Lectures notes for 2010 - KTH

Routing Tables
• Aggregate IP addresses (i.e., exploit CIDR)
• more specific networks (with longer prefixes)
• less specific networks (with shorter prefixes)
• ⇒ smaller routing tables
• If each routing domain exports (i.e., tells others) only a small set of
prefixes, this makes it easier for other routers to send traffic to it
• Unfortunately this requires clever address assignments
• Some mechanisms lead to increased fragmentation
• Due to limited availability of addresses long prefixes (particularily /24) are scattered
geographically
• Increasingly sites are connected to multiple ISPs (for redundancy) i.e., Multihoming
- thus they have addresses from several different subnetworks
• Current routing tables have ~157,975 entries [7] (of which a large
fraction are /24 prefixes) with a growth rate of “18,000 entries per
year”[8].
There are a limited number of prefixes for Class A + B + C networks (2,113,664). If the longest prefixes which a backbone
router had to deal with were /24, then a table with 16,777,216 entries would be sufficient (even without aggregation) - each
entry only needs to store the outgoing port number! This would allow a direct lookup in a memory of ~26Mbytes - with
upto 256 outgoing ports.
Maguire Routing Tables 2: 11 of 52
maguire@kth.se 2010.03.21 Internetworking/Internetteknik