Routing

Keep it simple with Route Summarization

Summarization lumps a series of subnets, or supernets as they are sometimes called, into a single address. This summarization of various subnets have several advantages. It can drastically reduce the size of your routing table and the recalculation of your network because your routing tables are so much smaller.

Unlike EIGRP and OSPF, RIP and IGRP protocols are two examples of automatic summarization because they do not send the subnet mask to the routing table so you can advertise 1 network instead of several networks.  Class C networks are usually subnetted by ‘borrowing’ host address bits and using them for a portion of the network.  You would enable router summarization with the auto-summary command in global configuration mode. An example of continuous subnets would be converting 211.30.192.0/24 through 211.30.195.0/24.

The end result that you would advertise would be 211.30.192.0/22.

Check out the article on IP Routing Protocols for more information.

Five ways that a Cisco router initiates a routing request out of an interface

As I’m sure you know, a switch switches and a router routes. Among other things, It routes packets of data and it routes requests. Listed below are five ways that a router will automatically initiate a routing request.
1. The router has been booted or rebooted.
2. A primary IP address was changed. An interface has several secondary IP addresses but only one primary IP address. If that IP address has been changed, the router will start a routing request out its interface.
3. Of course, if the interface goes down this will automatically cause a routing request.
4. If you make a change to the router’s configuration such as an IP address change.
5. If you issue the clear ip route command.

Four Routing Protocols that you should know

BGP: Border Gateway Protocol –  As defined in RFC 1771, BGP is an interdomain routing protocol. You would use BGP to exchange routing information between autonomous systems. It’s usually configured between two directly connected routers that belong to different autonomous systems and the routers must first become established neighbors. TCP Port 179 is reserved for the BGP protocol to establish connections with neighbors. For more information on BGP, see the article on Border Gateway Protocol documentation.

MBGP: Multiprotocol  Border Gateway Protocol – Bigger and better BGP. MBGP is an enhanced form of BGP which has IP routing information about other protocols. It also carries information about reachable and non-reachable destinations in the network layer (that’s Layer 3). For more, information, please see the MBGP article.

EIGRP: Enhanced IGRP  -  Just like MBGP is an enhanced form of BGP,  EIGRP provides  more compatibility than IGRP. EIGRP allows IGRP routes to be imported into EGRP, and vice versa, so it is possible to gradually add it into your existing IGRP network. For more information, see the EIGRP article.

RIP: Routing Information Protocol - A distance-vector protocol that uses hop count as its’ metric. This metric can be hops, bandwidth, delay, or multiple metrics.  Distance vector protocol periodically sends the neighboring routers its entire routing table to ensure network availability. RIP is used for routing traffic and is an interior gateway protocol (IGP), which means that it performs routing within a single autonomous system.  Just like IGRP, this is an old routing algorithm and could result in split horizon. For more, information, please see the RIP article.