Home » Routing

Routing

BGP Weight Attribute

BGP weight attribute is a Cisco proprietary path attribute that is local to the router and is used to influence local path selection on a certain router. BGP weight is the first in the list: BGP weight is the first tie breaker for best path selection on a Cisco router, the attribute is not propagated to other routers in the BGP updates and only found on Cisco routers. The path with the highest weight is selected as the best toward a destination. BGP weight values: The weight can be a number from 0 to 65,535. Paths that the router originates …

Read More »

BGP Attributes List

BGP Attributes are pieces of information that a BGP router attaches to describe different prefixes included in it’s BGP update messages. There is a variable sequence of BGP attributes in every update message except for those that carries only withdrawn routes. Each attribute is a TLV that consists of attribute type, attribute length and attribute value. Each attribute type consists of two octets, one for flag and the other is the type as described briefly in the following section. BGP attributes have different distinctive types that defines how routes are going to use and propagate a certain attribute to it’s …

Read More »

BGP Attributes: Local Preference Attribute

The local preference attribute is a well-know discretionary attribute. This means local preference must be recognized by all BGP implementations, but will not exist in all BGP update messages; specifically it will not exist in E-BGP update messages. BGP local preference attribute is one of the most used attributes in BGP real world to influence traffic. Local preference attribute is a four octet field of information that is used to inform internal peers about the autonomous system internal preference for an advertised route.

Read More »

BGP Attributes: Atomic Aggregate Atribute

Atomic aggregate is a Well-known Discretionary attribute; it must be recognized by all BGP implementations and does not have to exist in all BGP updates. The purpose of the attribute is to alert BGP speakers along the path that some information have been lost due to the route aggregation process and that the aggregate path might not be the best path to the destination. When some routes are aggregated by an aggregator, the aggregator does attache its Router-ID to the aggregated route into the AGGREGATOR_ID attribute and it sets the ATOMIC_AGGREGATE attribute or not; based on whether the AS_PATH information …

Read More »

BGP Attribute Types and Flags

BGP attributes is an interesting subject of study.  BGP is a very flexible and extensible protocol and I like that, let’s see how flexible is that protocol when it comes to attributes handling. We all know that BGP has four types of attributes as listed: Well-known mandatory. Well-known discretionary. Optional transitive. Optional non-transitive. I am not going to explain them in this post, they are explained everywhere over the internet. However, in short well-known attributes must be recognized by all BGP implementations, some of them are mandatory and must be included in all update messages, others are discretionary and may …

Read More »

How to: use IS-IS overload bit

Overload bit is special bit in the IS-IS LSP used to inform the network that the advertising router is not yet ready to forward transit traffic.  The overload bit was first intended for signaling overload or resource shortage on specific router for the rest of the network. You can use the command set-overload-bit intentionally on specific router to signal other routers not to use it as a transit hop in their SPF calculations. Typically this is done for a temporary situation like an overloaded router due to memory or processing shortage and released when the router recovers from the problematic …

Read More »