The Rules by Which Route Reflectors Propagate Updates

The following are the rules by which route reflectors propagate updates:

• If a route reflector receives multiple paths to the same destination, it chooses the best path.

• Routes received from a client are reflected to clients and nonclients by the route reflector. This excludes the originator of the route.

• Routes received from a nonclient are reflected to clients only by the route reflector.

• Routes received from EBGP-4 are reflected to clients and nonclients by the route reflector.

The command for configuring a route reflector is very straightforward. It is explained in the following syntax:

neighbor ip-address route-reflector-client

To remove a router as a client, issue the following command:

no neighbor ip-address route-reflector-client

Note that if all clients are removed, the route reflector loses its status and becomes a standard IBGP-4 router. If this happens, then the IBGP-4 routers need to be fully meshed, or the BGP-4 updates must be redistributed into an IGP. Table 9-2 breaks down the syntax of the command to configure a route reflector and identify the clients.

Figure 9-5 Clusters and Route Reflector Meshing

Figure 9-5 Clusters and Route Reflector Meshing

Table 9-2

Explanation of the Route Reflector Configuration Command

Syntax

Description neighbor

Identifies that the rest of the command is directed at a BGP-4 peer.

ip address

Is the IP address of the neighboring router being identified as a client.

route-reflector-client

Points to the client of the route reflector. Note that the client is not configured and is unaware of its change of status. It does nothing but continue to send updates to the route reflector, which forwards them unchanged to other clients.

Example 9-1 illustrates the concepts explained in this section. Use this example in conjunction with the network displayed in Figure 9-6.

Figure 9-6 Figure Supporting the Example 9-1

Figure 9-6 Figure Supporting the Example 9-1

Example 9-1 Configuration of a Route Reflector

New York# router bgp 100

! Create a connection to Router A, a client neighbor 167.55.22.2 remote-as 100

!make Router A as seen in Figure 9-6 a route reflector client neighbor 167.55.22.2 route-reflector-client ! connection to B

neighbor 167.55.11.1 remote-as 100 neighbor 167.55.11.1 route-reflector-client ! connection to San Francisco neighbor 167.55.77.7 remote-as 100 ! connection to Chicago neighbor 167.55.44.4 remote-as 100 neighbor 33.33.33.33 remote-as 300 RTB#

router bgp 100

! connection to New York neighbor 3.3.3.3 remote-as 100

Chicago#

router bgp 100

! connection to Router E

neighbor 167.55.66.6 remote-as 100

neighbor 167.55.66.6 route-reflector-client

! connection to Router F

neighbor 167.55.55.5 remote-as 100

neighbor 167.55.55.5 route-reflector-client

! connection to San Francisco neighbor 167.55.77.7 remote-as 100

! connection to New York neighbor 167.55.33.3 remote-as 100

After any BGP-4 configuration, it is necessary to reset the TCP session so that the changes can take effect. The command to do this is as follows:

clear ip bgp *

NOTE This command is described in detail in Chapter 8, in the section "Managing and Verifying the BGP-4 Configuration."

It is also important to verify that everything is working. Example 9-2 demonstrates the command that verifies the configuration.

Example 9-2 Example of the show ip bgp neighbors Command

New York# show ip bgp neighbors 167.55.44.4 BGP neighbor is 167.55.44.4, remote AS 100, Internal link Index 1, Offset 0, Mask 0x2 Inbound soft reconfiguration allowed BGP version 4, remote router ID 167.55.44.4 BGP state = Established, table version = 27, up for 00:06:12 Last read 00:00:12, hold time is 180, keepalive interval is 60 seconds Minimum time between advertisement runs is 30 seconds Received 19 messages, 0 notifications, 0 in queue Sent 17 messages, 0 notifications, 0 in queue Inbound path policy configured

Route map for incoming advertisements is testing Connections established 2; dropped 1 Connection state is ESTAB, I/O status: 1, unread input bytes: 0 Local host: 167.55.44.3, Local port: 11002 Foreign host: 167.55.44.4, Foreign port: 179

Enqueued packets for retransmit: 0, input: 0, saved: 0 Event Timers (current time is 0x530C294):

Timer

Starts

Wakeups

Next

Retrans

12

0

0x0

TimeWait

0

0

0x0

AckHold

12

10

0x0

SendWnd

0

0

0x0

KeepAlive

0

0

0x0

GiveUp

0

0

0x0

PmtuAger

0

0

0x0

iss: 133981889

snduna:

133982166

sndnxt: 13i

irs: 3317025518 rcvnxt: 3317025810

irs: 3317025518 rcvnxt: 3317025810

16093 0 ms sndwnd: delrcvwnd:

rcvwnd:

SRTT: 441 ms, RTTO: 2784 ms, RTV: 951 ms, KRTT minRTT: 0 ms, maxRTT: 300 ms, ACK hold: 300 ms Flags: higher precedence, nagle Datagrams (max data segment is 1460 bytes):

Rcvd: 15 (out of order: 0), with data: 12, total data bytes: 291 Sent: 23 (retransmit: 0), with data: 11, total data bytes: 276

16108 291

Table 9-3 describes the fields shown in Example 9-2.

Table 9-3 Explanation of the show ip bgp neighbors Command

Field Descriptions

BGP Neighbor IP address and autonomous system of the BGP neighbor. If the autonomous system numbers are the same, then internal BGP-4 (IBGP-4) is running between the neighbors; otherwise, external BGP-4 (EBGP-4) is in use.

BGP Version BGP version being used to communicate with the remote router. The neighbor's router ID (an IP address) is also specified.

BGP State

Internal state of this BGP connection.

Table 9-3 Explanation of the show ip bgp neighbors Command (Continued)

Field

Descriptions

Table Version

Indication that the neighbor has been updated with this version of the

primary BGP routing table.

Up For

Amount of time that the underlying TCP connection has been in

existence.

Last Read

Time that the BGP last read a message from this neighbor.

Holdtime

Maximum amount of time that can elapse between messages from the

peer or neighbor.

Keepalive Interval

Time period between sending keepalive packets, which maintain the

TCP connection.

Received

Number of total BGP messages received from this peer, including

keepalives.

Sent

Total number of BGP messages that have been sent to this peer,

including keepalives.

Notifications

Number of error messages that the router has sent to this peer.

Connections Established

Number of times that the router has established a TCP connection for

BGP-4 between the two peers.

Dropped

Number of times that a valid TCP connection has failed or been taken

down.

Connection State

State of BGP peer.

Unread Input Bytes

Number of bytes of packets still to be processed.

Local Host, Local Port

Peering address of local router, plus port.

Foreign Host, Foreign Port

Neighbor's peering address.

Event Timers

Table that displays the number of starts and wakeups for each timer.

Iss

Initial send sequence number.

Snduna

Last send sequence number that the local host sent but for which an

acknowledgment is outstanding.

Sndnxt

Sequence number that the local host will send next.

Sndwnd

TCP window size of the remote host.

irs

Initial receive sequence number.

Rcvnxt

Last receive sequence number that the local host has acknowledged.

Rcvwnd

Local host's TCP window size.

continues continues

Table 9-3 Explanation of the show ip bgp neighbors Command (Continued)

Field

Descriptions

Delrecvwnd

Delayed receive window. Data that the local host has read from the

connection but that has not yet been subtracted from the receive window

that the host has advertised to the remote host. The value in this field

gradually increases until it is larger than a full-sized packet, at which

point it is applied to the rcvwnd field.

SRTT

A calculated smoothed round-trip timeout.

RTTO

Round-trip timeout.

RTV

Variance of the round-trip time.

KRTT

New round-trip timeout (using the Karn algorithm). This field

separately tracks the round-trip time of packets that have been

retransmitted.

minRTT

Smallest recorded round-trip timeout (hard-wire value used for

calculation).

maxRTT

Largest recorded round-trip timeout.

ACK hold

Time that the local host will delay an acknowledgment to piggyback

data on it.

Flags

IP precedence of the BGP packets.

Datagrams Rcvd

Number of update packets received from neighbor with data.

With Data Total Data Bytes

Total bytes of data.

Sent

Number of update packets sent.

With Data

Number of update packets with data sent.

Total data bytes Total data bytes.

Total data bytes Total data bytes.

TIP When implementing clusters and route reflectors, select the route reflector carefully in accordance with the physical topology of the network. Keep the design simple, placing one route reflector in a cluster. When the logical cluster design is in place, configure one cluster at a time and one route reflector at a time. After the route reflector in the cluster is configured, remove the BGP-4 configuration that has the BGP-4 sessions between the clients.

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