Scenario 83 Answers

1 How many sessions are active?

There are two active sessions. In reading the large amount of information on the show ip bgp neighbor command, there is a line at the beginning of each session identifying the neighboring peer. The lines in this output screen are as follows:

BGP neighbor is 155.94.83.1, remote AS 2914, external link BGP neighbor is 144.39.228.49, remote AS 701, external link

2 What is the state of the sessions, and what do the states mean?

BGP state = Established, table version = 457046, up for 1w5d BGP state = Established, table version = 457055, up for 2w0d

Both the peers have established sessions. This means that they have a TCP session. They are now in a position to exchange routing tables and to synchronize their databases. The rest of the line indicates how many times the table has been updated and how long the session has been maintained. In this example, the first peer has had a session with the local router for one week and five days, while the second peer has been up for exactly two weeks. How many autonomous systems are there?

There are three autonomous systems in this configuration. The first peer belongs to autonomous system 2914, and the second belongs to 701. Because both of these neighbors belonging to their autonomous systems have an external BGP-4 session, there must be a third autonomous system, within which the local router resides.

BGP neighbor is 155.94.83.1, remote AS 2914, external link BGP neighbor is 144.39.228.49, remote AS 701, external link

Table 8-8, taken from the documentation set for BGP commands on the CCO Web page, explains the fields in the show ip bgp neighbors command.

Table 8-8 show ip bgp neighbors Field Descriptions

Field

Description

BGP Neighbor

IP address of the BGP neighbor and its autonomous system number. If

the neighbor is in the same autonomous system as the router, then the

link between them is internal; otherwise, it is considered external.

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 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 BGP last read a message from this neighbor.

Holdtime

Maximum amount of time that can elapse between messages from the

peer.

Keepalive Interval

Time period between sending keepalive packets, which help ensure that

the TCP connection is up.

Received

Number of total BGP messages received from this peer, including

keepalives.

Notifications

Number of error messages received from the peer.

Sent

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 and

that the two peers have agreed to speak BGP with each other.

Dropped

Number of times that a good 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.

continues continues

Table 8-8 show ip bgp neighbors Field Descriptions (Continued)

Field

Description

Local Host, Local Port

Peering address of local router, plus port.

Foreign Host, Foreign Port

Foreign host, foreign port.

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 has not received

an acknowledgment for.

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.

delrecvwnd

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

connection but that has not yet 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 a neighbor.

With Data

Number of update packets received with data.

Total Data Bytes

Total bytes of data.

Table 8-8 show ip bgp neighbors Field Descriptions (Continued)

Field

Description

Sent

Number of update packets sent.

With Data

Number of update packets with data sent.

Total Data Bytes

Total number of data bytes.

This chapter covers the following topics that you will need to master to pass the CCNP/ CCDP Routing exam:

• The scalability problems inherent in BGP-4.

• The concept and configuration of route reflectors.

• The concept and configuration of policy control using prefix lists.

• Methods of connecting to multiple autonomous systems.

• Redistribution between IGPs and EBGP.

• The Cisco commands for tuning BGP-4 and the Cisco commands for reviewing the configuration.

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