CCNP Recertification

Posted on Friday, 16th September 2005 by

One FAQ I see on CCIE lab lists is How do I load balance two paths in EIGRP without using the variance command? It’s an interesting problem, because the solution illustrates how EIGRP works. First of all, the variance command assigns a multiplier that says when two routes are “close enough” to load balance. Normally, [...]

Posted in Routing | Comments (5)

Posted on Tuesday, 13th September 2005 by sean

Scaling Internal BGP can be a challenge. Every iBGP router must have a peer with every other iBGP router within an AS. Why? BGP uses the AS-PATH attribute for loop prevention. However, the AS is only prepended between eBGP neighbours, so it will never appear in iBGP. However, this means that you must configure n-1 [...]

Posted in Routing | Comments (0)

Posted on Tuesday, 2nd August 2005 by sean

Last week I started in on BGP Route Aggregation, used to reduce the number of routes advertised in the global tables and to increase stability. This article will look at fine tuning the advertisements sent out when using the aggregate-address command. First, I left off one method of advertising an aggregate, which is creating a [...]

Posted in Routing | Comments (0)

Posted on Tuesday, 26th July 2005 by sean

Route aggregation happens where multiple routes are rolled up into a series of shorter prefixes (ie 4 /24′s become a /22). The idea is to reduce the number of routes that need to be processed. Often, the specifics don’t matter. If I’m advertising the 4 /24′s as above and they all go the the same [...]

Posted in Routing | Comments (0)

Posted on Friday, 22nd July 2005 by sean

BGP receives path updates from its peers, and then makes a decision as to which route is best. This is the route that is potentially transferred to the IP routing table and advertised to peers. The synchronization rule also plays a part in this decision, and is investigated more closely. Cisco has the BGP Best [...]

Posted in Routing | Comments (1)

Posted on Friday, 10th June 2005 by sean

Continuing on from the previous article, BGP Path Attributes, I’ll cover the remaining popular path attributes. Multi Exit Discriminator The MED is an optional non-transitive attribute, which is the metric of BGP. Though other protocols rely almost entirely on the metric to calculate routes, the BGP metric is only used when there are two exit [...]

Posted in Routing | Comments (0)

Posted on Wednesday, 8th June 2005 by sean

BGP Path attributes describe a prefix being advertised. A previous article mentioned AS-Path, which is an ordered list of the autonomous systems to the destination network. There are many more attributes that describe a route, letting downstream people make informed decisions about how to route a packet. Path attributes are sent in an UPDATE message, [...]

Posted in Routing | Comments (1)

Posted on Tuesday, 31st May 2005 by sean

Before I can get to BGP Path Attributes, I realized I have to cover the mecanics of BGP. The BGP 4 RFC covers it in detail, Riverstone has good coverage, but I’ll summarize it below. BGP State Machine BGP goes through six states to establish an adjacency. Idle – incoming connections are refused, and the [...]

Posted in Routing | Comments (0)

Posted on Monday, 30th May 2005 by sean

Every BGP autonomous system (AS) is identified by a 16 bit unsigned integer. An AS originates route advertisements consisting of a prefix and various attributes. This article will look at the AS numbers, which will then lead in to path attributes. Like IP address space, the issuing of AS numbers is controlled by a registry, [...]

Posted in Routing | Comments (0)

Posted on Saturday, 16th April 2005 by sean

BGP, the Border Gateway Protocol, is an exterior routing protocol. This means that it is used between two entities, rather than OSPF and EIGRP, which are used within an entity. In routing language, an entity is an Autonomous System, or an AS. The BCSI exam has several items relating to BGP. Describe the features and [...]

Posted in Routing | Comments (1)