Cisco Tips & Tricks

September 20, 2006

BGP- FAQ from Cisco’s website

Filed under: bgp, cisco, IP Routing, Router — ciscotips @ 8:52 pm

A pretty useful document for better understanding of BGP. 

 

Introduction

This document contains frequently asked questions (FAQs) about Border Gateway Protocol (BGP).

Q. How do I configure BGP with the use of a loopback address?A. The use of a loopback interface ensures that the neighbor stays up and is not affected by

malfunctioning hardware.

BGP uses the IP address configured on the physical interface directly connected to the BGP

peer as the source address when it establishes the BGP peering session, by default. Issue the

neighbor <ip address> update−source <interface> command in order to change this

behavior and configure the BGP that speaks to the router to establish peering with the use of a

loopback address as the source address.

Refer to Sample Configuration for iBGP and eBGP With or Without a Loopback Address for

more information.

Q. What is the order of preference of attributes when some or all are

applied to one neighbor in BGP?

A. The order of preference varies based on whether the attributes are applied for inbound

updates or outbound updates.

For inbound updates the order of preference is:

route−map 1.

filter−list 2.

prefix−list, distribute−list 3.

For outbound updates the order of preference is:

prefix−list, distribute−list 1.

filter−list 2.

route−map 3.

Note: The attributes prefix−list and distribute−list are mutually exclusive, and only one command (neighbor prefix−list or neighbor distribute−list) can be applied to each inbound

or outbound direction for a particular neighbor.

Q. What does a next hop of 0.0.0.0 mean in the show ip bgp command

output?

A. A network in the BGP table with a next hop address of 0.0.0.0 means that the network is

locally originated via redistribution of Interior Gateway Protocol (IGP) into BGP, or via a

network or aggregate command in the BGP configuration.

Q. What are the well known communities of the BGP community

attribute?

A. The community attribute is a transitive, optional attribute designed to group destinations in

a certain community and apply certain policies (such as accept, prefer, or redistribute). This

table shows the well known BGP communities.

Community Description
Local-AS Use in confederation scenarios to prevent sending packets outside the local autonomous system (AS).
no-export Do not advertise to external BGP (eBGP) peers. Keep this route within an AS.
no-advertise Do not advertise this route to any peer, internal or external.
none Apply no community attribute when you want to clear the communities associated with a route.
internet Advertise this route to the internet community, and any router that belongs to it.

.

Q. What formats can I use to configure the BGP community attribute?

A. In Cisco IOS® Software release 12.0 and later, you can configure communities in three

different formats called decimal, hexadecimal, and AA:NN. By default, IOS uses the older

decimal format. In order to configure and display in AA:NN, where the first part is the AS

number and the second part is a 2−byte number, use the ip bgp new−format global

configuration command.

Note: Although the community attribute can be represented in decimal, hexadecimal, or

AA:NN, it is still a 32−bit number. For example, any of these three configuration commands

specify the community 30:20 (AS 30, number 20):

set community 30:20 ¨

set community 0x1E0014 ¨

set community 1966100 ¨

Regardless of which command you use, the community displayed in the router configuration

file and the BGP table is 30:20.

Refer to the Community Attribute section of BGP Case Studies, and Using BGP Community

Values to Control Routing Policy in Upstream Provider Network for more information.

Q. How does BGP behave differently with auto−summary enabled or

disabled?

A. Auto−summary behavior has changed across Cisco IOS releases. Initially, auto−summarywas enabled by default. However, with Cisco bug ID CSCdu81680 ( registered customers only)

this behavior has changed. In the latest Cisco IOS, auto−summary is disabled by default.

When auto−summary is enabled, it summarizes the locally originated BGP networks to their

classfull boundaries. (Auto−summary is enabled by default in BGP). When auto−summary is

disabled, the routes introduced locally into the BGP table are not summarized to their

classfull boundaries. When a subnet exists in the routing table and the following three

conditions are satisfied, then any subnet of that classfull network in the local routing table

will prompt BGP to install the classfull network into the BGP table.

Classfull network statement for a network in the routing table ¨

Classfull mask on that network statement ¨

Auto−summary enabled ¨

For example, if the subnet in the routing table is 75.75.75.0 mask 255.255.255.0, and you

configure network 75.0.0.0 under the router bgp command, and auto−summary is enabled,

BGP introduces the classfull network 75.0.0.0 mask 255.0.0.0 in the BGP table.

If these three conditions are not all met, then BGP does not install any entry in the BGP table

unless there is an exact match in the local routing table.

Note: If the AS that performs BGP does not own the complete classfull network, Cisco

recommends that you disable auto−summary using the no auto−summary command under

router bgp.

Q. How can I verify if a BGP router announces its BGP networks and

propagates them to the global BGP mesh?

A. Use these commands to check if the IP blocks are announced to the directly connected

ISP:

The show ip bgp neighbors [address] advertise−routes command shows which

messages are being sent.

¨

The show ip bgp neighbors [address] routes command shows which messages are

being received.

¨

Note: The show ip bgp neighbors [address] advertise−routes command does not take into

account any outbound policies you may have applied. In future Cisco IOS versions the

command output will be changed to reflect the outbound policies.

In order to verify how the IP blocks get propagated to the global BGP mesh via the directly

connected ISP, log onto a route server on the Internet and look for the BGP entries of the

prefix in the route server.

Q. When and how should I reset a BGP session?

A. Clear a BGP session when you change the inbound/outbound policy for this session. Use

the clear ip bgp x.x.x.x soft out command to clear a BGP session in order to bring outboundpolicy changes into effect. Use the

clear ip bgp x.x.x.x command in order to clear a BGP

session to bring inbound policy changes into effect. If the neighbor has the soft

reconfiguration capability, you can use the clear ip bgp x.x.x.x soft in command.

Note: With Cisco IOS Software Release 12.0 and later, a new BGP Soft Reset Enhancement

feature is introduced. Refer to BGP Soft Reset Enhancement for more information.

Q. When I perform MD5 Authentication for BGP through a PIX, is there

anything special that needs to be done on the PIX?

A. Yes. When a BGP ‘neighbor … password …’ is configured, MD5 authentication is used on

the TCP psuedo−IP header, TCP header, and data (refer to RFC 2385 ). TCP uses this data,

which includes the TCP sequence and ACK numbers, and the BGP neighbor password, to

create a 128−bit hash number. The hash number is included in the packet in a TCP header

option field. By default, the PIX offsets the sequence number by a random value per TCP

flow. On the sending BGP peer, TCP uses the original sequence number to create the 128−bit

MD5 hash number and includes this hash number in the packet. When the receiving BGP peer

gets the packet, TCP uses the PIX modified sequence number to create a 128−bit MD5 hash

number and compares it to the hash number included in the packet. Because the TCP

sequence value was changed by the PIX, the hash is differentTCP on the BGP neighbor

drops the packet and logs an MD5 failed message similar to this:

%TCP−6−BADAUTH: Invalid MD5 digest from 10.28.0.9:1778 to 10.156.50.10:179

Use the norandomseq keyword to solve this problem and stop the PIX from offsetting the

TCP sequence number with this command:

static (inside,DMZ−ICE) 10.0.0.0 10.0.0.0 netmask 255.0.0.0 norandomseq

Q. What is an autonomous system (AS) number and how do I obtain

one?

A. AS numbers are globally unique numbers that are used to identify ASes, and which enable

an AS to exchange exterior routing information between neighboring ASes. An AS is a

connected group of IP networks that adhere to a single and clearly defined routing policy.

There are a limited number of available AS numbers. Therefore, it is important to determine

which sites require unique AS numbers and which do not. Sites that do not require a unique

AS number should use one or more of the AS numbers reserved for private use, which are in

the range from 64512 to 65535. Access the AS Number Registration Services Website to

obtain an AS number.

Q. What is the BGP path selection criteria?

A. BGP path selection criteria is documented in BGP Best Path Selection Algorithm.

Q. What is the difference between always−compare−med and

deterministic−med?

A. A complete explanation of the differences between these commands is documented in

How the bgp deterministic−med Command Differs from the bgp always−compare−med

Command.

Q. Do internal BGP sessions modify the next hop?

A. Internal BGP (iBGP) sessions preserve the next hop attribute learned from eBGP peers.

This is why it is important to have an internal route to the next hop. The BGP route is

otherwise unreachable. In order to make sure you can reach the eBGP next hop, include the

network that the next hop belongs to in the IGP or use the next−hop−self neighbor command

to force the router to advertise itself, rather than the external peer, as the next hop. Refer to

the BGP Nexthop Attribute section of BGP Case Studies for a more detailed explanation.

Q. Do eBGP sessions between confederations modify the next hop?

A. No, eBGP sessions between confederation sub−ASes does not modify the next hop

attribute. All iBGP rules still apply to have the whole AS behave as a single entity. The

metric and local preference values also remain unaltered among confederation eBGP peers.

Refer to the BGP Confederation section of BGP Case Studies for more information about

confederations.

Q. In eBGP sessions, which IP address is sent as the next hop?

A. In eBGP peering, the next hop is the IP address of the neighbor that announces the route.

However, when the route is advertised on a multi−access media (such as Ethernet or Frame

Relay), the next hop is usually the IP address of the router interface connected to that media,

which originated the route. Refer to the BGP Nexthop Attribute of BGP Case Studies for a

more detailed explanation.

Q. Does the route reflector change the next hop attribute of a reflected

prefix?

A. By default, the next hop attribute is not changed when a prefix is reflected by route reflector. However, you can use the neighbor next−hop−self command to change the

attribute of the next hop for prefixes reflected from an eBGP peer to any route reflector client.

Q. How can I announce a prefix conditionally to one ISP only when I lose

the connection to my primary ISP?

A. BGP advertises routes from its BGP table to external peers by default. The BGP

conditional advertisement feature provides additional control of route advertisement

depending on the existence of other prefixes in the BGP table. Normally, routes are

propagated regardless of the existence of a different path. The BGP conditional advertisement

feature uses the non−exist−map and advertise−map configuration commands to track routes

by the route prefix. If a route prefix is not present in the non−exist−map command, the routespecified by the

advertise−map command is announced. Refer to the Configuring BGP

Conditional Advertisement section of Configuring BGP for more information.

Q. How much memory should I have in my router to receive the

complete BGP routing table from my ISP?

A. The amount of memory required to store BGP routes depends on many factors, such as the

router, the number of alternate paths available, route dampening, community, the number of

maximum paths configured, BGP attributes, and VPN configurations. Without knowledge of

these parameters it is difficult to calculate the amount of memory required to store a certain

number of BGP routes. Cisco typically recommends a minimum of 128 MB of RAM in the

router to store a complete global BGP routing table from one BGP peer. However, it is

important to understand ways to reduce memory consumption and achieve optimal routing

without the need to receive the complete Internet routing table. Refer to Achieve Optimal

Routing and Reduce BGP Memory Consumption for more detailed information.

Q. What are the benefits of configuring BGP peer groups?

A. The major benefit of specifying a BGP peer group is that it reduces the amount of system

resources (CPU and memory) used in an update generation. It also simplifies BGP

configuration since it allows the routing table to be checked only once, and updates to be

replicated to all other in−sync peer group members. Depending on the number of peer group

members, the number of prefixes in the table, and the number of prefixes advertised, this can

significantly reduce the load. Cisco recommends that you group together peers with identical

outbound announcement policies. Refer to BGP Peer Groups for more detailed information.

Q. What is synchronization, and how does it influence BGP routes

installed in the IP routing table?

A. If your AS passes traffic from another AS to a third AS, BGP should not advertise a route

before all routers in your AS learn about the route via IGP. BGP waits until IGP propagates

the route within the AS and then advertises it to external peers. A BGP router with

synchronization enabled does not install iBGP learned routes into its routing table if it is not

able to validate those routes in its IGP. Disabling synchronization using the no synchronization command under router bgp prevents BGP from validating iBGP routes in

IGP. Refer to BGP Case Studies: Synchronization for a more detailed explanation.

Q. How do I know which Cisco IOS software release supports a

particular BGP feature?

A. Use the Cisco IOS Software Advisor ( registered customers only) to quickly find which Cisco

IOS software release supports your feature.

Q. How can I set the Multi Exit Discriminator (MED) value on prefixes

advertised to eBGP neighbors to match the IGP next hop metric?

A. The set metric−type internal route−map configuration command causes BGP to

advertise a MED that corresponds to the IGP metric associated with the next hop of the route.

This command is available in Cisco IOS Software Release 10.3 and later.

Q. What is the default BGP ConnectRetry timer, and is it possible to tune

the BGP ConnectRetry timer?

A. The default BGP ConnectRetry timer is 120 seconds. Only after this time passes does the

BGP process check to see if the passive TCP session is established. If the passive TCP

session is not established, then the BGP process starts a new active TCP attempt to connect to

the remote BGP speaker. During this idle 120 seconds of the ConnectRetry timer, the remote

BGP peer can establish a BGP session to it. Presently the Cisco IOS ConnectRetry timer

cannot be changed from its default of 120 seconds.

Q. What does r RIB−Failure mean in the show ip bgp command output?

R1> show ip bgp

BGP table version is 5, local router ID is 200.200.200.1

Status codes: s suppressed, d damped, h history, * valid, > best, i − internal,

r RIB−failure

Origin codes: i − IGP, e − EGP, ? − incomplete

Network Next Hop Metric LocPrf Weight Path

r> 6.6.6.0/24 10.10.13.3 0 130 0 30 i

*> 7.7.7.0/24 10.10.13.3 0 125 0 30 i

When BGP tries to install the bestpath prefix into Routing Information Base (RIB) (for

example, the IP Routing table), RIB might reject the BGP route due to any of these reasons:

Route with better administrative distance already present in IGP. For example, if a

static route already exists in IP Routing table.

¨

Memory failure. ¨

The number of routes in VPN routing/forwarding (VRF) exceeds the route−limit

configured under the VRF instance.

¨

In such cases, the prefixes that are rejected for these reasons are identified by r RIB Failure in the show ip bgp command output and are not advertised to the peers. This

feature was first made available in Cisco IOS Software Release 12.2(08.05)T.

Q. How can I redistribute internal BGP (iBGP) learned default−route

(0.0.0.0/0) route into EIGRP/OSPF/IS−IS?

A. The redistribution of iBGP routes into Interior Gateway Protocol (IGP)Enhanced Interior

Gateway Routing Protocol/Open Shortest Path First/Intermediate System−to−Intermediate

System (EIGRP/OSPF/IS−IS)can cause routing loops within the Autonomous System,

which is not recommended. By default, iBGP redistribution into IGP is disabled. Use the bgpredistribute−internal command to enable redistribution of iBGP routes into IGP.

Precautions should be taken to redistribute specific routes using route−maps into IGP. A

sample configuration for redistributing a iBGP learned default route 0.0.0.0/0 into EIGRP is

shown in this output. Configurations for OSPF/IS−IS are similar.

router bgp 65345

[…]

bgp redistribute−internal

!

router eigrp 10

[…]

redistribute bgp 65345 route−map check−def

!

ip prefix−list def−route seq 5 permit 0.0.0.0/0

!

route−map check−def permit 10

match ip address prefix−list def−route

 

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15 Comments »

  1. After a long time i’ve come across an excellent FAQ on BGP, its not only a quick reference but also adequately detailed. It also comprises of some questions which are normally considered trivial but quite important in many implementation scenarios. Thanks very much, your efforts are much appreciated.

    Comment by pravin — May 18, 2007 @ 11:51 pm

  2. really good

    Comment by mukesh — December 11, 2007 @ 1:06 pm

  3. Really good FAQ.

    Comment by bgp administrator — May 13, 2011 @ 5:39 pm

  4. Did you just copy this from Cisco? Why not read through it and extract the relevant info and then re-post with your take on the document. This way you will learn a lot more.

    Roger

    Comment by Cisco Support UK — December 5, 2011 @ 8:32 am

  5. Hi there…

    Thank you very much for this wonderful blog…

    I am preparing for my CCNP ROUTE exam and I feel BGP is the most confusing part….

    I have copied status codes from “show ip bgp” command output and its explanation. Could you please explain what are s, d and h (suppressed, dampened and history).

    Thank you very much for this wonderful blog…

    s—The table entry is suppressed. ???

    d—The table entry is dampened. ???

    h—The table entry history. ???

    *—The table entry is valid.

    >—The table entry is the best entry to use for that network.

    i—The table entry was learned via an internal BGP (iBGP) session.

    Comment by CCNA Trainer — August 1, 2012 @ 8:02 am

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