BGP Routing - Implementing Basic BGP Part 1
The lab is now setup as above, with OSPF being the interior routing protocol and BGP the external ... and well we will configure an internal BGP peering between R4 and R1 later.
Lets check out the commands at our disposal in BGP:
Router-5(config)#router bgp 6500
Router-5(config-router)#neighbor 10.1.45.1 ?
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
Check out all them options!!! wow boi!
First thing we need to do, in order to get started and get this relationship up and running is the remote-as command, this will get the conversation flowing with OPEN msgs to the neighbour statement we are configuring.
Router-5(config-router)#neighbor 10.1.45.1 remote-as 5500
Router-5#show ip bgp summ
BGP router identifier 109.170.1x7.xx, local AS number 6500
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd <--seq of the DB it has recieved
10.1.45.1 4 5500 0 0 0 0 0 never Active
Here we can see we have our configured neighbor .... totally dead (Up/Down column) at the moment, no messages sent or recieved. Notice the state/PfxRcd is set to ACTIVE, this means the exact same thing as in the EIGRP world:
ACTIVE = BAD
It is actively trying to bring the relationship up (whereas EIGRP was actively trying to find a backup route)
Lets configure his buddy;
Router-4(config)#router bgp 5500
Router-4(config-router)#neighbor 10.1.45.2 remote-as 6500
Now, lets see how long it takes for the relationship to come up!.... remember this is a SLOW protocol:
Router-5#show clock
21:48:35.778 GB Sat Nov 19 2011
Router-5#
013151: Nov 19 21:48:50.025: %BGP-5-ADJCHANGE: neighbor 10.1.45.1 Up
Okay, 15 seconds, that came up pretty quick (bit dissapointed in away as jeremy - the instructor was saying it can take as long as 60 seconds!)
Router-4#show ip bgp sum
BGP router identifier 10.1.45.1, local AS number 5500
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
10.1.45.2 4 6500 8 7 1 0 0 00:04:36 0
So notice we have msgs being sent and recieved, BUT the state/PfxRcd column is still 0, thats because with BGP you can establish a neighbour and not send/exchange any routes/information ..... SO different to what we are used to!! (so is statically configuring the neighbours too! lol BUT i guess if it did auto discover neighbours, imagine the issues ISPs would have, peoples home routers bringing up BGP, peering with them and then injecting routes all over the place LOL)
Now for iBGP, there is no funky command or any funny business, it will just be when we enter the remote-as command we enter "our" internal AS and the router will know it will be an iBGP speaker.
As the plan goes, we plan to get a iBGP peering going with routers 1&4, even though, they are not directly connected :0)
We also then need to advertise the loopbacks into the internal routing protocol (OSPF) so the rest of the network knows about them (otherwise the BGP messages would not make it from either iBGP speaker)
Router-4#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router-4(config)#inter loopback4
Router-4(config-if)#ip address 4.4.4.4 255.255.255.255
Router-4(config-if)#exit
Router-4(config)#router ospf 1
Router-4(config-router)#network 4.4.4.4 0.0.0.0 area 0
lets do the same for Router-1, but first double check he is now seeing this new loopback;
Router-1#show ip route
4.0.0.0/32 is subnetted, 1 subnets
O 4.4.4.4 [110/75] via 10.1.12.2, 00:01:02, Ethernet0/0
[110/75] via 10.1.13.2, 00:01:02, Serial0/0
Swweeet, lets do the loopback on this router too;
Router-1(config)#inter loopback1
Router-1(config-if)#ip address 1.1.1.1 255.255.255.255
Router-1(config-if)#exit
Router-1(config)#router ospf 1
Router-1(config-router)#network 1.1.1.1 0.0.0.0 area 0
lets configure the BGP part now on both routers;
Router-4(config)#router bgp 5500
Router-4(config-router)#neighbor 1.1.1.1 remote-as 5500
Router-1(config)#router bgp 5500
Router-1(config-router)#neighbor 4.4.4.4 remote-as 5500
BUT the neighbourship will NEVER form;
Router-1#show ip bgp summary
BGP router identifier 1.1.1.1, local AS number 5500
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
4.4.4.4 4 5500 0 0 0 0 0 never Active
BECAUSE .... the BGP packets will be hitting the neighbour with a source interface IP address which wont be the loopback (whichever OSPF determines the quicker path to the neighbour), therefore the relationship will not form as the BGP router has no idea who that interface is/belongs too, it wont be the statically configured IP in the neighbour command!!
So to fix this, we can use the update-source command, here we can specify the loopback of Router-4
Router-4(config)#router bgp 5500
Router-4(config-router)#neighbor 1.1.1.1 update-source loopback 4
lets go do the other router:
Router-1(config)#router bgp 5500
Router-1(config-router)#neighbor 4.4.4.4 update-source loopback 1
ah ha ...
Router-1#
000018: *Mar 1 01:52:30.823: %BGP-5-ADJCHANGE: neighbor 4.4.4.4 Up
Router-1#show ip bgp summ
BGP router identifier 1.1.1.1, local AS number 5500
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
4.4.4.4 4 5500 3 4 1 0 0 00:00:37 0
Notice, we now have no ACTIVE state and a 0 for the revision of the BGP database.
Over on router-4 we have the iBGP and eBGP neighbourships up;
Router-4#show ip bgp summ
BGP router identifier 10.1.45.1, local AS number 5500
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
1.1.1.1 4 5500 10 9 1 0 0 00:06:10 0
10.1.45.2 4 6500 52 51 1 0 0 00:48:19 0
just for clarification and sanity checking:
Router-4#show ip bgp neighbors
BGP neighbor is 1.1.1.1, remote AS 5500, internal link <---iBGP
Index 2, Offset 0, Mask 0x4
BGP version 4, remote router ID 1.1.1.1
BGP state = Established, table version = 1, up for 00:08:10
Last read 00:00:10, hold time is 180, keepalive interval is 60 seconds
Minimum time between advertisement runs is 5 seconds
Received 12 messages, 0 notifications, 0 in queue
Sent 11 messages, 0 notifications, 0 in queue
Prefix advertised 0, suppressed 0, withdrawn 0
Connections established 1; dropped 0
Last reset never
0 accepted prefixes consume 0 bytes
0 history paths consume 0 bytes
Connection state is ESTAB, I/O status: 1, unread input bytes: 0
Local host: 4.4.4.4, Local port: 11012
Foreign host: 1.1.1.1, Foreign port: 179
BGP neighbor is 10.1.45.2, remote AS 6500, external link <----eBGP
Index 1, Offset 0, Mask 0x2
BGP version 4, remote router ID 109.170.1x7.xx (my public IP range)
BGP state = Established, table version = 1, up for 00:50:50
Last read 00:00:50, hold time is 180, keepalive interval is 60 seconds
Minimum time between advertisement runs is 30 seconds
Received 54 messages, 0 notifications, 0 in queue
Sent 53 messages, 0 notifications, 0 in queue
Prefix advertised 0, suppressed 0, withdrawn 0
Connections established 1; dropped 0
Last reset never
0 accepted prefixes consume 0 bytes
0 history paths consume 0 bytes
BUT, we still have no BGP routes;
Router-4#show ip bgp
Becuase the neighbours and networks/routes are seperated in BGP unlike other routing protocols, whereby you have neighbours and you are told of the routes they know (via the network command under the routing protocol, which would tell the router what interfaces to advertise on and what networks to advertise into the process) ...
So with BGP, we have neighbour commands to bring up neighbour relationships BUT then to get the routing aspect we have to then apply the network commands, so they are totally seperate with BGP.
Right before we get BGP exchanging routes, lets just look at an example and at the same time tidy up our relationship with Router-5 (lets get it using a loopack interface tooo)
Router-5(config)#interface loopback5
Router-5(config-if)#ip address 5.5.5.5 255.255.255.255
Plus we need a static route to our neighbour, as if we were an ISP, we would not be running an interior routing protocol;
Router-5(config-if)#ip route 4.4.4.4 255.255.255.255 10.1.45.1
test;
Router-5#ping 4.4.4.4
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 4.4.4.4, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/3/4 ms
Now lets set a static on Router-4 to reach Router-5's (the ISP) loopback, as currently this router does not no anything about the 5.5.5.5 network;
Router-4(config)#ip route 5.5.5.5 255.255.255.255 10.1.45.2
Router-4#ping 5.5.5.5
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 5.5.5.5, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/3/4 ms
Lets just say for a moment, that say we had to links (T1 lines) going to the same ISP, we could have another static route on the ISP end and have that point to a 2nd loopback interface and then the cisco router would perform load balancing (see diagram above)
HOWEVER there is a rule for eBGP, and that is the neighbours MUST BE DIRECTLY CONNECTED, although we may be thinking well, yeah they are, the router has to pass through the routers interface to get to the loopback, and so see's it as being more than 1 hop away ...
SO, this can be fixed by apply the update-source command and setting it to the loopback 4.4.4.4 and then with this command:
Router-4(config-router)#neighbor 5.5.5.5 ebgp-multihop ?
<1-255> maximum hop count
<cr>
Router-4(config-router)#neighbor 5.5.5.5 ebgp-multihop 2
But again, be careful as for routing loops!! recommended is no more than 5!
RIGHT, so lets watch the neighbourship come up after i apply this on both routers;
Router-4(config)#router bgp 5500
Router-4(config-router)#neighbor 5.5.5.5 ebgp-multihop 2
Router-4(config-router)#neighbor 5.5.5.5 update-source loopback 4
Router-5(config)#router bgp 6500
Router-5(config-router)#neighbor 4.4.4.4 ebgp-multihop 2
Router-5(config-router)#neighbor 4.4.4.4 update-source loopback 5
Router-5(config-router)#
013233: Nov 19 23:22:28.467: %BGP-5-ADJCHANGE: neighbor 4.4.4.4 Up
Router-5#show ip bgp summ
BGP router identifier 109.170.1x7.xx, local AS number 6500
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
4.4.4.4 4 5500 3 4 1 0 0 00:00:57 0
SWEEET, without that multihop command this neighbour would never of come up.
RIGHT, lets get on with part 2
