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BGP Synchronization A BGP router with synchronization enabled will not advertise its iBGP learned routes to its eBGP peers unless it has learned or verified this route on its routing table through an IGP. This feature can be useful if in an AS not all routers are running BGP. The purpose of this post is to illustrate the difference between synchronization being enabled versus it being disabled. We will also take a look for some work arounds for the problems it creates. Take a look at the diagram below for the lab. R1 is on AS 1 while R2 and R3 is on AS 23. Now configure the router with the configuration below. R1 ! interface Serial1/0 ip address 192.168.12.1 255.255.255.0 serial restart-delay 0 ! router bgp 1 no synchronization bgp log-neighbor-changes neighbor 192.168.12.2 remote-as 23 no auto-summary R2 ! interface Serial1/0 ip address 192.168.12.2 255.255.255.0 serial restart-delay 0 ! interface Serial1/1 ip address 192.168.23.2 255.255.255.0 serial restart-d

TCL Ping Script Doing ping tests for lots of IP addresses can be tiring since you can't paste all the ping commands at the same time. You have to do it one at a time. All you need is Patience or you can opt for a TCL scripting language which is already available in Cisco IOS 12.2(25). At first, I thought you need to learn TCL scripting to be a CCIE but nah! :) All you need is the basics and if you are interested learning more about TCL you can click here . To access the tcl command line in Cisco router issue the "tclsh" command. For the ping script just modify the ip addresses what is shown below and it should be good. foreach address { 1.1.1.1 2.2.2.2 3.3.3.3 4.4.4.4 5.5.5.5 6.6.6.6 } { puts [ exec "ping $address" ] } The word "address" here is just a variable, you can substitute this with anything you want. Now lets try applying this to the Cisco router. R0#tclsh R0(tcl)#foreach address { +>(tcl)#1.1.1.1 +>(tcl)#2.2.2.2 +>(tcl)

BGP Best Path Selection Studying the BSCI for CCNP can be hard for the reason that you need to memorize some stuff like BGP Best Path Selection. Few months ago I found this mnemonic helpful to memorize the BGP Path Selection Process. It's easy as eating pancakes and it is We Love Oranges AS Oranges Mean Pure Refreshment. By taking note of the letters in caps, one will immediately remember how BGP selects the best path by the list below. “We Love Oranges AS Oranges Mean Pure Refreshment” W Weight  (Highest) L LOCAL_PREF  (Highest) O Originate  (local)  routes that are advertise through the "network" command or redistributed from an IGP. AS AS_PATH  (shortest) O ORIGIN Code  (IGP > EGP > Incomplete) M MED  (lowest) P Paths  (External > Internal) R RID  (lowest) Hi friends this is the easy way to remember path selection process of BGP

BGP Attributes Categories A quick copy-and-paste summary on BGP attribute categorization .  WELL-KNOWN, MANDATORY  AS-path:  A list of the Autonomous Systems (AS) numbers that a route passes through to reach the destination. As the update passes through an AS the AS number is inserted at the beginning of the list. The AS-path attribute has a reverse-order list of AS passed through to get to the destination. Next-hop:  The next-hop address that is used to reach the destination. Origin:  Indicates how BGP learned a particular route. There are three possible types -- IGP (route is internal to the AS), EGP (learned via EBGP), or Incomplete (origin unknown or learned in a different way). WELL-KNOWN, DISCRETIONARY  Local Preference:  Defines the preferred exit point from the local AS for a specific route. Atomic Aggregate:  Set if a router advertises an aggregate causes path attribute information to be lost. OPTIONAL, TRANSITIVE  Aggregator:  Specifies the router ID and

BGP AS-Path Prepending BGP is rich in features that you can have more control than on what IGP's offer however, you can only have control on how the traffic leaves your autonomous system and can't really control how other autonomous systems reach you. Other AS'es might have BGP policies that route the traffic in a way you don't intend it to go. You don't have control over those because, its their autonomous systems after all. However, there are work arounds which allow, an autonomous system affect the other autonomous systems, one of this is called BGP AS-path prepending. It is basically adding additional AS-paths by repeating your own AS number. Consider the diagram below. (Click image for a bigger view) By looking at the diagram, if you are familiar with BGP, the AS-path the networks from R4 will take towards R1 will be AS4, AS3 and then AS1. If all the attributes are set to the default values, most likely the AS-path attribute will determine which path to take.

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