How to Control Routing Table from exceeding its size theoretically or practically

Question

I understand that switches learn from incoming traffic and thus builds its switching table. Does that also apply for routers? If so, how routing table can avoid increasing the routing table size since this is an automatic process? Maybe cache algorithm!! I don't know .. Kindly, can an expert enlighten us about that.

Thank you.

Answer 1

I understand that switches learn from incoming traffic and thus builds its switching table. Does that also apply for routers?

No, routers only learn routes when they are advertised to them by other routers, by different routing protocols.

If so, how routing table can avoid increasing the routing table size since this is an automatic process?

Now though this is a semi-automatic process i.e. Once you have started running a protocol you are accepting all the routes being advertised by the peer router. So you can have certain methods by which you can limit number of routes and they mostly rotate around route-map:

• apply route-map(is a filter which you can apply between you and your peer) to accept only limited number of prefixes.
• apply route-map to accept only shorter prefixes and reject longer prefixes i.e. If you give /22 to your customer and you apply a route-map to accept no longer than /23 routes. You will have a maximum of 3 prefixes coming from that customer. A /22 and two /23.

Now as Ron said if you are planning to get internet routing table you need good CPU and RAM(greater than 4GB) because the routes are ever increasing on the internet and no you cannot apply caching mechanism because you are not asking for routes or routes are not constantly getting advertised by your peers( that you can monitor them and cache them) they are sent in updates(which maybe far and few and in large time intervals) by peers.

Hope this helps!

Answer 2

This depends on the router model and software. Some routers will just stop adding to the routing table, some will throw an exception and log the problem, and some just crash.

RAM is so cheap that modern routers are really only challenged by the full Internet routing table, which is huge. If you are designing a network, and you know that you will be requesting the full Internet routing table (fairly rare), you specify a router with the CPU and RAM to handle it.

Answer 3

I personally don't know if a way to do this, but with BGP hundreds of thousands of routes potentially get sent to routers with 2gb of RAM.

Tier 3 Routers would most likely get overloaded and crash, when sent over 1 million routes with potentially TB of data transferring through.

I therefore assume because there is no implementation in place to prevent overload crash that it applies to a switch as well.

Answer 4

If your are facing problem of increasing in size of routing table size possible options are

1. . Auto summaries the routes . This can reduce the size of routing table to optimal level . .2) change the existing routing from to OSPF inter area and divided networks into multiple areas to optimise size of routing table .