From the output you saved, describe how the BGP routers learn that a link is down. (Hint: Look at the BGP State field)

What will be an ideal response?

In the output, BGP State field contains this information. BGP is using TCP and if there is any link problem, TCP layer can notice this soon. If BGP State field is other than ‘Established’,
BGP connection to this neighbor is closed. In the output below, Router2’s neighbor Router1 (10.0.4.1) has BGP state of Active.
Also BGP has a KEEPALIVE message and this message is periodically exchanged between BGP peer. During the hold time of BGP, BGP router keeps this information. After expiring the hold time, if BGP router doesn’t get any KEEPALIVE message, it can also determine there is a problem on this link.
```
Router2#show ip bgp neighbors
BGP neighbor is 10.0.4.1, remote AS 100, external link
Index 1, Offset 0, Mask 0x2
BGP version 4, remote router ID 0.0.0.0
BGP state = Active , table version = 0
Last read 00:01:22, hold time is 30, keepalive interval is 10 seconds
Minimum time between advertisement runs is 30 seconds
Received 38 messages, 0 notifications, 0 in queue
Sent 39 messages, 0 notifications, 0 in queue
Prefix advertised 3, suppressed 0, withdrawn 0
Connections established 2; dropped 2
Last reset 00:01:23, due to User reset
0 accepted prefixes consume 0 bytes
0 history paths consume 0 bytes
No active TCP connection
BGP neighbor is 10.0.4.3, remote AS 300, external link
Index 2, Offset 0, Mask 0x4
BGP version 4, remote router ID 10.0.4.3
BGP state = Established, table version = 4, up for 00:01:07
Last read 00:00:07, hold time is 30, keepalive interval is 10 seconds
Minimum time between advertisement runs is 30 seconds
Received 42 messages, 0 notifications, 0 in queue
Sent 41 messages, 0 notifications, 0 in queue
Prefix advertised 3, suppressed 0, withdrawn 0
Connections established 2; dropped 1
Last reset 00:01:26, due to User reset
2 accepted prefixes consume 64 bytes
0 history paths consume 0 bytes
Connection state is ESTAB, I/O status: 1, unread input bytes: 0
Local host: 10.0.4.2, Local port: 11004
Foreign host: 10.0.4.3, Foreign port: 179
Enqueued packets for retransmit: 0, input: 0 mis-ordered: 0 (0 bytes)
Event Timers (current time is 0x1B185F8C):
Timer Starts Wakeups Next
Retrans 11 0 0x0
TimeWait 0 0 0x0
AckHold 10 2 0x0
SendWnd 0 0 0x0
KeepAlive 0 0 0x0
GiveUp 0 0 0x0
PmtuAger 0 0 0x0
DeadWait 0 0 0x0
iss: 1147100067 snduna: 1147100301 sndnxt: 1147100301 sndwnd: 16151
irs: 281557203 rcvnxt: 281557484 rcvwnd: 16104 delrcvwnd: 280
SRTT: 461 ms, RTTO: 3028 ms, RTV: 1053 ms, KRTT: 0 ms
minRTT: 4 ms, maxRTT: 300 ms, ACK hold: 200 ms
Flags: higher precedence, nagle Datagrams (max data segment is 1460 bytes):
Rcvd: 19 (out of order: 0), with data: 10, total data bytes: 280
Sent: 14 (retransmit: 0), with data: 10, total data bytes: 233
```