CCNP Routing Study Guide- P12 ppsx

We’ll add RouterB, which is at 172.16.2.1, and identify the network in which to advertise to our neighbor: RouterB config-router#neighbor 172.16.2.1 remote-as 63001The loopback IP addr

the source router An eBGP peer will have a different ASN than the source router.

Now let’s look at an example based on Figure 8.1 We’ll add RouterB, which is at 172.16.2.1, and identify the network in which to advertise to our neighbor:

RouterB (config-router)#neighbor 172.16.2.1

remote-as 63001The loopback IP address can be used for both iBGP and eBGP peers Addi-tional commands must be used when creating a peering session with a loop-back interface For iBGP sessions, the only additional command is the update-source command The available syntaxes are as follows:

neighbor [address | peer-group-name] update-source interface-type interface-number

The IP address of the loopback should be used for the peer address Since the loopback interface is being used as the source of the BGP session, the

interface-type should be entered as the loopback The

interface-number is the number of the loopback interface that is being used for BGP peering This is configured on the router using the loopback address.The following command adds networks and creates a route in the BGP table if the route is present in the IP table:

network network-numberLet’s look at an example adding our own network 63.78.0.0:

<cr>

RouterA(config-router)#network 63.78.0.0 mask 255.255.0.0

Again, network-number represents the network that is to be advertised

using the BGP process The IP network specified in the BGP network ment does not have to be directly connected to the router Network state-ments within the BGP protocol session allow BGP to advertise routes learned

state-by an IGP that are contained in the route table The network mask is applied because BGPv4 can support subnetting and supernetting When a logical BGP mesh is in place, each IGP session should have network statements con-figured for only those routes learned from the IGP Network statements should not be duplicated among internal BGP routers

BGP configuration can be very complicated Several different options may be configured to optimize BGP routing When only one link is used to peer with another AS or ISP, the configuration can be straightforward As more links are used, or multiple ISPs or autonomous systems are connected to a router, the configuration becomes increasingly complex.

Verifying BGP Configurations

After BGP is configured, several commands will allow us to verify the BGP configuration and troubleshoot the operation of BGP We can also use these commands to monitor the BGP process and its operations

Table 8.2 summarizes all of the commands that can be used to verify BGP

T A B L E 8 2 BGP Monitoring Command Summary

show ip bgp Shows all BGP configuration information for

the selected interface.

show ip bgp neighbors Shows all configured BGP neighbors It

pro-vides detailed statistics and information about each neighbor.

show ip bgp community Displays routes belonging to the specified

community.

The detailed use of some of these commands will be explained in the tion “Troubleshooting BGP.”

sec-In earlier versions of the Cisco IOS, in particular versions 11.1 and 11.3, some

of the show commands listed above can cause the router to reload Cisco became aware of the problem and has resolved it in later versions

Cisco has a configurable proprietary attribute that allows us to use weights as a metric in deciding the best route Let’s take a look at this attribute in the next section We’ll also see how to configure the MED attribute discussed in Chapter 7

Configuring BGP Route-Selection Attributes

BGP uses several metrics as criteria when selecting the best possible route to

a destination Each metric can be configured manually Other criteria that influence BGP route selection may also be configured

To quickly understand how BGP selects a route, review Figure 8.2 This figure summarizes the steps that the BGP process takes to choose the best route Ten different criteria are used in path selection, several of which are configurable

show ip bgp cidr-only Displays classless routes.

show ip bgp filter-list Displays AS path lists.

show ip bgp paths Displays all path information for the local

router.

show ip bgp peer-group Provides information on the members of the

specified peer group.

show ip bgp summary Shows the status of all BGP connections.

T A B L E 8 2 BGP Monitoring Command Summary (continued)

F I G U R E 8 2 BGP path-selection diagram

Is Next-hop reachable?

No Ignore route.

Yes Choose the path with the highest Weight.

Are Weights equal?

Choose the biggest Local Preference.

Yes

Are Local Preferences equal?

No

Was route originated

by local router?

Choose the shortest AS Path.

Are AS Paths equal?

Choose lowest Origin Code Select route.

Are Origin Codes equal?

Choose the lowest MED.

Are MEDs equal?

Choose External over Internal.

Are path types equal?

Choose closest IGP neighbor.

Are IGP neighbors equal?

Choose the path with the lowest Router ID.

Now let’s discuss some of these criteria separately We will also learn how

to configure them

Configuring the Atomic Aggregate Attribute

When using classless interdomain routing (CIDR), you can create aggregate routes to minimize the size of routing tables You can configure aggregate routes

in BGP by redistributing an aggregate route into BGP The Atomic Aggregate attribute can be configured using the aggregate-address command This command allows you to configure an aggregate or summary entry in the BGP table The command has several syntaxes Let’s look at the command and the possible syntaxes:

aggregate-address ip-address mask [summary-only] [as-set]

The ip-address and mask indicate the aggregate address to be created

By default, BGP advertises both aggregate routes and more specific routes

By using the summary-only syntax, the BGP router will advertise only the aggregate route If you use the as-set syntax, the BGP router will advertise the route as coming from your AS and will set the Atomic Aggregate attribute to show that information regarding the route may be missing

Configuring the Weight Attribute

The Weight attribute is a Cisco proprietary attribute used for path selection This attribute, which is also considered a metric, allows a system adminis-trator to manually assign a value to all paths learned from other BGP peers The larger the weight value, the more desirable the path

This metric is particularly helpful when a router is connected to multiple autonomous systems The weight assigned stays local to the router on which

it is configured When paths are learned from multiple sources, the Weight metric can be used to force BGP to select a specified interface over the others.This metric is configured using the following command from within the BGP routing session:

neighbor [ip-address | peer-group-name] weight weight

The ip-address is the IP address of the neighbor The peer-group-name

may be used when assigning weight to all routes learned via the BGP peer

group The weight value has a range from 0 to 65,535 The default value is

32,768

Configuring the Local Preference Attribute

The Local Preference attribute is used to assign metric values that are used

When a path is learned via two different border routers, both paths are advertised to other iBGP peers Either path is valid and can be used How-ever, if one path is to be used only as a backup route, you can set local pref-erence values on both routers.

The Local Preference attribute is configured by using the following command:

bgp default local-preference value

The command must be issued within the BGP session Configuration mode The configured values for the Local Preference range from 0 to 4,294,967,295 Higher values are preferred over lower values

Configuring in an NBMA Network

When you have a non-broadcast multi-access (NBMA) network in which the router you are configuring needs to advertise itself as the next hop to a des-tination, use the next-hop-self syntax for the neighbor command This allows the normal BGP process to override what it’s learned and forces updates to advertise this router as the next hop, even if there is another way

to the destination The command is as follows:

neighbor ip address | peer-group-name next-hop-self

Configuring MED

While the previous metrics inform local AS routers which path to select when leaving the AS, Multi-Exit Discriminators (MEDs) inform the neighboring

AS which link to use to receive traffic

MEDs are used when two autonomous systems are connected via multiple links or multiple routers MED values are not propagated to other autono-mous systems

Configuring MEDs is more complicated than configuring Weight or Local Preference values Because of the complexity of the configuration, more CPU resources are needed MEDs are set using route maps Route maps are a form

of access list Here is an example of a BGP configuration using MEDs:

Router1(config)#route-map ANEXAMPLE permit 10 Router1(config-rou)#match ip address 1

Router1(config-rou)#set metric 25 Router1(config-rou)#exit

Router1(config)#route-map ANEXAMPLE permit 20 Router1(config-rou)#exit

Router1(config)#access-list 1 permit 172.16.0.0

0.0.255.255 Router1(config)#^Z

Router1#

router bgp 63001 network 172.16.0.0 neighbor 172.16.1.1 remote-as 59000 neighbor 172.16.2.1 route-map ANEXAMPLE out

!

ip classlessaccess-list 1 permit 172.16.0.0 0.0.255.255route-map ANEXAMPLE permit 10

match ip address 1 set metric 25

!route-map ANEXAMPLE permit 20

!This configuration sets a MED of 25 for all networks belonging to 172.16.0.0 ASN 59000 will use this value Lower MED values are pre-ferred The second permit statement of the route-map ANEXAMPLE permits all other networks to be advertised but does not assign a MED value We’ll discuss route maps in more detail in Chapter 9

Clearing BGP Routes

The BGP configurations can easily be removed from the router using the clear ip bgp command Let’s look at the command and the available syn-taxes that are used in Privileged EXEC mode, and then we’ll explain each syntax:

clear ip bgp *|address [soft[in|out]

Using the * means that you wish to clear the entire BGP routing table You can use the soft syntax so that the router advertises all its routing updates again and the configuration is not cleared Using the address syntax instead

of the asterisk, only the network address identified is removed from the BGP table The in and out syntaxes are used with the soft syntax to identify that the triggered updates are to occur either on triggered inbound updates or outbound updates

Disabling BGP Synchronization

If all of the routers in your AS are running BGP, then there is no need to have synchronization turned on between BGP and your IGPs that are running When BGP Synchronization is turned on, the router will wait to learn about internal routes from an IGP instead of advertising routes learned by BGP With BGP Synchronization turned off, you can carry fewer IGP learned routes in the topology table and BGP can converge much more quickly To turn off BGP Synchronization, use the following command in BGP Config-uration mode:

Router1(config-router)# no synchronization

Troubleshooting BGP

The most important part of troubleshooting is verifying the status of the peering router When you issue the show ip bgp neighbors command, the basic troubleshooting information is displayed on the screen Let’s first take

a look at the command syntaxes and then view a problem configuration where the BGP peers have not synchronized

filter-list Display routes conforming to the

If the number of connections established keeps incrementing, there could be

a problem with the link between the two neighbors This output is from IOS version 12.0(5):

is 60 seconds Minimum time between advertisement runs is 30 seconds Received 0 messages, 0 notifications, 0 in queue Sent 0 messages, 0 notifications, 0 in queue Prefix advertised 0, suppressed 0, withdrawn 0 Connections established 0; dropped 0

Last reset never

0 accepted prefixes consume 0 bytes

0 history paths consume 0 bytes External BGP neighbor not directly connected

No active TCP connection2514#

Now let’s look at the same router with the connection established:

2514#show ip bgp neighbors

BGP neighbor is 172.16.2.1, remote AS 63001, external

link

Index 1, Offset 0, Mask 0x2

BGP version 4, remote router ID 172.16.2.1

BGP state = Idle, table version = 0

Last read 00:00:07, hold time is 180, keepalive interval

Minimum time between advertisement runs is 30 seconds Received 4582 messages, 0 notifications, 0 in queue

Sent 3552 messages, 0 notifications, 0 in queue

Prefix advertised 0, suppressed 0, withdrawn 0

Connections established 1; dropped 0

Last reset never

0 accepted prefixes consume 0 bytes

0 history paths consume 0 bytes

External BGP neighbor not directly connected

BGP version 4, remote router ID 172.16.2.1

BGP state = Established, table version = 508, up for

Sent 5703 messages, 0 notifications, 0 in queue

Inbound path policy configured

Outbound path policy configured

Incoming update AS path filter list is 10

Outgoing update AS path filter list is 1

Connections established 1; dropped 0 Last reset never

No of prefix received 10 Connection state is ESTAB, I/O status: 1, unread input

bytes: 0 Local host: 172.16.65.1, Local port: 179 Foreign host: 172.16.65.10, Foreign port: 29768

Enqueued packets for retransmit: 0, input: 0 mis-ordered: 0 (0 bytes)

Event Timers (current time is 0x14322791): Timer Starts Wakeups Next Retrans 5677 1 0x0

TimeWait 0 0 0x0

AckHold 5578 4246 0x0

SendWnd 0 0 0x0

KeepAlive 0 0 0x0

GiveUp 0 0 0x0

PmtuAger 0 0 0x0

DeadWait 0 0 0x0

iss: 1337567913 snduna: 1337679159 sndnxt: 1337679159 sndwnd: 15066 irs: 4270375806 rcvnxt: 4270482004 rcvwnd: 15548

delrcvwnd: 836

SRTT: 309 ms, RTTO: 708 ms, RTV: 45 ms, KRTT: 0 ms minRTT: 4 ms, maxRTT: 453 ms, ACK hold: 300 ms Flags: passive open, nagle, gen tcbs

Datagrams (max data segment is 1460 bytes):

Rcvd: 11252 (out of order: 0), with data: 5579, total data

bytes: 106216 Sent: 9996 (retransmit: 1), with data: 5675, total data

bytes: 111245 Router#

A great deal of information is provided by the show ip bgp neighbor command When a peering relationship has trouble getting established, use this command to see if the TCP connection has failed This will give you a starting point for troubleshooting.

When the problem seems to be route information-oriented, you can use the following command:

show ip bgp regexp regular-expressionUse this command to see which routes are being learned from the neigh-boring AS If the neighboring AS is not receiving given routes from your AS, you can use the following command to see what you are advertising to the AS:show ip bgp neighbor address advertised-routes

A quick summary command can be used to verify connectivity via BGP:show ip bgp summary

These are just a few of the commands that you can use when shooting BGP Many other commands and procedures can be used to accom-plish this task, but they are beyond the scope of this book

trouble-Using Debug with BGP

The debug ip bgp command can be used to display events as they occur The only drawback to this command is that not only does the BGP process being used to advertise ASNs across the Internet use considerable processing power, but the debug command is assigned a high priority on the router and can kill your processing power To stop all debugging on a router, use the

short summary of the debug commands in Table 8.3

T A B L E 8 3 The Debug Commands Related to BGP

debug ip bgp dampening Displays BGP dampening events as they

occur.

debug ip bgp events Displays all BGP events as they occur.

debug ip bgp keepalives Displays all events related to BGP keepalive

packets.

debug ip bgp updates Displays information on all BGP update

packets.

In Chapter 7 we looked at how BGP is used, when to use BGP, when not

to use BGP, and the type of protocol BGP is, and we focused on the attributes sent in BGP update messages In this chapter, we looked at how to enable BGP, identify the network number the router belongs to so it can be adver-tised to its neighbors, identify the BGP neighbors, and assign the Weight attribute

In addition, we covered how to configure several other attributes, such as the Next-hop, MED, Atomic Aggregate, and Local Preference attributes Then we covered the show commands that can be used to verify the config-uration and troubleshoot problems that might arise in the configuration

of BGP

As you may have noticed while looking at some of the output that appeared throughout the chapter, BGP has many command syntaxes that make configuring BGP very complex—much more complex than we will cover in the following Hands-on Lab section below In Chapter 9, we will cover many more aspects of BGP, including the addition of filters and policies, route flapping, and using BGP in large-scale networks

Key Terms

Before taking the exam, make sure you are familiar with the following term:peer group

Commands Used in This Chapter

in BGP and CIDR addressing

bgp default preference

local-Allows you to assign a Local Preference attribute value in the range of 0 to 4,294,967,295 Higher values are preferred over lower values

of routes from the BGP table

allow you to identify the internal and external neighbors and assign different metrics to each.

associated with the local router

between the IGPs and BGP for faster convergence

Displays AS path lists

4. What command would you use to identify a CIDR aggregate address?

5. What command displays all the BGP path information learned by the router?

7. If your AS resides in the network 63.78.0.0 and you use a 24-bit net mask, how do you identify this in the BGP configuration?

sub-8. What command allows you to view all events related to BGP updates?

9. What command can be used to view routes belonging to a specific community?

10. What command can be used to view the status of all connections?