TCAM, and FIB tables are I'll give you a short rundown.
CAM - Content Addressable Memory: This table holds all of the MAC address information the switch has. So if a packet arrives with the destination of 000.0a9.3b9.01c then it looks that MAC address up in the CAM table. If it finds a match it spits the packets out the corresponding interface. If that MAC was not found then it relies on a "best effort" way of forwarding where it repeats that packet on every interface on the switch (excluding the one interface the packet originated from) hoping to find it's match. CAM table's can be manually populated (Putting every mac off of each interface into the CLI manually) or it can automatically populate it's table my adding the source address of ethernet frames and it's corresponding interface to the CAM table.
TCAM - Ternary Content Addressable Memory: There are two TCAM tables. One for access lists (ACLs) and one for quality of service (QOS). It takes the access list you put in and creates sort of a "matching string" to match incoming packets to. This table is populated when you add or edit access lists and/or quality of service policies.
FIB - Forwarding Information Base: The FIB (Which is really fun to say) is the "Layer 3 CAM" if you will. It contains the routes the switch knows about. This table is populated either when you manually add a route or if routes are updated via a routing protocol.
This is what I've learned/refreshed today.
Hope this helps,
Matt
hi matt
ReplyDeletethanks for the explanation but could you give a more detailed info about the TCAM.
cause the book has a very difficult to understand explanation, where is yours is simple and clear.
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