Border Gateway Protocol BGP4 (2)

(1)

159.334 Computer Networks

Border Gateway Protocol – BGP4 (2)

Professor Richard Harris

(2)

Presentation Outline

(3)

Computer Networks

Computer Networks--1/1/33

Learning Objectives

You will be able to:

Discuss the operation of the BGP4 protocol

(4)

References

Tanenbaum, “Computer Networks”, 4th _Edition

Forouzan, “Data Communications and Networking”, 4th

Edition

Cisco CCNA1 Module 10 - part 1

Stallings, William 2000 ‘Data and Computer Communications’, Prentice Hall, Sixth Edition

(5)

BGP Path Selection Overview

Unlike other routing protocols, BGP doesn’t use shortest path technology.

BGP is regarded as a path vector protocol since it works by sending route advertisements.

A route advertisement indicates the reachability of a network, viz:

A network address and a netmask representing a block of contiguous IP addresses.

It also contains the list of all the transit AS’s that must be used to reach the announced network.

(6)

BGP Attributes – 1

Unlike other routing protocols, BGP does not use a single routing metric to decide the preferred

route.

It uses multiple parameters that have different degrees of significance as well as differences in the way they are distributed within BGP.

These metric parameters are called path attributes.

Attributes are classified into two types:

Well-known

(7)

Computer Networks Computer Networks--1/1/77 159.334 Computer Networks 159.334 Computer Networks

BGP Attributes – 2

Attributes Well-known Optional

(8)

Summary of Attributes

The following attributes have been identified for use with BGP: AS-Path Origin Next-hop Weight Local Preference

(9)

Simplified Operation of BGP Router[3]

Inbound filter Attribute manipulation Inbound filter Attribute manipulation Inbound filter Attribute manipulation Outbound filter Attribute manipulation Outbound filter Attribute manipulation Outbound filter Attribute manipulation BGP routing table BGP Decision Process 1. Highest LOCAL-PREF 2. Shortest AS-PATH 3. Lowest MED

4. eBGP over iBGP

5. Nearest IGP neighbour

Forwarding table ….. Peer₁ Peer_N ….. Peer₁ Peer_N Selects acceptable advertisements Select best route

based on decision list. Place in the forwarding table

(10)

Brief Review of Attributes

The following slides will review what is meant by the key attributes that have been identified for use with BGP.

At the conclusion of these discussions, a hierarchical structure will be presented that shows in more detail the policy based approach used to identify routes employed by BGP.

(11)

Computer Networks--1/1/1111 159.334 Computer Networks

AS-Path Attribute – 1

As a BGP update passes through an AS, that AS number gets appended as an update.

So an arriving update at a router will have passed

through a number of AS’s and these will be stored in the AS-path

The AS-path attribute is a well-known mandatory

(12)

Sequence of AS’s a route has traversed.

Provides a mechanism for loop detection.

Policies may be applied based on the AS path.

Local AS added only when sent to external peer.

* Shortest AS path preferred

(13)

Origin

This attribute is included in all BGP routing updates and its purpose is to indicate the origin of the path

information. It is permitted to have one of only three values, viz:

IGP: Network was redistributed from and IGP to BGP EGP: The origin is EGP (almost obsolete now)

INCOMPLETE: The origin is unknown.

(14)

Next-hop

It is the IP address of the next hop associated with a BGP update.

The Next-hop attribute is a well-known and mandatory attribute. Router A will advertise

(15)

Weight

This influences path selection from a router when more than one route to the destination is present.

It is configurable on a per-neighbour basis but has no significance outside of that router.

It is classified as an optional attribute and is not transitive

The default weight on a Cisco router for a route that is originated by the local router is 32,768 and it is 0 for all other routes.

(16)

Local Preference

An often-used attribute, local_pref (normally 100) overrides AS_PATH, and is transitive throughout your network. It is never

advertised to an eBGP peer.

For example, you can

express the policy “prefer private interconnects” by setting the local_pref to 150

and leaving all other peers at

(17)

Metric (Multi Exit Discriminator –

MED)

Indication to external peers of the preferred path into an AS.

Affects routes with the same AS path. Advertised to external neighbours

Usually based on IGP metric

* Note that the lowest MED is preferred

The MED (multi-exit discriminator) is a commonly used attribute. It comes after the AS_PATH in

evaluation sequence.

(18)

Community

This attribute is a means of grouping a set of

destinations so that a common policy can be applied to them.

The attribute is an optional transitive attribute. (If a

router receives this and doesn’t understand it it simply passes it on transparently in its own update.)

(19)

Computer Networks Computer Networks--1/1/1919 159.334 Computer Networks 159.334 Computer Networks

BGP Attributes – Summary

Attributes Well-known Optional

Mandatory Discretionary Transitive Non-transitive AS-Path

Origin Next-hop

(20)

BGP Route Selection - Detail

The following hierarchical rule set is followed for BGP: The path is ignored if the next hop is inaccessible

Highest weight is preferred (if this optional proprietary attribute is in use on the router).

Highest local preference is preferred.

Prefer a route that has originated from the router that is advertising it.

Shortest AS path.

This means the path with the least number of AS entries in it.

Lowest Origin Code (IGP > EGP > INCOMPLETE). Lowest Metric or MED value

External path is preferred to internal. The administrative distance for an EGP path is 20 and 200 for an IBGP route

(21)

Comment on Selection Criteria

These criteria act as filters, and the Nth _{criterion is}

only evaluated if more than one route has passed the (N – 1)th _criterion.

It should be noted that most BGP implementations allow the network administrator to optionally

disable some of the criteria of the BGP decision process.

(22)

Control of Outgoing Traffic [3] - 1

Two approaches have been used:

1. Use local_pref to be manipulated based on passive or active measurements.

These solutions usually measure the load on each inter-domain link, and some rely on active measurements to evaluate the

performance of inter-domain paths.

Based on these measurements and some knowledge of the Internet topology (obtained either through a central server or from the BGP router to which they are attached), they attach appropriate values of the local_pref attribute to indicate which route should be

(23)

Control of Outgoing Traffic [3] - 2

As shown in the slide depicting BGP router operation, the BGP decision process will select the nearest IGP neighbour when comparing several equivalent routes received via iBGP.

For example, suppose router R₂₇ receives one packet whose destination is R₄₅: The

BGP decision process of router R₂₇will compare two routes toward R₄₅, one

received via R₂₈ and the other received via R₂₆.

By selecting router R₂₈as the exit border router for this packet, AS2 will ensure that this packet will consume as few resources as possible inside its own network.

If a transit AS relies on a tuning of the weights of its intra-domain routing protocol this tuning will indirectly influence its outgoing traffic.

R₅₁ R₁₁ R₁₄ R13 R₁₂ R21 R₂₇ R₂₂ R₂₃ R₂₅ R₂₈ R₂₄ R₂₆ R₃₁ _R 32 R₃₃ _R 34 R₃₅ R36 R₆₁ R₄₃ R₄₄ R₄₅ R₄₂ R₄₁ AS 5 AS 1 AS 2 AS 3 AS 4 AS 6

(24)

Control of Incoming Traffic [3] – 1

The first method that can be used to control the traffic that enters an AS is to rely on selective advertisements and announce different route advertisements on different links.

Another method would be to allow an AS to indicate a ranking among the various route

advertisements that it sends. Based on the utilisation of the length of the AS-path as the third criteria in the BGP decision

process, a possible way to

influence the selection of routes by a distant AS is to artificially

increase the length of the AS path

(25)

Border Gateway Protocol BGP4 (2)