Let’s now consider common BGP Routing issues
for example, we might have incorrect network
statement, let’s double check to make sure that we have enter the network
statement correctly, let’s make sure that we haven’t transpose
to couple of numbers in the IP Address in the example but something that bit more
unique to BGP is a concept of the “Next Hop
Router”
A big point to keep in
mind is that when “an advertisement comes into Autonomous
System and it’s advertised over IBGP connections to Routers all belonging to
the same Autonomous System, by default the next Hop Router information is not updated”
and it’s possible that we got a Router residing in this Autonomous System that
has a no way to get to the next Hop Address that’s been advertised to in this BGP
advertisement.
One way we could
address that is to “Redistribute BGP into the IGP”,
we could alternately set up the static Route to tell the Router how to get to
that Next-Hop-Address however, we could go into the BGP Router Configuration of
a Router and say “Neighbor” and then we give the neighbors
IP Addresses or Peer Group name and then we say “Next-Hop-Self”
It’s going to cause a
Router to advertise its IP Address as the Next Hop IP Address when
it’s sending a BGP Routing update to an IBGP neighbors,
those are few different ways we might be able to address an issue where a
Router within an Autonomous System is not able to reach the next Hop Route
advertised into BGP Advertisement.
Similar to EIGRP we could have a Split-Horizon issue which say’s if a Route was learned on an interface
that interface is not going to advertised the Route back out and in a point to
multipoint network maybe like frame-relay that could be an issue.
We could have Filtered Routes, we might be doing some
Route Filtering maybe intentionally or maybe there is more believable Route
like a Static Route in IP Routing Table and
it’s possible that BGP is taking a Sub Optimal Route because of Inappropriate path Attributes
Infect to
illustrate that bit further, let’s go out to
interface and Troubleshoot a very real world issue when you got an enterprise
network connected out to more than One ISP and by default we are taking a
sub-optimal path and let’s see how to fix that.
Notice that Router R2 has a connection of 768K out of ISP1, it’s got a
1.544Mbps connection out to ISP2 and clearly, we would prefer to use the ISP
that had a highest bandwidth we would prefer to use ISP2, let’s see what we using
right now though, if we do a
Ø R2#show ip route
I am using 9.9.9.9
as an example, of some internet destination, that’s actually a Loopback
interface on my INET Router on picture, and it
looks like to get there i am gonna go via 198.51.100.2, that’s actually ISP1
that’s the slower link and why is that? let’s take a look at BGP
Table.
Ø R2#show ip bgp
BGP knows about two
different ways to get there it knows that we could get there via ISP2 or ISP1
but notice the (greater than >) sign,
this is telling us that we were using ISP1 and
that’s this enterprise network Autonomous System 65001 as it going out to the
internet, how does it look from the internet as coming back in,
let’s go to the INTERNET Router and let’s do a
Ø INET#show ip bgp
Let’s look at one of
Routes inside the Enterprise Autonomous System, 192.0.2.0
as an example, i can get their couple of different ways but notice the Greater
than “>” sign is it looks like from the prospective of the internet we gonna
go via ISP1 again we going via the slower link and underline causes most likely
a lower Router ID(Next Hop Address) and in the Real World, we
probably not gonna be able to do any work on the ISP Router we have to do our
work on Enterprise Router in this case Router R2, and we want to be able to do Configuration just on R2 to
influence both Outbound and Inbound Path Selection.
To influence the Outbound Path Selection let’s use the Local
Preference, notice that we do not
currently have a Local-Preference set for Routes that we learned via the ISP1
and ISP2 Routers, let’s change and Higher Local Preference values are preferred, to do this we create a couple
of Route-Maps. I am gonna create a Route-map for ISP1
Ø
R2(config)#route-map ISP1
Ø R2(config-route-map)#set
local-preference 100
I am going to
apply this to Routes that I am learning from ISP1, let’s create another
Route-Map for ISP2 and i am gonna set the higher Local-Preference value to make
it more preferable
Ø R2(config)#route-map ISP2
Ø R2(config-route-map)#set
local-preference 200
Now to make this take
effect we have to apply the Route Maps as a part of the neighbor statement in
Router Configuration mode for BGP
Ø R2(config)#router bgp 65001
Ø R2 (config-router)#neighbor
198.51.100.2 route-map ISP1 in
I want to apply that
in Inbound direction another word, as i learned Routes from that neighbor, i
want to apply the ISP1 Route-Map which is going to assign those Routes to a
local Preference of 100, let’s do something similar for the other neighbor for ISP2.
Ø Router(config-router)#neighbor
198.51.100.6 route-map ISP2 in
To make this take effect, let’s reset
the BGP Process
Ø R2#clear ip bgp * soft
Ø R2#show ip bgp
And look at this
now in order to get to this 9.9.9.9/32 network, we gonna go via ISP2, notice
the Greater than “>” sign why is that?
It’s because of the Local-Preference there is a Local-Preference
of 200 to use ISP2, there is a Local-Preference of 100 use ISP1,
we’ve now influenced Outbound Path Selection to
prefer the ISP that has more bandwidth however, we now need to Influence Inbound Path Selection, host
out on the internet are still coming into us based on the shortest Autonomous
System Path so, let’s do this.
Let’s say that we want
to Prepend to that Autonomous System Path some additional Instances of our own
Autonomous System, that’s what i mean, let’s create another Route-Map
Ø
R2(config)#route-map ASPATH
ü
This is name, i made up that’s not some sort of Cisco iOS Keyword
I am
setting the Autonomous System Path Attribute such that, i am prepending additional
instances of the local Autonomous System, i am gonna add couple of additional
instances
Ø R2(config-route-map)#set
aspath prepend 65001 65001
And i am only
going to apply this Route-map to ISP1, ISP1 is going to appear to have more
Autonomous System to transit in order to get into the Enterprise Network.
Ø R2(config)#router bgp 65001
Ø Router(config-router)#neighbor
198.51.100.2 route-map ASPATH out
This time it gonna be
Outbound direction when i am sending Route advertisements to ISP1, i am going
to be prepending two additional instances of my own Autonomous System to the
ASPATH, let’s do a Soft Reset of BGP again.
Ø R2#clear ip bgp * soft
And
let’s go out to the INTERENT Router and see how it looks coming back in the
Enterprise Network
Notice before, when we looking into the Enterprise network we are going via
ISP1 the Autonomous System path had the same length where they went via ISP1 or
ISP2, now we should update that
such that, it gonna be a longer path to go via
ISP1 let’s once again do a
Ø INET#show ip bgp
Look at this now get to this internal 192.0.2.0 network inside of our
enterprise network the best Path indicating is with Greater than “>” sign,
it’s ISP2 why is that?
Well now the Autonomous System Path via ISP2 is significantly Shorter then
the path via ISP1 and the reason is, we Prepended a couple of additional
instances of our local Autonomous System to the AS Path that was being
advertised out the ISP1 and that’s a very real world look at how
we could Troubleshoot a scenario where we have a
sub-optimal path when an Enterprise network is connecting out to more than 1
ISP remember, we need to influence both Outbound and Inbound Path Selection.
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