BGP Path Attributes (PA)

小弟把自己看CISCO的官方介紹PA的簡報其中的文字列表並排序如下，也附上自己的心智圖給朋友們參考:
Refer link: http://www.cisco.com/c/dam/en_us/training-events/le31/le46/cln/qlm/CCIP/bgp/understanding-bgp-path-attributes-3/player.html

BGP Path Attributes (PA)

Define: BGP metrics attached to a BGP route.

Page-2_Objectives:

Upon completing this module, you should be able to do the following:

List BGP path attributes and the functionality of each attribute

-Describe the concept of BGP path attributes.

-Explain the difference between mandatory and discretionary wellknow BGP attributes.

-Explain the difference between nontransitive and transitive optional BGP attributes.

-Describe the functionality of the AS-Path attribute

-Describe the functionality of the next-hop attribute.

Page-3_BGP Path Attributes:

-BGP metrics are called path attributes

-BGP attributes are categorized as "well-known" and optional.

Each BGP update consists of one or more IP subnets and a set of attributes attached of them.

Some of these attributes must be recognized by all BGP implementations.

These attributes are called "well-known BGP attributes" Attributes that are not well-known are called "optional".

These could be attributes that are specified in a later extension of BGP or even in private vendor extensions that are not documentd in a standard document.

Page-4_Well-known BGP attributes:

Well-known attributes are divided into mandatory and discretionary.

-Mandatory well-known attributes must be present in all update messages.

-Discretionary well-known attributes are optional; they could be present in update messages.

-All well-known attributes are propagated to other neighbors.

There are three specific well-known attributes that must be present on every update.

There are the next-hop, AS-path, and origin attributes and are referred to as "mandatory well-known attributes."

Other well-known attributes may or may not be present. depending on the circumstances under which the updates are sent and the desired routing policy.

The well-known attributes that could be present, but are not required, are called "discretionary well-known attributes."

When a router receives a BGP update. it analyzes the attached attributes and compares them with the attributes and compares them with the attributes received from a different source.

The router then decide which source has the best is propagated, along with its well-known attributes, to other BGP neighbors.

Page-5_Mandatory Well-Known BGP Attributes

1- The origin of a BGP route

--i Route originated in an IGP

--e Route originated in EGP

--? Route was redistributed into BGP

2- AS-Path

Sequence of AS number through which the network is accessible.

information about an IP subnet passes over an autonomous system border. When a router first originates a route in BGP, the AS-Path attribute every time information about an IP subnet passes over an AS boundary, the transmitting AS prepends its own AS number to the AS-Path.You can trace the sequence of autonomous systems using the AS-Path attribute.

3- Next-hop

IP address of the next-hop router

A router modifies the next-hop attribute as a route passes throught a network. This attribute indicates the IP address of the next-hop router.

Page-6_Discretionary Well-Known BGP Attributes:

1- Local preference:

--Used for consistent routing policy within AS

Local preference is using in the route selection process.

This attribute is carried only within an AS.

Router prefers a route with a local preference value to a route with a low value.

By default, routes receivced from a peer AS are tagged with the local preference set to a value of 100 before they are entered into the local AS.

If this value is changed through BGP configuration. the BGP selection process is influenced. Because all routers within an AS receive the attribute along with the route, a consistent routing decision is made throughout the AS.

2- Atomic aggregate:

--Informs the neighbor AS that the originating router aggregated routes.

The atomic aggregate attribute is attached to a route created from route summarization (Called "aggregation" in BGP). This attribute signals that information present in the original routing update may have been lost when the updates were summarized into a single entry.

Page-7_Optional BGP Attributes

Optional BGP attributes are transitive or non-transitive

1- Transitive optional attributes:

-- Propagated to other neighbors if not recognized; Partial bit set to indicate that the attribute was not recognized.

2- Non-transitive

-- Discarded if not recognized.

Recognized optional attributes are propagated to other neighnors based on their meaning (not constrained by transitive bit)

When a router receives an update containing an optional attribute, it checks whether its implementation recognizes the attribute.

If it does, the router should know how to handle it and whether to propagate it

if the router does not recognize the attribute, the BGP implementation should look for the transitive bit in the attribute code.

Some attributes, although not recognized by the router, might still be helpful to upstream routers and should be propagated.

These attributes ( called "transitive optional attributes" ) are propagated even when they are not recognized.

If a router propagates an unknown transitive optional attribute, it sets an additional bit in the attribute header, called the "partial bit" to indicate that at least one of the routers in the path did not recognize the meaning of a transitive optional attribute.

Other attributes, Called "non-transitive optional attributes", might be of no values to upstream routers if a router earlier in the path does not recognize them. Router that do not recognize these attr

Page-8_Optional BGP Attributes (Cont.)

Non-transitive attributes

1- Multi-exit discriminator

-- Used to discriminate between multiple entry points to a single AS.

Transitive attributes

1- Aggregator

-- Specifies IP address and AS number of the router that performed route aggregation.

2- Community

-- Used for route tagging,

One of the non transitive optional attributes is the multi-exit discriminator (MED) attribute, which also influences the BGP router selection process, Whenever there are several links between two adjacent AS, one AS can use the MED attribute to tell another AS to prefer one of the links for specific destinations.

Aggregator identifies the AS and the router within that AS that created a route summarization, or aggregate.

Community is a numerical value that can be attached to certain routes as they pass a specific point in the network. The community value can be examined by other routers at different points in the network for filtering or route selection. BGP configuration may cause routes with a specific community value to be treated differently than others.

Page-9_AS-Path Attribute

-- The AS-path attribute is empty when a local route is inserted in the BGP table.

-- The AS number of the sender is prepended to the AS-path attribute when the routing update crosses AS boundary.

-- The receiver of BGP routing information can use the AS-path attribute to determine through which AS the information has passed.

-- An AS that receives routing information with its own AS number in the AS path silently ignores the information.

The AS-path attribute is modified by an edge router every time information about a particular IP subnet passes over an AS border.

1.When a router first originates a route in BGP, the AS-path attribute is empty.

2.The local AS number is prepended to the AS path each time the route crosses an AS boundary.

There are several consequences of this behavior:

When you examine BGP routes, the AS path can be interpreted as the equence of AS a route must follow to reach the intended network.

The AS that originally injected the route into BGP is always found at the rightmost end of the AS path. it is easy to distinguish local routes from routes received from other AS - BGP routes with an empty AS path were injected into BGP from within the local AS.

The AS-path attribute is also used to avoid routing loops. When a router receives a BGP update, it checks the AS-path attribute and looks for its own AS number.

If it finds that number in the AS path, the route has already crossed the local AS and the router has a routing information loop. The route is silently ignored.

Page-13_Next hop attribute

1- indicates the next-hop IP address used for packet forwarding.

2- Usually set to the IP address of the sending External Border Gateway Protocol (BGP) router.

3- Can be set to a third-party IP address to optimize routing.

The BGP next-hop attribute identifies the IP address a router uses to forward packets toward their destination, and that is also announced in BGP routing updates. In most cases, sending routers set the next-hop attribute to its own IP address. There are cases, however, where the next-hop address points to a third router.

#CCNP
#Cisco
#MED
#Path Attributes
#AS-Path
#Next-hop
#2017

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