The Border Gateway Protocol (BGP) is the routing protocol used to exchange routing information across the Internet. It makes it possible for ISPs to connect to each other and for end-users to connect to more than one ISP. BGP is the only protocol that is designed to deal with a network of the Internet’s size, and the only protocol that can deal well with having multiple connections to unrelated routing domains.

 

How BGP Selects Paths

A router running Cisco IOS Release 12.0 or later does not select or use an iBGP route unless both of the following conditions are true:

•The router has a route available to the next hop router:

•The router has received synchronization via an IGP (unless IGP synchronization has been disabled).

BGP bases its decision process on the attribute values. When faced with multiple routes to the same destination, BGP chooses the best route for routing traffic toward the destination. The following process summarizes how BGP chooses the best route.

1. If the next hop is inaccessible, do not consider it.

This decision is why it is important to have an IGP route to the next hop.

2. If the path is internal, synchronization is enabled, and the route is not in the IGP, do not consider the route.

3. Prefer the path with the largest weight (weight is a Cisco proprietary parameter).

4. If the routes have the same weight, prefer the route with the largest local preference.

5. If the routes have the same local preference, prefer the route that was originated by the local router.

For example, a route might be originated by the local router using the network bgp router configuration command, or through redistribution from an IGP.

6. If the local preference is the same, or if no route was originated by the local router, prefer the route with the shortest autonomous system path.

7. If the autonomous system path length is the same, prefer the route with the lowest origin code (IGP < EGP < INCOMPLETE).

8. If the origin codes are the same, prefer the route with the lowest MED metric attribute.

This comparison is only made if the neighboring autonomous system is the same for all routes considered, unless the bgp always-compare-med router configuration command is enabled.

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Note The most recent Internet Engineering Task Force (IETF) decision regarding BGP MED assigns a value of infinity to the missing MED, making the route lacking the MED variable the least preferred. The default behavior of BGP routers running Cisco IOS software is to treat routes without the MED attribute as having a MED of 0, making the route lacking the MED variable the most preferred. To configure the router to conform to the IETF standard, use the bgp bestpath med missing-as-worst router configuration command.

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9. Prefer the external BGP (eBGP) path over the iBGP path.

All confederation paths are considered internal paths.

10. Prefer the route that can be reached through the closest IGP neighbor (the lowest IGP metric).

The router will prefer the shortest internal path within the autonomous system to reach the destination (the shortest path to the BGP next hop).

11. If the following conditions are all true, insert the route for this path into the IP routing table:

–Both the best route and this route are external.

–Both the best route and this route are from the same neighboring autonomous system.

–The maximum-paths router configuration command is enabled.

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Note eBGP load sharing can occur at this point, which means that multiple paths can be installed in the forwarding table.

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12. If multipath is not enabled, prefer the route with the lowest IP address value for the BGP router ID.

The router ID is usually the highest IP address on the router or the loopback (virtual) address, but might be implementation-specific.

For more info, go to Cisco docs.

Cisco Games Arcade : On Cisco website, you can find very interesting Cisco online games. The games purpose is to teach a newbie in his journey to learning networking. Games include the following most popular games:

1)     The Cisco Mind Share Game

2)     Subnet Troubleshooting Game

3)     Cisco Multiplayer Challenge

4)     The Binary Game

5)     Cisco Edge Quest

6)     Network Defenders

7)     SAN Rover

8 )     Secure Volunteer

9)     The Cisco Subnet Game

10) Subnet Slingshot

11) Unified Communication Simulation Challenge

12) Wireless Explorer

13) IPC Rockin’ Retailer

14) Peter Packet

15) The Realm

 

You can find the above games on below Cisco page:

https://cisco.hosted.jivesoftware.com/docs/DOC-2608

 

Learning is Fun !

The output of “sh ip route eigrp” is shown in the below snapshot:

 

 See the Routing table and you will find all the 10 subnets in 10.0.0.0 network mentioned separately as shown below; (this 10 subnets include all the networks including the static routes and others in addition to EIGRP.So, don’t confuse

The difference between “span vlan 2 cost 10” and “span cost 10” is that the first command only applies the cost to vlan 2 while the second one does this change for all the existing VLANs. Its shown below:

RESULT of “spanning-tree cost 10” command

 

 

RESULT of “spanning-tree vlan 2 cost 10” command

 There is a bug in DYNAGEN that after restarting a switch when we create VALN, it gives the following error:

%  not enough space on flash to store vlan database. Trying squeeze … First create squeeze log by erasing the entire device

% error squeezing flash – (Missing or corrupted log)

To solve this problem, we need to erase flash. After this, you can create new VLANs successfully. Remember , after removing the flash:vlan.dat file, the current configuration of VLANs remain there until the switch has been restarted. So, just save running configuration to statup before closing the Dynagen. This will create a new vlan.dat file having old as well as new config. The following snapshot shows the flash erasing process: 

 

If the configuration revision of the client switch is more than or equal to the switch from which it receives VTP advertisements, it will not update its VTP database. To reset the configuration revision, just change the VTP domain to some dummy domain and then change it back to the correct domain. This will make the configuration revision back to zero. Below snapshot explains the solution to this problem:

 

MAC Reduction Feature: Switch has to allocate a different MAC address for each VLAN on which spanning-tree is enabled. Sometimes the numbers of MAC Addresses are less than the number of VLANs available. To fix this, we use MAC address reduction feature. In this we add “System ID extension” in the Bridge ID. So, the new priority becomes Priority = System ID Extension (VLAN ID) + Priority. One important thing, the Bridge priority can be anywhere between 0 & 65535 with MAC reduction feature disabled. But when MAC reduction feature is enabled, the priority can only be a multiple of 4096.

As obvious by name “MANUAL” method of IP assignment is shown up when you assign an IP address of a particular interface yourself using ip address command. It’s given in the below snapshot:

On the other hand “SLARP” is totally different. SLARP stands for Serial Line Address Resolution Protocol. When a router is reloaded, it sends DHCP DISCOVER requests in order to find a DHCP Server. If it cannot find a DHCP Server, it will search a BOOTP Server. And if it fails to find BOOTP Server, it sends RARP or SLARP requests. RARP will be used in LAN environments while SLARP is used in serial lines for assigning an IP address to a interface. Check the below snapshot for SLARP IP assignment: