From the BGP Intro, here are the points relevant to this post:
- BGP Peers in the same AS are iBGP (Internal) Peers
- BGP Peers NOT in the same AS are eBGP (External) Peers
- The underlying connection that forms BGP Peerings is connection oriented
- The connection runs on TCP port 179
- BGP Adjacencies = BGP “Peerings”
- eBGP is recommended by Cisco to be directly connected devices, while iBGP Peers often are not directly connected devices
- No information is exchanged between BGP Speakers until they are Peers
With that, since I already have R1 and R5 connected, lets make some eBGP Peers!
R1(config)#router bgp ?
<1-65535> Autonomous system number
R1(config)#router bgp 100 ?
<cr>
R1(config)#router bgp 100
R1(config-router)#
And so it begins! Nothing fancy there, and wanted to show exactly that, with the “router bgp (AS #) command there is no frills but getting into router configuration mode.
So the first step to configuration, is entering a “neighbor” command, which we see here:
R1(config-router)#neighbor ?
A.B.C.D Neighbor address
WORD Neighbor tag
X:X:X:X::X Neighbor IPv6 address
R1(config-router)#neighbor 172.12.15.5 ?
activate Enable the Address Family for this Neighbor
advertise-map specify route-map for conditional advertisement
advertisement-interval Minimum interval between sending BGP routing updates
allowas-in Accept as-path with my AS present in it
capability Advertise capability to the peer
default-originate Originate default route to this neighbor
description Neighbor specific description
disable-connected-check One-hop away EBGP peer using loopback address
distribute-list Filter updates to/from this neighbor
dmzlink-bw Propagate the DMZ link bandwidth
ebgp-multihop Allow EBGP neighbors not on directly connected
networks
fall-over session fall on peer route lost
filter-list Establish BGP filters
inherit Inherit a template
local-as Specify a local-as number
maximum-prefix Maximum number of prefixes accepted from this peer
next-hop-self Disable the next hop calculation for this neighbor
next-hop-unchanged Propagate the iBGP paths’s next hop unchanged for
this neighbor
password Set a password
peer-group Member of the peer-group
prefix-list Filter updates to/from this neighbor
remote-as Specify a BGP neighbor
remove-private-as Remove private AS number from outbound updates
route-map Apply route map to neighbor
route-reflector-client Configure a neighbor as Route Reflector client
send-community Send Community attribute to this neighbor
shutdown Administratively shut down this neighbor
soft-reconfiguration Per neighbor soft reconfiguration
timers BGP per neighbor timers
translate-update Translate Update to MBGP format
transport Transport options
ttl-security BGP ttl security check
unsuppress-map Route-map to selectively unsuppress suppressed
routes
update-source Source of routing updates
version Set the BGP version to match a neighbor
weight Set default weight for routes from this neighbor
R1(config-router)#neighbor 172.12.15.5
I stopped the command here to point out, this is not your usual “neighbor …” command, as there is no <cr> at the end here – We have to input something. I took the liberty of highlighting in red the modifier required for initial neighbor configuration:
R1(config-router)#neighbor 172.12.15.5 remote-as ?
<1-65535> AS of remote neighbor
R1(config-router)#neighbor 172.12.15.5 remote-as 500 ?
nlri Specify nlri type supported by neighbor
shutdown Administratively shut down this neighbor
<cr>
R1(config-router)#neighbor 172.12.15.5 remote-as 500
R1(config-router)#
I just wanted to see what was beyond the neighbor number, I found the “shutdown” command to remotely Administratively shutdown a neighbor was mischievously funny.
So now R1 is set in BGP AS 100 with a Neighbor statement points at R5’s interface IP, so logically if we go do the same thing to R5 we’re in business:
R5(config)#router bgp 500
R5(config-router)#neighbor 172.12.15.1 remote-as 100
R5(config-router)#
*Apr 8 01:50:59.199: %BGP-5-ADJCHANGE: neighbor 172.12.15.1 Up
R5(config-router)#
Success! I didn’t think that simple basic configuration would actually bring up that neighbor relationship, but sure enough it did.
So I’m kind of going off on my own to find some show commands, lets look at some different output of “show” commands to see what we find:
R5#sh bgp ?
* All scopes
all All address families
ipv4 Address family
ipv6 Address family
l2vpn Address family
nsap Address family
vpnv4 Address family
vpnv6 Address family
vrf VRF scope
R5#sh ip bgp ?
A.B.C.D Network in the BGP routing table to display
A.B.C.D/nn IP prefix <network>/<length>, e.g., 35.0.0.0/8
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
extcommunity-list Display routes matching the extcommunity-list
filter-list Display routes conforming to the filter-list
import Display route topology import / export activity
inconsistent-as Display only routes with inconsistent origin ASs
injected-paths Display all injected paths
ipv4 Address family
ipv6 Address family
l2vpn Address family
labels Display Labels for IPv4 NLRI specific information
neighbors Detailed information on TCP and BGP neighbor connections
nsap Address family
oer-paths Display all oer controlled paths
paths Path information
peer-group Display information on peer-groups
pending-prefixes Display prefixes pending deletion
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
topology Routing topology instance
update-group Display information on update-groups
version Display prefixes with matching version numbers
vpnv4 Address family
vpnv6 Address family
| Output modifiers
<cr>
Woah! That right there is “sh bgp” vs “sh ip bgp” which the latter has just a ton more options up front, but also “sh ip bgp” has the <cr> making it by itself a legal command!
So just wanted to demonstrate the difference between those two, I am finding “sh (something)” vs “sh ip (something)” differences to be very useful to know the differences in which command gets you which output.
Moving along, the first full show command to look at your bgp neighbors, is as follows:
R5#sh ip bgp neigh
BGP neighbor is 172.12.15.1, remote AS 100, external link
BGP version 4, remote router ID 1.1.1.1
BGP state = Established, up for 00:07:46
Last read 00:00:46, last write 00:00:41, hold time is 180, keepalive interval is 60 seconds
Neighbor sessions:
1 active, is not multisession capable
Neighbor capabilities:
Route refresh: advertised and received(new)
Four-octets ASN Capability: advertised
Address family IPv4 Unicast: advertised and received
Multisession Capability: advertised
Message statistics:
InQ depth is 0
OutQ depth is 0
Sent Rcvd
Opens: 1 1
Notifications: 0 0
Updates: 1 0
Keepalives: 10 10
Route Refresh: 0 0
Total: 12 11
Default minimum time between advertisement runs is 30 seconds
For address family: IPv4 Unicast
Session: 172.12.15.1
BGP table version 1, neighbor version 1/0
Output queue size : 0
Index 1
1 update-group member
Sent Rcvd
Prefix activity: —- —-
Prefixes Current: 0 0
Prefixes Total: 0 0
Implicit Withdraw: 0 0
Explicit Withdraw: 0 0
Used as bestpath: n/a 0
Used as multipath: n/a 0
Outbound Inbound
Local Policy Denied Prefixes: ——– ——-
Total: 0 0
Number of NLRIs in the update sent: max 0, min 0
Address tracking is enabled, the RIB does have a route to 172.12.15.1
Connections established 1; dropped 0
Last reset never
Transport(tcp) path-mtu-discovery is enabled
Graceful-Restart is disabled
Connection state is ESTAB, I/O status: 1, unread input bytes: 0
Connection is ECN Disabled, Mininum incoming TTL 0, Outgoing TTL 1
Local host: 172.12.15.5, Local port: 179
Foreign host: 172.12.15.1, Foreign port: 54105
Connection tableid (VRF): 0
Maximum output segment queue size: 50
Enqueued packets for retransmit: 0, input: 0 mis-ordered: 0 (0 bytes)
Event Timers (current time is 0x2B85F0):
Timer Starts Wakeups Next
Retrans 11 0 0x0
TimeWait 0 0 0x0
AckHold 9 8 0x0
SendWnd 0 0 0x0
KeepAlive 0 0 0x0
GiveUp 0 0 0x0
PmtuAger 0 0 0x0
DeadWait 0 0 0x0
Linger 0 0 0x0
ProcessQ 0 0 0x0
iss: 3920965672 snduna: 3920965944 sndnxt: 3920965944 sndwnd: 16113
irs: 4259821679 rcvnxt: 4259821915 rcvwnd: 16149 delrcvwnd: 235
SRTT: 231 ms, RTTO: 769 ms, RTV: 538 ms, KRTT: 0 ms
minRTT: 0 ms, maxRTT: 300 ms, ACK hold: 200 ms
Status Flags: passive open, gen tcbs
Option Flags: nagle, path mtu capable
IP Precedence value : 6
Datagrams (max data segment is 1460 bytes):
Rcvd: 21 (out of order: 0), with data: 10, total data bytes: 235
Sent: 21 (retransmit: 0, fastretransmit: 0, partialack: 0, Second Congestion: 0), with data: 12, total data bytes: 271
Packets received in fast path: 0, fast processed: 0, slow path: 0
fast lock acquisition failures: 0, slow path: 0
R5#
I’ll cut out the unnecessary bottom half of output that we aren’t concerned with (yet) moving forward, but I did want to post the output in it’s entirety. A lot of good information right at the top like the RID, Remote IP, Remote AS #, and updates both sent and received.
Now the status currently shows Active in the above “sh bgp nei” output, but I want to go back in time and look at BGP from R1’s perspective before it formed a Peering with R5:
R5(config)#router bgp 500
R5(config-router)#no neighbor 172.12.15.1 remote-as 100
R5(config-router)#
*Apr 8 02:11:36.307: %BGP_SESSION-5-ADJCHANGE: neighbor 172.12.15.1 IPv4 Unicast topology base removed from session Neighbor deleted
*Apr 8 02:11:36.307: %BGP-5-ADJCHANGE: neighbor 172.12.15.1 Down Neighbor deleted
R5(config-router)#
ASR#1
[Resuming connection 1 to r1 … ]
*Mar 30 23:03:18.177: %BGP-5-ADJCHANGE: neighbor 172.12.15.5 Down Peer closed the session
R1#sh ip bgp nei
BGP neighbor is 172.12.15.5, remote AS 500, external link
BGP version 4, remote router ID 0.0.0.0
BGP state = Active
Last read 00:00:40, last write 00:00:40, hold time is 180, keepalive interval is 60 seconds
Message statistics:
InQ depth is 0
OutQ depth is 0
Sent Rcvd
Opens: 1 1
Notifications: 0 0
Updates: 0 1
Keepalives: 23 25
Route Refresh: 0 0
Total: 24 27
Default minimum time between advertisement runs is 30 seconds
I stopped the output there, shortly to be touched on again, but let’s compare R5’s up and good state vs R1’s now down and bad state with “sh bgp nei”:
Good
BGP state = Established, up for 00:07:46
Bad
BGP state = Active
The Active state with BGP indicates a BGP Peer connection that doesn’t yet fully exist, and actually I will list off the different states before going any further:
- Active = (Explained above)
- Idle = Initial state of BGP Peering, if you see a connection stay in Idle status, make sure the neighbors IP address is correct, and that the local router knows how to reach it
- Connect = Follows Idle status, in this state a TCP connection request has been sent but a response has not yet been received. If the TCP connection completes BGP moves to an “OpenSent” state, if it does not it goes back to “Active”
- OpenSent = The BGP speaker has received an “Open” message from the peer, in this state BGP determines if the peer in the same BGP AS or a different (iBGP vs eBGP)
- OpenConfirm = The BGP speaker is waiting for a keepalive, and when one is received, the state moves to “Established”
- Established = Peering is complete and update packets are exchanged
That is the order of Peering status from beginning of a connection to the ending (hopefully) of being successful in becoming “Established.”
Now back to the second half of the “sh ip bgp nei” command on R1, I’ll give it the full output as it is much shorter when it’s not established:
R1#sh ip bgp nei
BGP neighbor is 172.12.15.5, remote AS 500, external link
BGP version 4, remote router ID 0.0.0.0
BGP state = Active
Last read 00:20:53, last write 00:20:53, hold time is 180, keepalive interval is 60 seconds
Message statistics:
InQ depth is 0
OutQ depth is 0
Sent Rcvd
Opens: 1 1
Notifications: 0 0
Updates: 0 1
Keepalives: 23 25
Route Refresh: 0 0
Total: 24 27
Default minimum time between advertisement runs is 30 seconds
For address family: IPv4 Unicast
BGP table version 1, neighbor version 0/0
Output queue size : 0
Index 1, Offset 0, Mask 0x2
1 update-group member
Sent Rcvd
Prefix activity: —- —-
Prefixes Current: 0 0
Prefixes Total: 0 0
Implicit Withdraw: 0 0
Explicit Withdraw: 0 0
Used as bestpath: n/a 0
Used as multipath: n/a 0
Outbound Inbound
Local Policy Denied Prefixes: ——– ——-
Total: 0 0
Number of NLRIs in the update sent: max 0, min 0
Connections established 1; dropped 1
Last reset 00:20:57, due to Peer closed the session
No active TCP connection
R1#
At the very bottom of the output, is very good information, as to when the connection dropped / reason / Any active TCP Connections, so you can see if the router ever “Established” a BGP connection with a BGP Peer.
So I will through the configuration back on R5, and point out one more thing for this small but important start to a lab:
R5(config-router)#neighbor 172.12.15.1 remote-as 100
R5(config-router)#^Z
R5#
*Apr 8 02:42:07.635: %BGP-5-ADJCHANGE: neighbor 172.12.15.1 Up
R5#sh ip bgp nei ?
A.B.C.D Neighbor to display information about
X:X:X:X::X IPv6 neighbor to display information about
| Output modifiers
<cr>
R5#sh ip bgp nei 172.12.15.1
BGP neighbor is 172.12.15.1, remote AS 100, external link
BGP version 4, remote router ID 1.1.1.1
BGP state = Established, up for 00:00:24
Last read 00:00:24, last write 00:00:24, hold time is 180, keepalive interval is 60 seconds
Neighbor sessions:
1 active, is not multisession capable
Neighbor capabilities:
Route refresh: advertised and received(new)
Four-octets ASN Capability: advertised
Address family IPv4 Unicast: advertised and received
Multisession Capability: advertised
Message statistics:
InQ depth is 0
OutQ depth is 0
Sent Rcvd
Opens: 1 1
Notifications: 0 0
Updates: 1 0
Keepalives: 2 3
Route Refresh: 0 0
Total: 4 4
Default minimum time between advertisement runs is 30 seconds
I just wanted to point out in red, that the “external link” in this command is going to be how we can tell if it’s eBGP or iBGP is with this command, and also that the command can be narrowed down per neighbor but it is “sh ip bgp nei …” and NOT “sh bgp nei”:
R5#sh bgp neigh ?
% Unrecognized command
R5#sh bgp neigh
This is very important to note, at the top “sh bgp …” leaves a lot of verification and troubleshooting commands to be desired, or hidden behind the initial modifiers. I looked into the show bgp commands and they seemed a bit advanced for what I am studying so I won’t bother going into them beyond that, but important to note if you are typing a verification command it will begin with “sh ip bgp …”
So to summarize what is needed for a eBGP Peering to succeed:
- “router bgp (local AS #)” in global config
- “neighbor (IP address) remote-as (Remote AS #)
- Both routers to know how to reach the other
- “sh ip bgp nei” for verification
That’s all I got for this beginning lab, next up we will add an iBGP Peer, TGIF!
The DEVNET GRIND! 