To troubleshoot with Border Gateway Protocol (BGP), you must gather information about the protocol and how it is functioning. As with other protocols, the two main sources of information are the show command and the debug command, so the place to start is by reviewing the show commands that you can use with BGP.
The BGP show command is surprisingly show ip bgp, which displays information about your router, including its Router ID and the networks that are visible with their metric, weight, and route flag. This command shows you whether the routes are internal (on the inside of your network) or stale (not updated recently and possibly down).
Notice that the RouterID is the address that was associated with the loopback interface, as with the OSPF protocol (covered in the previous chapter). Also as with the OSPF protocol, BGP uses the highest IP address of its connected interfaces as its RouterID, which you can force by using a loopback interface. To review both the OSPF protocol and how to configure the loopback interface.
Router2>enable
Password:
Router2#show ip bgp
BGP table version is 2, local router ID is 192.168.255.254
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 192.168.6.0 192.168.1.2 0 0 100 i
The show ip bgp command has many options for viewing additional information. One of these options is neighbors. The command reveals the following information:
BGP version that is in use.
Neighbors that are visible. Although the command can be run with a neighbor specified.
BGP messages that have been seen.
BGP connections to other routers.
Event timer summary.
Router2>enable
Password:
Router2#show ip bgp neighbors
BGP neighbor is 192.168.1.2, remote AS 100, external link
BGP version 4, remote router ID 192.168.6.2
BGP state = Established, up for 00:11:11
Last read 00:00:11, hold time is 180, keepalive interval is 60 seconds
Neighbor capabilities:
Route refresh: advertised and received(old & new)
Address family IPv4 Unicast: advertised and received
Message statistics:
InQ depth is 0
OutQ depth is 0
Sent Rcvd
Opens: 1 1
Notifications: 0 0
Updates: 0 1
Keepalives: 14 14
Route Refresh: 0 0
Total: 15 16
Default minimum time between advertisement runs is 30 seconds
For address family: IPv4 Unicast
BGP table version 2, neighbor version 2
Index 1, Offset 0, Mask 0x2
1 update-group member
Sent Rcvd
Prefix activity: ---- ----
Prefixes Current: 0 1 (Consumes 48 bytes)
Prefixes Total: 0 1
Implicit Withdraw: 0 0
Explicit Withdraw: 0 0
Used as bestpath: n/a 1
Used as multipath: n/a 0
Outbound Inbound
Local Policy Denied Prefixes: -------- -------
Bestpath from this peer: 1 n/a
Total: 1 0
Number of NLRIs in the update sent: max 0, min 0
Connections established 1; dropped 0
Last reset never
Connection state is ESTAB, I/O status: 1, unread input bytes: 0
Local host: 192.168.1.240, Local port: 11001
Foreign host: 192.168.1.2, Foreign port: 179
Enqueued packets for retransmit: 0, input: 0 mis-ordered: 0 (0 bytes)
Event Timers (current time is 0x75ECBF0):
Timer Starts Wakeups Next
Retrans 15 0 0x0
TimeWait 0 0 0x0
AckHold 14 8 0x0
SendWnd 0 0 0x0
KeepAlive 0 0 0x0
GiveUp 0 0 0x0
PmtuAger 0 0 0x0
DeadWait 0 0 0x0
iss: 2844915809 snduna: 2844916121 sndnxt: 2844916121 sndwnd: 16073
irs: 1529136828 rcvnxt: 1529137192 rcvwnd: 16021 delrcvwnd: 363
SRTT: 259 ms, RTTO: 579 ms, RTV: 320 ms, KRTT: 0 ms
minRTT: 0 ms, maxRTT: 300 ms, ACK hold: 200 ms
Flags: active open, nagle
IP Precedence value : 6
Datagrams (max data segment is 1460 bytes):
Rcvd: 20 (out of order: 0), with data: 14, total data bytes: 363
Sent: 25 (retransmit: 0, fastretransmit: 0), with data: 15, total data bytes: 311
Other show ip bgp options include the following:
Router2>enable
Password:
Router2#show ip bgp ?
A.B.C.D IP prefix <network>/<length>, e.g., 35.0.0.0/8
A.B.C.D Network in the BGP routing table to display
all All address families
cidr-only Display only routes with non-natural netmasks
community Display routes matching the communities
community-list Display routes matching the community-list
dampening Display detailed information about dampening
filter-list Display routes conforming to the filter-list
inconsistent-as Display only routes with inconsistent origin ASs
injected-paths Display all injected paths
ipv4 Address family
ipv6 Address family
labels Display Labels for IPv4 NLRI specific information
neighbors Detailed information on TCP and BGP neighbor connections
nsap Address family
paths Path information
peer-group Display information on peer-groups
prefix-list Display routes matching the prefix-list
quote-regexp Display routes matching the AS path "regular expression"
regexp Display routes matching the AS path regular expression
replication Display replication status of update-group(s)
rib-failure Display bgp routes that failed to install in the routing
table (RIB)
route-map Display routes matching the route-map
summary Summary of BGP neighbor status
template Display peer-policy/peer-session templates
update-group Display information on update-groups
vpnv4 Address family
| Output modifiers
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dummies
Source:http://www.dummies.com/how-to/content/troubleshooting-border-gateway-protocol-bgp.html
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