BGP - Longer AS-path
November 14th, 2008 in
BGP,
IP IGP Routing | tags:
as-path,
BGP In this post I will talk about BGP and the advertisement of routes. You can see the diagram used for this network below.
Router PE_AS2 and CE1_AS3 peer
Router PE_AS2 and REMOTE_CE peer
Router REMOTE_CE and CE2_AS3 peer
Router CE2_AS3 and CE1_AS3 peer
On router PE_AS2 I will configure two loopback ip-addresses and advertise them in BGP.
You can see the configuration of the routers below :
What I want to show is the way the loopback addresses of router PE_AS2 get advertised throughout the network. Especially how router CE1_AS3 receives the routes from router PE_AS2 and from router CE2_AS3 when the connection between router PE_AS2 to CE1_AS3 breaks down.
PE_AS2 :
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hostname PE_AS2
!
interface Loopback1
ip address 1.1.1.1 255.255.255.255
!
interface Loopback2
ip address 2.2.2.2 255.255.255.255
!
interface FastEthernet0/0
description connection to CE1 Fa0/0
ip address 10.0.0.2 255.255.255.0
duplex auto
speed auto
no shut
!
interface FastEthernet1/0
description connection to RemoteCE Fa0/0
ip address 30.0.0.1 255.255.255.0
duplex auto
speed auto
no shut
!
router bgp 2
no synchronization
bgp log-neighbor-changes
network 1.1.1.1 mask 255.255.255.255
network 2.2.2.2 mask 255.255.255.255
timers bgp 10 30
neighbor 10.0.0.1 remote-as 3
neighbor 30.0.0.2 remote-as 1
no auto-summary
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RemoteCE :
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hostname RemoteCE
!
interface FastEthernet0/0
description connection to PE Fa1/0
ip address 30.0.0.2 255.255.255.0
duplex auto
speed auto
no shut
!
interface FastEthernet1/0
description connection to CE2 Fa0/0
ip address 20.0.0.1 255.255.255.0
duplex auto
speed auto
no shut
!
router bgp 1
no synchronization
bgp log-neighbor-changes
timers bgp 10 30
neighbor 20.0.0.2 remote-as 3
neighbor 30.0.0.1 remote-as 2
no auto-summary
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CE2_AS3 :
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hostname CE2_AS3
!
interface FastEthernet0/0
description connection to RemoteCE Fa0/1
ip address 20.0.0.2 255.255.255.0
duplex auto
speed auto
no shut
!
interface FastEthernet1/0
description connection to CE1 Fa0/1
ip address 192.168.0.2 255.255.255.0
duplex auto
speed auto
no shut
!
router bgp 3
no synchronization
bgp log-neighbor-changes
timers bgp 10 30
neighbor 20.0.0.1 remote-as 1
neighbor 192.168.0.1 remote-as 3
neighbor 192.168.0.1 next-hop-self
no auto-summary
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CE1_AS3 :
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hostname CE1_AS3
!
interface FastEthernet0/0
description connection to PE Fa0/0
ip address 10.0.0.1 255.255.255.0
no shut
duplex auto
speed auto
!
interface FastEthernet1/0
description connection to CE2 Fa1/0
ip address 192.168.0.1 255.255.255.0
duplex auto
speed auto
no shut
!
router bgp 3
no synchronization
bgp log-neighbor-changes
timers bgp 10 30
neighbor 10.0.0.2 remote-as 2
neighbor 192.168.0.2 remote-as 3
neighbor 192.168.0.2 next-hop-self
no auto-summary
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As you can see the only networks advertised in this example are the loopback ip-addresses of PE_AS2. Now let’s take a look in router CE1_AS3 bgp table :
CE1_AS3 :
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CE1_AS3#sh ip bgp sum
BGP router identifier 192.168.0.1, local AS number 3
BGP table version is 5, main routing table version 5
2 network entries using 234 bytes of memory
2 path entries using 104 bytes of memory
2/1 BGP path/bestpath attribute entries using 248 bytes of memory
1 BGP AS-PATH entries using 24 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 610 total bytes of memory
BGP activity 2/0 prefixes, 4/2 paths, scan interval 60 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
10.0.0.2 4 2 69 70 5 0 0 00:10:51 2
192.168.0.2 4 3 70 69 5 0 0 00:10:55 0
CE1_AS3#sh ip bgp
BGP table version is 5, local router ID is 192.168.0.1
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
*> 1.1.1.1/32 10.0.0.2 0 0 2 i
*> 2.2.2.2/32 10.0.0.2 0 0 2 i
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As you can see router CE1_AS3 learns only routes via PE_AS3 although it also has a peering with router CE2_AS3. Now let’s look at router’s CE2_AS3 bgp table :
CE2_AS3 :
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CE2_AS3#sh ip bgp sum
BGP router identifier 192.168.0.2, local AS number 3
BGP table version is 5, main routing table version 5
2 network entries using 234 bytes of memory
4 path entries using 208 bytes of memory
3/1 BGP path/bestpath attribute entries using 372 bytes of memory
2 BGP AS-PATH entries using 48 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 862 total bytes of memory
BGP activity 2/0 prefixes, 4/0 paths, scan interval 60 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
20.0.0.1 4 1 91 91 5 0 0 00:14:33 2
192.168.0.1 4 3 89 90 5 0 0 00:14:13 2
CE2_AS3#sh ip bgp
BGP table version is 5, local router ID is 192.168.0.2
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
*>i1.1.1.1/32 192.168.0.1 0 100 0 2 i
* 20.0.0.1 0 1 2 i
*>i2.2.2.2/32 192.168.0.1 0 100 0 2 i
* 20.0.0.1 0 1 2 i
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When we look at what router CE2_AS3 advertises onto his neighbor CE1_AS3 we get :
CE2_AS3 :
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CE2_AS3#sh ip bgp nei 192.168.0.1 adv
Total number of prefixes 0
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So what we can conclude here is that when a router like router CE1_AS3 in this case receives the same routes from two peers but from one peer with a longer AS-path it doesn’t advertise it on to the peer which advertises it with a shorter AS-path (like CE1_AS3 in this case).